Article

Evidence for the subducting lithosphere under Southern Vancouver Island and Western Oregon from teleseismic P wave conversions

January 1981
Journal of Geophysical Research Atmospheres 86(NB5):3857-3866

January 1981
86(NB5):3857-3866

Authors:

The University of Memphis

Long-period teleseismic P waves recorded at VIC (Victoria, British Columbia) and COR (Corvallis, Orgeon) show anomalously large Ps conversions and later arriving P-to-S reverberations not observed from typical continental crustal sections or from previously proposed structures for these stations determined from refraction surveys. The timing and large amplitude of the Ps phase, relative to direct P, suggests a high velocity-contrast interface at 45 to 50 km depth under VIC and COR forming the base of a distinct low velocity zone. This interface is proposed to be the oceanic Moho which is being subducted under North America. Off azimuth Ps recorded at COR is consistent with a 20° eastward dip for the interface. Horizontal particle motion at both sites show evidence for lateral heterogeneity in local crustal structure. The distinct low velocity zone and its negative gradient with depth has important consequences for refraction interpretation in the region since the usual assumption of increasing velocity with depth is violated. Crustal thicknesses derived from such misinterpretation may be overestimated. In principle, this type of structure suggests a solution for the Vancouver Island crustal thickness problem in which the observed positive Bouguer gravity anomaly is inconsistent with the 50 km thick crustal thickness derived from previous refraction work.

Dipping Structure under Dourbes, Belgium, Determined by Receiver Function Modeling and Inversion

Article

Feb 1995

Teleseismic broadband P and S waves recorded at the NARS station NE06 (Dourbes, Belgium) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. Azimuthally varying radial and tangential components have been observed on 38 recordings after vector rotation of horizontal P waves into the ray direction. The tangenital P waves attain amplitudes comparable to the radial components from the east with negative polarity and west with positive polarity, but tend to be zero in the north and south, suggesting major discontinuities in the crust dipping southward. The SH wave from the east contains a large SPmP phase, an S-to-P conversion at the free surface and then reflected back to the surface from the Moho. The polarity of this SPmP phase presents further evidence for a southward-dipping Moho. We employ ray theory for three-dimensionally dipping interfaces to compute the P-wave response. Linear inverse theory with smoothness constraints is applied to the simultaneous inversions of P-wave receiver functions for four different backazimuths. Through the progressive change of interface strike and dip and the inversion of layer shear-wave velocities, a dipping crustal model that is consistent with both the observed waveforms and results of previous local geophysical surveys has been determined. The results suggest a large velocity contrast in the shallow structure near the surface, another major interface at a depth of 12 km with dip of 10°, and a seismically transparent unit below the interface. The interface at a depth of 12 km reportedly emerges at the Midi fault 50 km north of the station NE06.

2000JB000039

Data

Full-text available

Jan 2016

Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

Article

Full-text available

Jan 1989

The Pacific Coastal States form a complex geologic environment in which the crust and lithosphere have been continuously reworked. We divide the region tectonically into the southern transform regime of the San Andreas fault and the northern subduction regime, and summarize the geophysical framework with contour maps of crustal thickness, lithospheric and seismicity cross sections, and results from site-specific geophysical studies. The uniformity of crustal thickness (30 ± 2 km) in southern California is remarkable, and appears to be primarily the result of crustal extension in the Mojave Desert and ductile shear of the lower crust along the plate transform boundary. Southern California seismicity defines a broad zone of deformation that extends from the Borderland to the Mojave Desert (about 300 km). The geophysical framework of central and northern California records magmatism and accretion associated with the Mesozoic and Cenozoic subduction, late Cenozoic transform faulting, and in the Basin and Range to the east, extension. The crust thickens from about 20 km at the coast to as much as 55 km in the Sierra Nevada, and thins to about 30 km in the Basin and Range. Cross sections of the crust show that seismic velocities and densities vary significantly over short distances perpendicular to the coast, reflecting processes that include the accretion of oceanic sediments and igneous crust, and significant lateral motion of crustal blocks. Maximum hypocentral depths in central California become deeper as the crust thickens to the west, but seismicity is low beneath the Great Valley and Sierra Nevada, which together appear to form a relatively undeforming block. The lower crust of the Pacific Coastal States has a high average seismic velocity (6.7 km/sec or greater), which probably is the product of tectonic underplating of oceanic crust and/or magmatic underplating by a basaltic melt. The geophysical framework of the subduction regime is dominated by the subduction of the Gorda and Juan de Fuca plates, arc magmatism in the Cascade Range, and plateau volcanism and rifting in the back arc. As defined by earthquake hypocenters, the Juan de Fuca plate dips at a shallow angle (3°) within 50 km of the trench, increases to 10° beneath the continental shelf and coastal province, and plunges more steeply (25° dip) a short distance west of the Cascade Range. Whereas a true continental Moho exists from the Cascade Range to the east, the Moho is that of the subducting oceanic lithosphere west of the range. Crustal thickness increases from about 18 km at the coast to about 42 km beneath the Cascades Range, a distance of about 200 km. The crustal velocity structure and crustal thickness of the Cascades Range is relatively uniform along its axis. The velocity structure shows high velocities (greater than 6.5 km/sec) at all depths greater than 10 km, indicating rocks of an intermediate-to-mafic composition, and a relatively low upper-mantle velocity of 7.7 ±0.1 km/sec, indicating high temperatures. Seismological studies at the volcanic centers of the Cascades indicate that the dimensions of subsurface magmatic systems are small, on the order of a few kilometers. Some 1 to 6 km of Miocene and younger basaltic extrusives cover much of the back arc, thereby obscuring most of the pre-Miocene geology. However, geophysical data demonstrate the importance of Mesozoic compression and Cenozoic (particularly Eocene) extension, accompanied by magmatic underplating of the crust.

Receiver function study of the Cascadia megathrust: Evidence for localized serpentinization

Article

Full-text available

Jul 2009
GEOCHEM GEOPHY GEOSY

We characterize the crustal and upper mantle structure of the Cascadian fore arc using receiver function analysis of data from long-running observatories located along the North American margin in Oregon and Washington. We identify coherent SV-polarized phases consistent with P-S conversion from the top of the subducting Juan de Fuca plate throughout the area west of the volcanic arc. In many instances we also find evidence for deeper features, likely associated with the subducted oceanic crust. At many locations SH-polarized energy in receiver functions is associated with the surface of the subducting plate. These SH-polarized phases display significant directional variation and show clear polarity reversals, consistent with the presence of seismic anisotropy. Previously, a layer of serpentinized material within the megathrust has been proposed to explain an anisotropic, low-velocity, high Poisson's ratio layer identified by receiver function analysis at Corvallis, Oregon (Global Seismic Network station COR). Our new results suggest that this feature is widespread in Cascadia, but the specific receiver function signature varies. This anisotropic supraslab layer is supported by receiver function estimates beneath seismic stations which, similar to COR, are ∼100 km from the coast and ∼40 km above the slab surface. However, the orientation of rock fabric implied by the anisotropy varies greatly from station to station. Modeling with synthetic seismograms supports the existence of serpentinite beneath station GNW (Green Mountain, Washington). Closer to the coast, where the slab is shallower, an anisotropic layer is likely beneath some stations, but the evidence is less conclusive. We propose that the anisotropic signature beneath the inland stations is related to serpentinization of the mantle wedge at greater depths, best explained by the presence of the polymorph antigorite. Hypothetically suctioned upward by slab rollback, a deformed sliver of serpentinite-rich rock above the slab interface would likely display the observed large spatial variation in anisotropic symmetry axis within the layer, consistent with our observations.

Regional crustal structure and tectonics of the Pacific Coastal States; California, Oregon, and Washington

Article

Full-text available

Jan 1989

Walter D. Mooney

Evidence of a Low‐Velocity Zone in the Upper Mantle Beneath Cumbre Vieja Volcano (Canary Islands) Through Receiver Functions Analysis

Article

Full-text available

May 2024
GEOPHYS RES LETT

Plain Language Summary Oceanic volcanic islands have a complex internal structure. Our study is important to understand volcanic activity in its geodynamic context better. The last eruption occurred in the Cumbre Vieja volcanic complex on La Palma Island (2021) and had an unexpected magnitude in terms of lava volume, explosivity, and a significant impact on the economy and society of the island. This study used receiver function analysis to image the lithosphere up to a depth of 50 km. This seismological technique exploits the recordings of teleseismic events (i.e., earthquakes located between 30 and 90° from the stations) for this purpose. We used an advanced inversion technique to study the structure of the crust and the upper mantle beneath the volcano. The most relevant finding is a zone of anomalous low seismic velocities extending from 13 to 37 km depth. This zone is likely to host partial melt and magma chambers and extends over a much wider and thicker volume than expected from previous studies. Previous studies also identified two magma chambers: the upper one, located at about 10–15 km, and the lower one, beneath 25 km depth. These findings provide new insights into the current paradigms about the internal active oceanic volcanic islands.

Earth Crust Imaging of Malay Peninsula Using Earthquake Data and Ambient Noise Singal for Seismic Hazard Assessments

Thesis

May 2019

Abdul Halim Abdul Latiff

The subsurface model of the Earth's crust and uppermost mantle layers within this region is relatively few. The models produced from various techniques are crucial in investigating the cause of Bukit Tinggi earthquakes that occurred in between 2007 to 2009. In this study, the 1-D crustal velocities beneath eight seismometer stations in Malay Peninsula were investigated by adopting the joint inversion of the P-wave receiver functions and surface wave dispersions techniques. In addition, the 3-D crustal velocities were determined through the investigation of P-wave arrival time from teleseismic tomography as well as surface wave arrival inferred from cross-correlation of ambient noise signal. The generated 1-D and 3-D models of Malay Peninsula velocity were then used for seismic hazard assessment in the Peninsular Malaysia. To incorporate the local site effect, 28 localities within the Bukit Tinggi fault zone were investigated through the horizontal-to-vertical spectral ratio (HVSR). From the result, the models produced from the 1-D inversion indicate that the Moho boundary is getting shallower towards the North of Malay Peninsula. In addition, high seismic velocity was determined along the main Titiwangsa Range as distinctive Western-Central/Eastern belts separation clearly identified from 3-D velocity tomography. Furthermore, the ground acceleration analysis indicates that the occurrence of a large and destructive earthquake in the peninsular is highly unlikely, as deduced from the development of integrated seismic hazard analysis of Peninsular Malaysia.

Seismic imaging of intra-crustal low velocity layer beneath the Kishtwar region, North-West Himalaya, India using receiver function technique

Article

Jan 2022
HIMAL GEOL

The converted wave seismological data across the Doda-Kishtwar region in the Kashmir seismic gap of NorthWest Himalaya has been used in this study. The results based on inversion of receiver functions (RFs) reveals, for the first time, the crustal velocity-structure with a Low-Velocity Layer (LVL). The crustal thickness varies from 47 km to 58 km from south to north, and the Main Himalayan Thrust exists at depth between 21 to 26 km. The LVL with its top lying at 11 to 13 km extends downward up to a depth of 29 km, and shows Vp/Vs of 1.9. This high value of Vp/Vs may be explained by the presence of fluid/fractional melt in the depth range between 10 to 15 km. We suggest that the LVL and hence the fluids/melts could be one of the possible reasons for the generation of upper-to-mid-crustal earthquakes. Interestingly, the LVL coincides with the focal depths of most of the earthquakes, observed in this region.

Tracing Geophysical Indicators of Fluid‐Induced Serpentinization in the Pampean Flat Slab of Central Chile

Article

Full-text available

Sep 2019
GEOCHEM GEOPHY GEOSY

An apparent gap in the Andean volcanic arc in the Pampean section of the subduction zone in Chile (~28°–33°S) marks a section of flat slab subduction. In this tectonic environment, the fate of fluids released from the subducting Nazca slab remains uncertain and the degree oftheir interaction with the basal layer ofthe continental lithosphere is poorly understood. Results ofa RF investigation and forward modeling effort at three long‐running stations of the Chilean National Seismic Network allow us to constrain the position of the subducting Nazca slab and to address the physical properties of the interplate contact zone in Central Chile. Our observations suggest a transition in seismic character, from a weakly anisotropic contact in the normally subducting section north ofthe flat slab region to a strongly anisotropic plate contact within the flat slab region. We attribute this change to a transition from sheared olivine to serpentinized peridotite generated as a result of fluid release across the flat slab. This interpretation is supported by forward modeling synthetic RFs at each of the stations. We propose that the identified layer extends across the flat slab region, acting as a mineral reservoir that captures and, possibly, transports fluids from the dehydrating Nazca Plate as it subducts below South America. We note that the Ps converted phase at the slab interface at southernmost station GO04 suffers a 2‐s discontinuity at 180° back azimuth, consistent with a 15–20‐km scarp or kink in the Nazca slab to the south of the station.

Crustal thicknesses and Poisson's ratios beneath the Chuxiong-Simao Basin in the Southeast Margin of the Tibetan Plateau

Article

Full-text available

Jan 2019

In the southeast margin of the Tibetan plateau, low-velocity sedimentary layers that would significantly affect the accuracy of the H-κ stacking of receiver functions are widely distributed. In this study, we use teleseismic waveform data of 475 events from 97 temporary broadband seismometers deployed by ChinArray Phase I to obtain crustal thicknesses and Poisson’s ratios within the Chuxiong-Simao Basin and adjacent area, employing an improved method in which the receiver functions are processed through a resonance-removal filter, and the H-κ stacking is time-corrected. Results show that the crustal thickness ranges from 30 km to 55 km in the study area, reaching its thickest value in the north and thinning toward the south and southwest. The apparent variation of crustal thickness around the Red River Fault supports the view of southeastern escape of the Tibetan Plateau. Relatively thin crustal thickness in the zone between Chuxiong City and the Red River Fault indicates possible uplift of mantle in this area. The positive correlation between crustal thickness and Poisson’s ratio is likely to be related to lower crust thickening. Comparison of results obtained from different methods shows that the improved method used in our study can effectively remove the reverberation effect of sedimentary layers.

Crustal Thickness and Velocity Structure of Malay Peninsula Inferred from Joint Inversion of Receiver Functions and Surface Waves Dispersion

Article

Jan 2019
J Southeast Asian Earth Sci

Shear wave velocity model beneath CBJI station West Java, Indonesia from joint inversion of teleseismic receiver functions and surface wave dispersion

Article

Full-text available

Mar 2018

Earthquake signal observations around the world allow seismologists to obtain the information of internal structure of the Earth especially the Earth's crust. In this study, we used joint inversion of receiver functions and surface wave group velocities to investigate crustal structure beneath CBJI station in West Java, Indonesia. Receiver function were calculated from earthquakes with magnitude more than 5 and at distance 30°-90°. Surface wave group velocities were calculated using frequency time analysis from earthquakes at distance of 30°- 40°. We inverted shear wave velocity model beneath the station by conducting joint inversion from receiver functions and surface wave dispersions. We suggest that the crustal thickness beneath CBJI station, West Java, Indonesia is about 35 km.

Array analysis of the large-aperture array of the 1988–89 PASSCAL Basin and Range Passive-Source Seismic Experiment

Article

Mar 1994

The 1988–89 PASSCAL Basin and Range Passive-Source Seismic Experiment consisted of two array deployments, the large- and small-aperture arrays. The large-aperture array consisted of seven stations with three-component mid-period seismometers, and independently triggered prototype PASSCAL data recorders with GOES clocks. The array spatial distribution and the synchronized data have permitted us to employ simple beamforming operations to improve receiver-function estimation from single teleseismic events. The beamforming reinforces the coherent teleseismic P wave and the deterministic portions of the P-wave coda, primarily generated by interactions at major lithospheric interfaces. Microseismic background noise is incoherent across the array, and is attenuated by beamforming. Beamforming also diminishes those portions of the coda due to scattering by inhomogeneities, that appear random on the scale of the array aperture. The enhanced signal-to-noise ratio of the teleseismic recording dramatically improves the single-event receiver function deconvolution results.

Cascadia subduction slab heterogeneity revealed by three-dimensional receiver function Kirchhoff migration: Cascadia subduction slab heterogeneity

Article

Full-text available

Jan 2017

We present a 3-D model of upper mantle seismic discontinuity structure below Cascadia using a receiver function Kirchhoff migration method. A careful analysis of the primary and multiple reverberated phases allows imaging of the Juan de Fuca plate dipping below the North American continent. The subducting slab is observed as an eastward dipping signal at all latitudes. We associate this signal with a thermal gradient between the slab and surrounding mantle, rather than a sharp chemical discontinuity. Our model also shows along-strike variations in the dipping angle and strength of this signal. To the southern and northern ends of the subduction system, the signal is clearly observed down to ~300km. However, beneath central Oregon, this structure is missing below ~150km depth. We propose that this gap is due to weakening of the slab beneath central Oregon possibly caused by deformation and hydration combined with plume-slab interaction processes after subduction.

Statistics and Frequency-Domain Moveout for Multiple-Taper Receiver Functions

Article

Full-text available

Aug 2016

The multiple-taper correlation (MTC) algorithm for the estimation of teleseismic receiver functions (RFs) has desirable statistical properties. This paper presents several adaptations to the MTC algorithm that exploit its frequency-domain uncertainty estimates to generate stable RFs that include moveout corrections for deeper interfaces. Narrow-band frequency averaging implicit in spectral cross-correlation restricts the MTC-based RF estimates to resolve Ps converted phases only at short delay times, appropriate to the upper 100 km of Earth's lithosphere. The Ps conversions from deeper interfaces can be reconstructed by the MTC algorithm in two ways. Event cross-correlation (ECC) computes a cross-correlation of single-taper spectrum estimates for a cluster of events rather than for a set of eigenspectrum estimates of a single P coda. To extend the reach of the algorithm, prestack moveout corrections in the frequency domain preserves the formal uncertainties of the RF estimates, which are used to weight RF stacks. Moving-window migration (MWM) retains the multiple-taper approach, but cross-correlates the P-polarized motion with time-delayed SH and SV motion to focus on a Ps phase of interest. The frequency-domain uncertainties of bin-averaged RFs do not translate directly into the time domain. A jackknife over data records in each bin-stack offers uncertainty estimates in the time domain while preserving uncertainty weighting in the frequency-domain RF stack.

A Multi-Objective Optimization Framework for Joint Inversion

Article

Full-text available

Mar 2016

Different geophysical data sets such as receiver functions, surface wave dispersion measurements, and first arrival travel times, provide complementary information about the Earth structure. To utilize all this information, it is desirable to perform a joint inversion, i.e., to use all these datasets when determining the Earth structure. In the ideal case, when we know the variance of each measurement, we can use the usual Least Squares approach to solve the joint inversion problem. In practice, we only have an approximate knowledge of these variances. As a result, if a geophysical feature appears in a solution corresponding to these approximate values of variances, there is no guarantee that this feature will still be visible if we use the actual (somewhat different) variances. To make the joint inversion process more robust, it is therefore desirable to repeatedly solve the joint inversion problem with different possible combinations of variances. From the mathematical viewpoint, such solutions form a Pareto front of the corresponding multi-objective optimization problem.

Teleseismic constraints on the geological environment of deep episodic slow earthquakes in subduction zone forearcs: A review

Article

Jan 2016
TECTONOPHYSICS

More than a decade after the discovery of deep episodic slow slip and tremor, or slow earthquakes, at subduction zones, much research has been carried out to investigate the structural and seismic properties of the environment in which they occur. Slow earthquakes generally occur on the megathrust fault some distance downdip of the great earthquake seismogenic zone in the vicinity of the mantle wedge corner, where three major structural elements are in contact: the subducting oceanic crust, the overriding forearc crust and the continental mantle. In this region, thermo-petrological models predict significant fluid production from the dehydrating oceanic crust and mantle due to prograde metamorphic reactions, and their consumption by hydrating the mantle wedge. These fluids are expected to affect the dynamic stability of the megathrust fault and enable slow slip by increasing pore-fluid pressure and/or reducing friction in fault gouges. Resolving the fine-scale structure of the deep megathrust fault and the in situ distribution of fluids where slow earthquakes occur is challenging, and most advances have been made using teleseismic scattering techniques (e.g., receiver functions). In this paper we review the teleseismic structure of six well-studied subduction zones (three hot, i.e., Cascadia, southwest Japan, central Mexico, and three cool, i.e., Costa Rica, Alaska, and Hikurangi) that exhibit slow earthquake processes and discuss the evidence of structural and geological controls on the slow earthquake behavior. We conclude that changes in the mechanical properties of geological materials downdip of the seismogenic zone play a dominant role in controlling slow earthquake behavior, and that near-lithostatic pore-fluid pressures near the megathrust fault may be a necessary but insufficient condition for their occurrence.

The Northeast Pacific Ocean and Margin

Chapter

Jan 1988

R. P. Riddihough

The northeast Pacific Ocean floor and the adjacent continental margin has occupied a critical position in the development of plate tectonic theory. The magnetic surveys conducted from the USS Pioneer in the late 1950s provided (Fig. 1), with their publication in 1961 (Raff and Mason, 1961), the first glimpse of the remarkable magnetic anomaly “stripes” which were to lead to a coherent time scale of magnetic field reversals, confirmation of seafloor spreading processes, and a solution to the enigma of continental drift-plate tectonics. J. Tuzo Wilson and F. Vine in 1965 first identified the Juan de Fuca Ridge as a spreading ridge and recognized the symmetry in the magnetic anomaly pattern which was the key to its interpretation (Vine and Wilson, 1965). The enormous apparent offsets required by these anomalies led Wilson (1965a,b) to propose the mechanism of transform faults. Throughout the 1960s the region was the subject of numerous studies and investigations, culminating in the thesis and publication of Atwater (1970), which effectively provided the framework and foundation of almost all subsequent work on the tectonics of the region.

Simulated ground motions for hypothesized Mw = 8 subduction earthquakes in Washington and Oregon

Article

Feb 1991

The amplitude and duration of strong ground motions from hypothesized Mw = 8 subduction zone thrust earthquakes in the Puget Sound-Portland region were estimated using a semi-empirical method. The simulation procedure assumes the rupture surface may be represented by a grid of fault elements. Scattering and attenuation structure are empirically modeled by the use of corrected accelerograms from Mw ~ 7 Michoacan, Mexico, and Valparaiso, Chile, aftershocks for the fault element source functions. Fault models for the Puget Sound and Portland regions and seismic velocity structure models have been adapted from published regional studies. -from Authors

Nature of the low velocity zone in Cascadia from receiver function waveform inversion

Article

Jan 2011

Ralf Theodor Johannes Hansen

3-D structure of the Rio Grande Rift from 1-D constrained joint inversion of receiver functions and surface wave dispersion

Article

Sep 2014
EARTH PLANET SC LETT

The Southern terminus of the Rio Grande Rift region has been poorly defined in the geologic record, with few seismic studies that provide information on the deeper Rift structure. In consequence, important questions related to tectonic and lithospheric activity of the Rio Grande Rift remain unresolved. To address some of these geological questions, we collect and analyze seismic data from 147 EarthScope Transportable Array (USArray) and other seismic stations in the region, to develop a 3-0 crust and upper mantle velocity model. We apply a constrained optimization approach for joint inversion of surface wave and receiver functions using seismic S wave velocities as a model parameter. In particular, we compute receiver functions stacks based on ray parameter, and invert them jointly with collected surface wave group velocity dispersion observations. The inversions estimate 1-D seismic S-wave velocity profiles to 300 km depth, which are then interpolated to a 3-D velocity model using a Bayesian kriging scheme. Our 3-D models show a thin lower velocity crust anomaly along the southeastern Rio Grande Rift, a persistent low velocity anomaly underneath the Colorado Plateau and Basin and Range province, and another one at depth beneath the jemez lineament, and the southern RGR.

Crustal and Upper Mantle Velocity Structure in the Vicinity of the Eastern Tennessee Seismic Zone Based Upon Radial P-Wave Transfer Functions

Article

Dec 2014

Teleseismic transfer function analysis is used to investigate crust and upper mantle velocity structure in the vicinity of the active eastern Tennessee seismic zone (ETSZ). The ETSZ is associated with the New York – Alabama (NY-AL) magnetic lineament, a prominent aeromagnetic anomaly indicative of Grenville-age, basement structure. Radial component, P-wave transfer functions for ten short-period stations operated by the Center for Earthquake Research and Information (CERI) are inverted for velocity structure. Velocity profiles are also determined for three broadband stations by converting the instrument response to that of an S-13 short-period seismometer. Distinct differences in the velocity profiles are found for stations located on either side of the NY-AL magnetic lineament; velocities west of the lineament are lower than velocities to the east of the lineament in the upper 10 km and in the depth range 30 to 50 km. A gradational Moho boundary is found beneath several stations located in the Valley and Ridge province. A Moho boundary is absent at four Valley and Ridge stations located east of the magnetic lineament and south of 35.5°N.

The fate of the downgoing oceanic plate: Insight from the Northern Cascadia subduction zone

Article

Dec 2014
EARTH PLANET SC LETT

In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40–70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the "plate boundary" as a characteristic "velocity-jump" at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

Scattered waves from low-frequency earthquakes and plate boundary structure in northern Cascadia

Article

Aug 2013

We use 3-D waveform modeling of LFEs (low-frequency earthquakes) to investigate their relation to plate boundary structure along a linear transect in northern Cascadia. To account for crustal velocity heterogeneity, a smoothed 3-D model of subduction zone structure is assembled that incorporates constraints from regional tomographic and plate boundary models. Scattered phases within LFE waveforms are identified based on synthetic predictions that incorporate thrust mechanisms aligned parallel to a dipping plate boundary atop a high-Vp/Vs low-velocity zone (LVZ). Scattering for near-vertical paths is dominated by S-to-P/S-to-S reflections/conversions from the LVZ. The modeling suggests that LFEs lie at or very close to the plate boundary and that the LVZ structure is laterally heterogeneous but broadly consistent with results from teleseismic analysis.

A preliminary study of crustal structure in Taiwan region using receiver function analysis

Article

Oct 2004

Selected teleseismic data observed at temporary and permanent broad-band stations have been analysed using the receiver function method in order to investigate the very complex crustal structure in Taiwan region. Very significant azimuthal variations of radial and transverse receiver function responses from broad-band stations could be attributed to, among other things, the sampling of incoming seismic waves across the nearby subduction zone, a subsurface dipping interface, or a localized anisotropic region. A mid-crust discontinuity, interpreted as the Conrad discontinuity, can be identified at 18-20 km depth beneath TATO and TPUB stations in the Western Foothills, but is absent beneath the two nearby stations SSLB and TDCB in the Central Mountain Range. The separation of upper and lower crust beneath the Western Foothills and the steady increase in crustal velocity as a function of depth across the entire thicker crust beneath the Central Mountain Range suggest that the tectonic evolution of the crust may be significantly different for these two adjacent regions. Although a `thin-skinned' model may be associated with the tectonic evolution of the upper crust of the Western Foothills and Western Coastal Plain, a `thick-skinned' or `lithospheric deformation' model can probably be applied to explain the crustal evolution of the Central Mountain Range. A trend of crustal thinning from east (50-52 km) to west (28-32 km) is in very good agreement with the results from two east-west-trending deep seismic profiles obtained using airgun sources. The thinner crust (20-30 km) beneath TWB1 station in northeastern Taiwan can be associated with the high-heat-flow backarc opening at the western terminus of the Okinawa trough behind the subduction of the Philippine Sea plate. The relatively simple crustal structure beneath KMNB station, offshore southeastern China, depicts typical continental crust, with the Moho depth at 28-32 km. An apparent offset of the thickest Moho beneath NACB station from the topographic high in the central Central Mountain Range suggests that the Taiwan orogeny has probably not reached its isostatic status.

Is a cation ordering transition of the Mg-Fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the Indian Craton?

Article

Full-text available

Dec 2012

We use first-principles molecular dynamics simulations to study the behavior of cation ordering in the non-equivalent octahedral sites of Mg-Fe olivine solid solutions. Our theoretical calculations confirm the previous experimental finding that Mg2+ and Fe2+ can invert their octahedral site occupancy at a critical temperature. Assuming that the site preference of Fe changes discontinuously between two states in which it is completely restricted to either M1 or M2 sites, we have calculated the transition temperature, Tt, between the two extreme states. Under ambient pressure Tt is calculated to be 520°C that agrees fairly with the experimental finding in which, however, the ordering state changed discontinuously over a much smaller range of the site occupancy of Fe. Tt is found to be pressure sensitive, showing an increase by 30 to 100°C per unit GPa, depending upon the iron content. Using the Indian continental geotherm, we estimate a depth of around 75 Km corresponding to the calculated transition pressure and temperature of cation ordering, which matches well with the depth for the Hales discontinuity marked by a jump of shear wave velocity by ˜4%. For olivine solid solutions with 12.5% iron, the ordering transition increases Vs from 4.5 to 4.7 Km/s. Both the inferences, viz. depth of discontinuity and magnitude of velocity increase find support from the modeling of teleseismic earthquake waveforms recorded over broadband seismographs on the Dharwar Craton. This leads us to infer that the cation ordering transition in ferromagnesian olivine might be a potential factor for the Hales discontinuity.

Evidence for two upper mantle sources driving volcanism in Central Kamchatka

Article

Mar 2012
EARTH PLANET SC LETT

Volcanoes of the Central Kamchatka Depression (CKD) form the most active arc volcano system in the world. Volcanoes of the CKD are positioned ~ 170 km above the subducting Pacific Plate, in excess of the typical global value of ~ 100 km for arc volcanism. We present results of a combined geophysical and petrological study of the main volcanic center in the CKD, and argue for the presence of a second contributing melt source within the mantle wedge. This region of melt generation is separate from the fluid fluxed region above the subducting Pacific Plate; it may explain the presence of the active CKD arc in its current location.Results of receiver function imaging of the upper mantle beneath CKD reveal a distinct area of low velocities at approximately 110 km depth that is clearly distinct from the crust of the subducting Pacific Plate. Results of petrological modeling suggest presence of pyroxenite source melt contribution to CKD lavas, alongside previously described peridotite source melts. We contend that our results advance the notion that melting at two separate sources, rather than the simple flux-induced melting within the mantle wedge, drives volcanoes of the CKD.

Imaging the Indian lithosphere beneath Eastern Himalayan region

Article

Nov 2011

Lithospheric thickness is an important parameter to understand the nature of collision and subduction between the Indian and Asian tectonic plates. In this study, we apply the S receiver function technique to data from a network of broad-band stations in the northeast India and Eastern Himalayan regions and image the geometry of Indian Plate collision. This analysis reveals clear S-to-p conversions from the Moho and Lithosphere--Asthenosphere boundary (LAB) in the various tectonic units of the study region. The Indian lithosphere is found to be only 90 km thick beneath the Shillong plateau deepening to 135 km on either side suggestive of a lithospheric upwarp related to the plateau uplift. The lithosphere thickens northward, with values reaching ˜180 km beneath the Eastern Himalaya. The trend of the LAB north of the foredeep region indicates that the Indian Plate plunges beneath the Eastern Himalaya. The consistent northward-dipping character of the Indian Plate suggests that the Indian Plate is traceable until it gets subducted beneath Tibet just south of Bangong suture zone. The deepening of the LAB and its correlation with the topographic elevation is in agreement with homogeneous thickening of the lithosphere in response to compressive forces due to the continental collision of India with Asia.

Seismic Velocity Structure of the Southern Cascadia Subduction Zone

Article

Full-text available

Dec 2005

The goal of this study is to examine seismic velocity structure within the southern Cascadia subduction zone in order to place bounds on thermal and compositional variations across the region. These fundamental constraints are necessary to enable a better understanding of the thermal, mechanical, and lithologic structure of the Cascadia subduction system. Data used in this study were recorded by the Cascadia Array (CASC), a dense linear array of broadband seismometers with 4-8 km spacing that spanned from the western coast of central Oregon 300 km inland. We used a multi-channel cross correlation technique to accurately determine relative delay times for 157 regional and teleseismic P wave events, resulting in 2906 individual measurements. We assumed the IASP91 velocity model to correct raw relative delay times for moveout and also applied an elevation correction. The distribution of events provides good azimuthal coverage and excellent incidence angle coverage for many backazimuthal ranges. Clear variations in relative delay times exist across the region. Maximum peak-to-peak variations in delay times are ~2 seconds. The most conspicuous feature of the data is a relative delay time anomaly recorded at stations around 122.5 W longitude in which first arrivals are 0.5-1.0 s early relative to the mean relative delay time. This feature is consistently observed for events from all backazimuths; however, the magnitude and exact location of the peak varies as a function of backazimuth. Events arriving from the east show the greatest magnitude of variation (~1.5-2.0 s) while events arriving from the west show the least variation (~0.5-1.0 s). Preliminary analysis of S wave delay times show the same general pattern, but with greater peak-to-peak variations of up to twice that of the P data. Using these relative delay time data, we have generated relative velocity perturbation models using a linear inversion applying the VanDecar [1990] method. Results of these preliminary models are consistent with the raw data which exhibit low velocities flanking a central region of higher than average velocities near 122.5 W longitude. We are currently evaluating the possible causes for the observed rapid increase in velocity with slower flanking velocities. Strong seismic anisotropy has also been observed in data from this array and likely accounts for some, but not all, of the variation observed in the delay times. Preferential propagation of seismic energy along the subducting Juan de Fuca slab could also account for the early relative arrival times. Other possibilities for the observed variations include structure due to a serpentinized mantle wedge or thermal variations within the subduction zone.

Receiver Functions from Multiple-Taper Spectral Correlation Estimates

Article

Full-text available

Dec 2000

Teleseismic P waves are followed by a series of scattered waves, par- ticularly P-to-S converted phases, that form a coda. The sequence of scattered waves on the horizontal components can be represented by the receiver function (RF) for the station and may vary with the approach angle and azimuth of the incoming P wave. We have developed a frequency-domain RF inversion algorithm using multiple-taper correlation (MTC) estimates, instead of spectral division, using the pre-event noise spectrum for frequency-dependent damping. The multitaper spectrum estimates are leakage resistant, so low-amplitude portions of the P-wave spectrum can contribute usefully to the RF estimate. The coherence between vertical and hor- izontal components can be used to obtain a frequency-dependent uncertainty for the RF. We compare the MTC method with two popular methods for RF estimation, time- domain deconvolution (TDD), and spectral division (SPD), both with damping to avoid numerical instabilities. Deconvolution operators are often biased toward the frequencies where signal is strongest. Spectral-division schemes with constant water- level damping can suffer from the same problem in the presence of strong signal- generated noise. Estimates of uncertainty are scarce for TDD and SPD, which impedes developing a weighted average of RF estimates from multiple events. Multiple-taper correlation RFs are more resistant to signal-generated noise in test cases, though a "coherent" scattering effect, like a strong near-surface organ-pipe resonance in soft sediments, will overprint the Ps conversions from deeper interfaces. The MTC RF analysis confirms the broad features of an earlier RF study for the Urals foredeep by Levin and Park (1997a) using station ARU of the Global Seismographic Network (GSN), but adds considerable detail, resolving P-to-S converted energy up to f 4.0 Hz.

Correction to “Anomalous seismic structure beneath the Klyuchevskoy Group, Kamchatka”

Article

Jul 2010

The Klyuchevskoy Group is among the most active volcanic features on Earth, yet its position within the Kamchatka subduction zone is hard to explain with a simple tectonic mechanism. The geochemistry of Klyuchevskoy Group lavas is typical for volcanic products resulting from flux melting in the mantle wedge, yet the depth to the subducting Pacific plate beneath it is much larger than the average depth of volatile release in subduction zones. We present new seismological constraints on the upper mantle structure beneath the Klyuchevskoy Group and identify a planar dipping seismic feature in the mantle wedge which is ∼100 km deep and appears to be sharply bounded. We hypothesize that this upper mantle structure may be the true source of melts erupting at the Klyuchevskoy Group, and that the presence of this feature may explain both the extraordinary productivity of the Klyuchevskoy Group and its unusual location.

Seismological evidence for intra-crustal low velocity and thick mantle transition zones in the north-west Himalaya

Article

May 2023
J EARTH SYST SCI

The Himalayan region witnesses several natural hazards like earthquakes and landslides due to the continental collisions between the Indian and Eurasian plates. This has given rise to extreme topographic variations throughout the Himalayan belt. The Kumaun–Garhwal region is a classic example of such geological consequences and is prone to several earthquakes. High-quality three-component teleseismic waveform data recorded at seven seismological stations operated by the Wadia Institute of Himalayan Geology (WIHG) are used to investigate the detailed subsurface structure of the crust, the intra-crustal low-velocity layer (LVL), and the upper mantle discontinuities beneath the Kumaun–Garhwal, north-west Himalaya. The results, derived from the inversion of individual station's stacked P-receiver functions (PRFs) using the neighbourhood algorithm approach, show that the crustal thickness varies from 44 to 54 km beneath the study region. The depth of LVL observed beneath six stations from individual and stacked PRFs, varies from 9 to 24 km. The LVL zone with a high Vp/Vs ratio may be due to fluid or partial melt, leading to shallow seismic activity within the study region. The presence of fluid or partial melts in the LVL may be due to the shear heating, cooling, and decompression. The 2D PRF migration image depicts a thick mantle transition zone due to the elevated 410 km discontinuity with respect to the global average values predicted by the IASP91 velocity model. The present research suggests that this might be due to the colder transition zone in this region, indicating the cool underthrust Indian plate with respect to the ambient mantle has reached down to the upper mantle transition zone.

Source parameters of the 1949 magnitude 7.1 south Puget Sound, Washington, earthquake as determined from long-period body waves and strong ground motions

Article

Oct 1987

Teleseismic P, SH, and SV first motions and SH to SV amplitude ratios recorded at eight teleseismic receivers from the 1949 magnitude 7.1 Olympia, Washington, earthquake in combination with data from three stations at regional distances were utilized in a grid testing routine to constrain focal mechanism. Identification of the pP phase places the event at 54 km depth. Distinct pulses, assumed to be source effects, are observed in the far-field waveforms. Analysis of these pulses for directivity made possible discrimination between the fault and auxiliary planes. The plane taken to represent the fault surface strikes east-west ± 15°, dips 45° ± 15° to the north, and has nearly pure left-lateral slip. The preferred source model has an eastward propagation of 40 km. Surface reflections of successive source pulses suggest an upward component of propagation of 5 km. Bounds on the earthquake location and rupture of the 13 April event were determined using depth and source mechanism constraints from the teleseismic study and characteristics of local strong ground motion recordings. The 9-sec S-instrument trigger time seen in the Seattle acceleration recordings places the event at least 60 km from Seattle. Strong motion velocity at the Olympia Highway Test Laboratory is characterized by an impulsive and rectilinear S wave. The low amplitude of the vertical component of initial S motion suggests that either the epicenter is within 5 km of the Olympia Highway Test Laboratory for a pure incident SV wave or located along an azimuth of N159° if the wave is SH. The combined constraint of minimum distance from Seattle and the S polarization angle implied by the teleseismic data focal mechanism places the initiation of rupture 5 to 10 km north to north-northwest of the Olympia Highway Test Laboratory at 47.13°N, 122.95°W. This is approximately 20 km west of previously determined epicenters. The T axis, gently dipping to the southeast, supports other evidence that the Juan de Fuca plate dips to the southeast in a zone between segments of the plate north and south of the event's location. The fault plane's slip is taken to indicate that subduction is still active beneath Washington and that motion of the two segments is probably independent.

An Earthquake Nest in Cascadia

Article

Aug 2019

We investigate an isolated cluster of temporally persistent, intraslab earthquakes (ML<3.2) at >60 km depth below the Georgia Strait in southern British Columbia that is unique in Cascadia and meets the criteria for identification as an earthquake nest. A total of 129 relocated hypocenters define two northwest‐dipping structures in the subducting Juan de Fuca mantle within an ∼30×10×10 km3 volume. Focal mechanisms for 15 events represent a mix of strike‐slip and reverse faulting, and a stress regime of down‐dip tension and plate‐normal compression, consistent with a previous regional study. Converted seismic phases inferred to originate at the boundaries of subducted oceanic crust are observed at several receivers and are consistent with a local slab depth of ∼45 km, shallower than some JdF plate models. The geographical isolation of the nest within the confines of an extrapolated propagator wake suggests that its location is controlled by this pre‐existing and presumably hydrated structure.

Seismic-Wave Velocity Structure of the Subducting Plate Boundary Estimated from Later Phases

Article

Dec 1989

Toru MATSUZAWA

Many later phases such as ScSp, PS, SP and SS converted or reflected at the upper boundary of a descending oceanic plate are frequently observed in a subduction zone. The location of the upper boundary has been estimated to be just above the deep seismic zone by the use of these phases. Moreover, from the analyses of the later phases such as PS and channel waves, it has been found that a low velocity layer exists on the surface of the plate. The most probable interpretation of the low velocity layer, at least in the shallow depths, is the descending oceanic crust.

Shear wave velocity structure in West Java, Indonesia as inferred from surface wave dispersion

Conference Paper

Feb 2016

We investigated the crust and upper mantle of West Java, Indonesia by measuring the group velocity dispersion of surface waves. We analyzed waveform from four teleseismic earthquake recorded at three 3-component broadband seismometers. We analyzed fundamental mode of Rayleigh and Love waves from vertical, radial, and transverse components using multiple filter technique. We inverted the measured group velocity to obtain shear wave velocity profile down to 200 km depth. We observed low shear wave velocity zone at depth of about 20 km. Shear velocity reduction is estimated to be 18% compared to the upper and lower velocity layer. The low velocity zone might be associated with the subducting slab of Indo-Australian Plate as similar characteristics of low velocity zones also observed at other subducting regions.

Application of microseismic data to constraining inversion for velocity structure beneath stations in sedimentary area

Article

Jan 2011

The joint inversion of receiver functions and seismic surface wave dispersion has become a common technique to study the velocity structure beneath stations. The results can simultaneously match the two data sets of different resolution so that the non-uniqueness of solution has been effectively suppressed in a certain extent. However, for the present distribution of seismic stations, due to many factors, there are less effective short-period surface wave dispersion data. The horizontal resolutions of seismic tomography are low, so it is difficult to reflect the real dispersion characteristics of the local media near stations. The spatial extents reflected by these surface wave dispersions and receiver functions at different depths are also different, especially in the sedimentary region with complex lateral variations, where receiver functions and surface wave dispersion can not simultaneously achieve a good fitting result. So the constraint data, which can reflect information of media in the same spatial extent, should be used as far as possible for joint inversion. In May of 2009, we laid a small-aperture seismic array for micro-seismic observation near two mobile seismic stations in a sedimentary region on the plateau of northern Hebei province and in North China Plain, and obtained high-frequency phase velocity dispersion curves by spatial autocorrelation method. The micro-seismic dispersion curves are used as constraint on shallow velocity structure for the joint inversion of receiver functions and surface waves in sedimentary region. The inversion result shows that the constraint of high-frequency phase velocity dispersion curves may obviously improve the stability and reliability of the inversion for velocity structure in sedimentary region.

Determination of crustal structure beneath a broadband station in Malaysia using receiver function analysis

Article

Jan 2011

We performed receiver function analyses in order to investigate crust structure beneath a broadband seismic station in Malaysia. We selected 11 teleseismic events from the Malaysian National Seismic Network Database whose magnitudes are in the range between 5.8 and 7.1 with good signal-to-noise ratios. We compared the observed receiver functions to the synthetic receiver functions computed for a model for the site of station 1PM taken from the global crust model, CRUST 2.0. There is a significant difference between them. This is likely to be due to the thin sedimentary layer in the crust model, and our comparison suggests that such a sedimentary layer does not exist beneath the station site. Then, we applied a genetic algorithm to perform inversion. We modeled the crust and uppermost upper mantle with six major layers: sediment layer, basement layer, upper crust, middle crust, lower crust and uppermost mantle. The Moho depth is relatively well constrained and is estimated to be about 35km. The thin sedimentary layer is not obtained in the inversion result, although a thin layer with relatively low S wave velocity is obtained. This study is a preliminary attempt to determine velocity structures and obtain an appropriate velocity model for Malaysia. Further accumulation of data and stacking using a larger dataset will be useful to construct such a model by application of the data analysis procedure of this study.

Construction of Shear Wave Models by Applying Multi-Objective Optimization to Multiple Geophysical Data Sets

Article

Mar 2015

For this work, our main purpose is to obtain a better understanding of the Earth’s tectonic processes in the Texas region, which requires us to analyze the Earth structure. We expand on a constrained optimization approach for a joint inversion least-squares (LSQ) algorithm to characterize a Earth’s structure of Texas with the use of multiple geophysical data sets. We employed a joint inversion scheme using multiple geophysical data sets for the sole purpose of obtaining a three-dimensional velocity structure of Texas in order to identify an ancient rift system within Texas. In particular, we use data from the USArray, which is part of the EarthScope experiment, a 15-year program to place a dense network of permanent and portable seismographs across the continental USA. Utilizing the USArray data has provided us with the ability to image the crust and upper mantle structure of Texas. We prove through numerical and experimental testing that our multiobjective optimization problem (MOP) scheme performs inversion in a more robust, and flexible matter than traditional inversion approaches.

Seismological Constraints on the Velocity Structure and Fate of Subducting Lithospheric Slabs: 25 Years of Progress

Article

Full-text available

Dec 1994
ADV GEOPHYS

Thorne Lay

Upper mantle structure from teleseismic P wave arrivals in Washington and northern Oregon ( USA).

Article

Feb 1986
J GEOPHYS RES

Teleseismic P wave travel time residuals are used to detect lateral velocity heterogeneities in the upper mantle beneath Washington and northern Oregon. The results of an inversion for three-dimensional velocity variations resolves an east dipping high-velocity zone that we interpret as the subducting Juan de Fuca plate. The plate is characterized by 3–8% higher velocities than those in the surrounding upper mantle. Inversion of the travel time data and ray trace modeling indicate that the plate extends to a depth of 200–300 km. The plate dips at a moderate angle of 45° to the east-northeast beneath the central Washington Cascade Range north of Mount Rainier, with 5% faster velocities than the surrounding upper mantle. Beneath the North Cascade Range of Washington, the plate strikes to the northwest and has 6–8% faster velocities than the upper mantle to the west. South of 47°N, beneath the Cascade Range in southern Washington and northern Oregon, the plate dips steeply to the east and has 3–4% faster velocities than the surrounding upper mantle. Based on changes in the geometry and velocity structure of the subducted Juan de Fuca plate east of about 123°W, we propose that the subducted slab is segmented into three sections beneath Washington and northern Oregon.

Feasibility Analysis of Short-Period Seismograph Receiver Function—An Example of Hotan Seismic Array, Xinjiang

Article

Jul 2013

Using the teleseismic waveform data recorded by 9 sub-stations (including 1 set of broadband and 9 sets of short-period seismographs) of Hotan Seismic Array with an aperture about 3 km in 3 years, we studied and analysed the stability and reliability of short-period seismograph receiver function. By comparative analysis of the receiver function waveforms isolated from teleseismic waveform data recorded by short-period and broadband seismograph, the following results are obtained. (1) Receiver functions of short-period and broadband seismograph are very consistent and high stability, they have good linear correlation (correlation coefficient 0.9), but Ps seismic phases have small amplitude difference (about 20%), whenever the Gaussian coefficient value is 1.5 or 2.5. (2) The short-period seismograph receiver functions can replace broadband seismograph receiver functions, if a method using the seismic phase arrival time of receiver functions is adopted (such as grid-stacking-search method). (3) The result may cause wrong interpretation (e.g., the lower crust low-velocity layer) if only the waveform data of short-period seismograph receiver functions are used to inverse for crustal S-wave velocity structure, because there exist large velocity deviations (about 0.3 km/s) from lower crust to upper mantle, due to lack of low frequency information under 0.155 Hz and the nonlinear amplitude response under 1 Hz frequency band with short-period seismographs; so additional data sensitive to wave velocity value (such as surface wave dispersion) are needed for joint analysis.

Constraints on the Moho in Japan and Kamchatka

Article

Dec 2013
TECTONOPHYSICS

This review collects and systematizes in one place a variety of results which offer constraints on the depth and the nature of the Moho beneath the Kamchatka peninsula and the islands of Japan. We also include studies of the Izu–Bonin volcanic arc. All results have already been published separately in a variety of venues, and the primary goal of the present review is to describe them in the same language and in comparable terms.For both regions we include studies using artificial and natural seismic sources, such as refraction and reflection profiling, detection and interpretation of converted-mode body waves (receiver functions), surface wave dispersion studies (in Kamchatka) and tomographic imaging (in Japan). The amount of work done in Japan is significantly larger than in Kamchatka, and resulting constraints on the properties of the crust and the uppermost mantle are more detailed.Japan and Kamchatka display a number of similarities in their crustal structure, most notably the average crustal thickness in excess of 30 km (typical of continental regions), and the generally gradational nature of the crust–mantle transition where volcanic arcs are presently active.

The Moho in subduction zones

Article

Dec 2013
TECTONOPHYSICS

M. G. Bostock

The Moho in subduction zones exists in two distinct forms, one associated with the subducting oceanic plate and second with the overriding plate. The seismic expression of both forms is linked to the nature of a landward dipping, low-velocity zone (LVZ) that has been detected in a majority of subduction zones about the globe and that approximately coincides with Wadati–Benioff seismicity. We review seismic studies that constrain the properties of the LVZ in Cascadia where it has been extensively studied for over a quarter century. A model in which the LVZ is identified with hydrated pillow basalts and sheeted dikes of oceanic crustal Layer 2, is consistent with available geological and geophysical data, and reconciles previously conflicting interpretations. In this model, the upper oceanic crust is hydrated through intense circulation at the ridge and becomes overpressured upon subduction as a result of metamorphic dehydration reactions combined with an impermeable plate boundary above and a low porosity gabbroic Layer 3 below. The resulting seismic velocity contrast (approaching 50% for S-waves) significantly overwhelms that of a weaker, underlying oceanic Moho. At greater depths, oceanic crust undergoes eclogitization in a top-down sense leading to gradual disappearance of the LVZ. The large volume change accompanying eclogitization is postulated to rupture the plate boundary allowing fluids to penetrate the cooled, forearc mantle wedge. Pervasive serpentinization and free fluids reduce velocities within the wedge, thereby diminishing, erasing or even inverting the seismic contrast associated with the Moho of the overriding plate. This model is tested against observations of LVZs and forearc mantle structure worldwide.

Nature of the low velocity zone in Cascadia from receiver function waveform inversion

Article

Jul 2012
EARTH PLANET SC LETT

A dipping low-velocity zone (LVZ) is a ubiquitous structural element of subduction zones worldwide. In this study we map seismic attributes characterizing the LVZ beneath the Cascadia subduction zone from northern Vancouver Island to northern California using receiver function waveform inversion. Throughout this region, the LVZ is characterized by high VP/VSVP/VS ratios (mean=2.77), strong S-velocity contrasts (∼∼ 50%) and thicknesses averaging 3.37 km. The LVZ is immediately underlain by a second, weaker layer exhibiting moderate VP/VSVP/VS ratios (mean=1.85) with mean thickness of 4.62 km. We interpret the combined structure in terms of subducting oceanic crust, based on classical structural/petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores. The LVZ is identified with pervasively hydrated, high porosity pillow basalts and sheeted dikes of Layer 2 with possible contributions from sediments (Layer 1). Fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above, and a low porosity, gabbroic/mafic-cumulate dominated Layer 3 below.

Crustal thickness estimate at AAE (Addis-Ababa, Ethiopia) and NAI (Nairobi, Kenya) using teleseismic P-wave conversions

Article

Feb 1985
TECTONOPHYSICS

Long-period teleseismic P waves recorded at AAE (Addis Ababa, Ethiopia) and NAI (Nairobi, Kenya) show comparable Ps conversions on the radial component of ground motion. The timing and amplitude of the Ps conversions are modeled with synthetic seismograms to get an estimate of crustal thickness under both stations. Instrument response and effective source time function are removed from the data using a P-wave equalization procedure. The timing of the Ps conversions, relative to direct P, suggests that both stations have similar crustal thickness. Using constraints on crustal velocities determined by previous surface wave dispersion and travel-time studies, the Ps-P timing suggests a crust of 41 km thickness. This agrees reasonably well with previous crustal estimates. Tangential wave forms exhibit large amplitudes and are consistent at most backazimuths. However, these wave forms could not be explained with models containing simple planar dipping interfaces. The crustal thickness of 41 km taken in conjunction with the close proximity of the stations to the rift zone suggests that crustal thinning is localized to the rift itself.

Seismic Evidence for an Ancient Rift Beneath the Cumberland Plateau, Tennessee: A Detailed Analysis of Broadband Teleseismic P Waveforms

Article

Sep 1984

Broadband receiver functions developed from teleseismic P waveforms recorded on the midperiod passband of Regional Seismic Test Network station RSCP are inverted for vertical velocity structure beneath the Cumberland Plateau, Tennessee. The detailed broadband receiver functions are obtained by stacking source-equalizd horizontal components of teleseismic P waveforms. The resulting receiver functions are most sensitive to the shear velocity structure near the station. A time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin, flat-lying, homogeneous layers. Lateral changes in structure are identified by examining azimuthal variations in the vertical structure. The results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the East Continent Gravity High located northeast of RSCP. The results from events arriving from the northeast show a high-velocity midcrustal layer not present in results from the southeast azimuth. This velocity structure can be shown to support the idea that this feature is part of a Keweenawan rift system. Another interesting feature of the derived velocity models is the indication that the crust-mantle boundary beneath the Cumberland Plateau is a thick, probably laminated transition zone between the depths of 40 and 55 km, a result consistent with interpretations of early refraction work in the area.

Seismic structure across the active subduction zone of western Canada

Article

Jan 1985

The Vancouver Island Seismic Project was conducted in 1980 to study the structure of the subducting oceanic Juan de Fuca plate and the overriding continental America plate. The principal seismic refraction line (line I) was a 350-km onshore-offshore profile perpendicular to the continental margin. An array of 32 receivers was located on the America plate on the mainland and across Vancouver Island and extended offshore with three ocean bottom seismometers (OBS's). Two shots were fired at the eastern end of the line, and 17 shots were located along the westermost 100 km of the profile. Control for the interpretation of the onshore-offshore profile was provided by a reversed refraction profile along the length of Vancouver Island and by a marine refraction profile recorded on the OBS's. Modeling of the seismic structure of this complex region utilized an iterative inversion method for travel times from explosions in which shots at several locations are recorded on the same set of receivers and utilized an algorithim based on asymptotic ray theory for the calculation of synthetic seismograms through two-dimensional media. The major features of the refraction structural model are that (1) the oceanic lithosphere dips at 3° or less beneath the continental slope, so the bend in the subducting slab occurs landward of the foot of the slope (2) the oceanic lithosphere dips at 14°-16° beneath the continental shelf until it passes beneath the continental Moho at 37 km depth below western Vancouver Island, (3) an upper mantle reflector may correspond to the base of the subducting lithosphere, and (4) a segment of high-velocity material above the downgoing crust, with velocity 7.7 km/s and depth range 20-25 km, may represent a remnant of subducted lithosphere, perhaps detached when the subduction zone jumped westward to its present position.

Shear wave constraints on a deep crustal reflective zone beneath Vancouver Island

Article

Nov 1991

Analysis of teleseismic receiver functions recorded in the northern Cascadia subduction zone show that large P-to-S converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors (the ``E'' zone) beneath central Vancouver Island. These data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey. Modeling of the amplitudes, polarities, and arrival times of the P-to-S converted phases demonstrates that the E zone is a prominent low-velocity layer, with an S velocity 1.0+/-0.2 km/s lower than the regions above and below. The E zone dips 7°+/-5° in the direction N10°E+/-20° and extends from depths of 36.5 to 41 km at ALB-B on central Vancouver Island. The estimated S velocity contrast, when considered with refraction P velocities, yields a Poisson's ratio of 0.34 for this zone. Given the uncertainty in DeltaVp and DeltaVs, this value may range from 0.27 to 0.38. The low P and S velocities and high Poisson's ratio combined with electrical resistivity measurements suggest a porosity of 0.1-1.9% and crack aspect ratios of 0.001-0.01, values consistent with a region containing thin, fluid-saturated cracks and thus supportive of the interpretation of this feature as a major shear zone.

P to S converted wave observations at Long Valley Caldera

Article

Nov 1988

We report observations of P to S converted teleseismic body waves recorded with seismic stations located on the resurgent dome of Long Valley caldera. We observe strong conversions occurring at boundaries that could be the tops of small magma bodies at depths of 4–9 kilometers beneath the resurgent dome. This technique could prove to be an extremely powerful tool in the search for magma bodies for geothermal energy development. Our results from a two month deployment of triggered, digital teleseismic recorders are discussed.

Atwater, T. M. Implications of plate tectonics for the Cenozoic evolution of western North America. Geol. Soc. Am. Bull. 81, 3513-3536

Article

Full-text available

Dec 1970

Tanya Atwater

Opinions presented in this publication do not reflect official positions of the Society. worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. other forums for the presentation of diverse opinions and positions by scientists the posting includes a ...

Seismicity, earthquake mechanisms, and tectonics along the western coast of North America, from 42°N to 61°N

Article

Oct 1974

Umesh Chandra

Seismicity maps of the region are presented for the periods January 1900 to June 1973 and January 1961 to June 1973. Focal mechanism solutions for 24 earthquakes (mb ≧ 5.5) occurring between January 1963 to July 1973 have been determined. Four earthquakes which occurred in the vicinity of the Gorda ridge have a component of normal faulting, thus identifying it as a spreading ridge. However, departure of the trend of the axis of tension, determined in the focal mechanism solutions for these earthquakes, from being perpendicular to the trend of the ridge axis indicates that the tectonics of the Gorda basin is complex. The existence of the Blanco fracture zone and the Queen Charlotte Islands fault as dextral transform faults, east block moving south, is confirmed. Evidences derived from the focal mechanism solutions indicate that the Revere-Dellwood and Sovanco fracture zones are also right-lateral transform faults. The northern extent of the Queen Charlotte Islands fault probably terminates near 54.5°N, 135.2°W or 57.5°N, 136°W. Focal mechanism solutions for an earthquake occurring near Vancouver Islands and another near the Puget Sound region in Washington indicate normal faulting and suggest the presence of a downgoing slab of lithosphere in this region. The interpretation is that the former results from tension caused by the bending of the plate as it dips under the continent and the latter results from tension within the sinking slab. Two recent earthquakes which occurred on July 1 and July 3, 1973 at a latitude of about 58°N, have thrust fault solutions and together with the seismicity data suggest a zone of underthrusting of the lithospheric plate between latitudes 58°N to 59.5°N. The possibility of a third zone of subduction north of latitude 59.5°N between longitudes 139°W to 142°W is indicated.

Crustal refraction profile, Oregon coast range

Article

Dec 1966

Seven temporary and three permanent seismograph stations recorded seismic waves to a distance of about 270 km from a 110,000 pound quarry blast detonated near Depoe Bay, Oregon. The recording stations were in a north-south line along the northern coast range of Western Oregon and Washington. The travel-time data indicate an apparent shallow crustal thickness (about 16 km) for this region. The time versus distance data were not continuous beyond 130 km from the source which may have resulted from any combination of the following causes: (1) insufficient source energy; (2) lateral geological variations; and/or (3) a subcrustal negative velocity gradient.

Direction of faulting in some of the larger earthquakes of 1949*

Article

Jan 1954

The Dominion Observatory is carrying out a program to investigate the direction of faulting in large earthquakes through the study of first motion at distant stations. In this, the third paper of the series, solutions are obtained for fifteen earthquakes, with focal depths varying down to 600 km. For the first time, data from PP and P′ are used in obtaining solutions. Seven of the earthquakes considered lie in the North Pacific Ocean, four in South America, two in the southwest Pacific and two in central Asia. The two earthquakes in central Asia apparently occurred on steeply dipping normal faults, but most of the circum-Pacific shocks are due to transcurrent faulting. No attempt is made to correlate the results with structure. The attempt will be postponed until more earthquakes have been reduced.

Early Tertiary Rifting in Western Oregon-Washington: GEOLOGIC NOTES

Article

Jan 1978
AAPG BULL

Robert McWilliams

Seafloor spreading along the Kula-Farallon plate boundaries may be the explanation for geographic separation and different ages of the submarine basalts in western Oregon-Washington. Eocene strata of the Columbia arc are interpreted to be Tertiary seafloor that was wedged between the Juan de Fuca and North American plates when, in early Refugian time, subduction changed from its Mesozoic-early Tertiary location to the present location.

Direction of faulting in some of the larger earthquakes of the southwest Pacific 1950–1954

Article

Jan 1956

J. Hodgson

Implications of plate tectonics for the Cenozoic evolution of western North America

Article

Tanya Atwater

Crustal refraction profile, Oregon Coast Range

Article

Jan 1966

J.W. Berg

Determination of the subducting lithosphere boundary by use of converted phases

Article

Aug 1977

ScSp arrivals, resulting from ScS-to-P conversions at upper boundaries of downgoing plates in subduction regions, have been identified on Japanese and South American seismograms. Analysis of the ScSp arrivals provides information about the sharpness and the location of these boundaries. It is inferred from ScSp arrivals that the boundaries are characterized by velocity contrasts of at least 5 per cent over transition zones of at most a few kilometers. In general, locations of the boundaries inferred from ScSp observations agree with locations inferred from seismicity except beneath central Peru. In central Peru the seismicity at depth is diffuse. It has been proposed on the basis of that seismicity that the downgoing plate dips at a gentle 10°; the ScSp observations, however, suggest that the Nazca plate has a dip of ∼30°.

Glorified optics and wave propagation in nonplanar structure

Article

Oct 1978

Waves propagating in varying nonplanar structure can produce many interesting phenomena, such as focusing, caustics, and triplications. A high-frequency technique based on the first-motion approximation, referred to as glorified optics, has been developed to generate synthetic seismograms for these types of problems. The technique, in its simplest form, uses the spreading rate of a beam with transmission and reflection coefficients along each possible ray path. The time behavior of each arrival is either that of the original pulse or its Hilbert transform depending on the position of caustics. The geophysically interesting structure of a soft basin over a half-space is investigated in detail by this method. Synthetic seismograms appropriate for various locations are compared with the results of finite difference and finite element methods. The technique appears rich in insight and should prove very useful in modeling problems.

Corvallis, Oregon, crustal and upper mantle receiver structure from teleseismic P and S waves

Article

Jun 1977

Charles Langston

Structure under Corvallis, Oregon, was examined using long-period Ps and Sp conversions and P reverberations from teleseismic events as recorded at the WWSSN station COR. A distinct low-velocity zone in the uppermost mantle is inferred by modeling these phases in the time domain using a data set composed of six deep and intermediate-depth earthquakes. The lower boundary occurs at 45-km depth and has S and P velocity contrasts of 1.3 and 1.4 km/sec, respectively. The material comprising the low-velocity zone has a Poisson ratio of at least 0.33 and is constrained by the average P and S travel times determined from the converted phases. The top of the earth model conforms to previously published refraction results.

Modeling Crustal Structure Through the Use of Converted Phases in Teleseismic Body-Wave Forms

Article

Jun 1977

By comparing records of the radial component of motion of teleseismic P waves to records of the vertical component, it is possible to identify S phases within the P wave form. These phases are generated by the mechanism of P to S conversion at discontinuities in velocity under the receiving station. Similar phases of the S to P converted type appear as precursors to the direct SV arrival. Models for the crustal structure can be tested by generating synthetic seismograms for both components of motion of both the P and SV waves and comparing with the data. The technique has been used to model the crustal structure at WWSSN stations CAR and COR. It has also been used to check a recently proposed model for the crustal structure in eastern Canada which contains a large low-shear-velocity zone at the base of the crust. This study indicates that the crustal structure in eastern Canada is highly non-uniform with perhaps few features common to the whole region. Finally, the technique is used to identify several stations in the WWSSN which appear to be located on highly anomalous structure.

The crust-mantle transition in the Bering Sea

Article

Jan 1968

Don Helmberger

A plane layered oceanic crustal model is investigated. A point source and receiver are located in the oceanic fluid layer. The Mohorovi~i~ discontinuity is treated as a layered transition zone. De Hoop's modification of Cagnlard's method is used to obtain the exact transient response which is convolved with the source pulse shape and the instrumental system function to yield theoretical seismog rams. When the boundary is an interface or a thin layered transition zone the head wave is a small pure refraction which is followed by a strong reflection. As the transition zone grows in thickness the head wave receives both refracted and reflected energy and becomes the dominant feature. These results compare favorably with observations made in the Bering Sea. This study suggests that the transition zone is thin, probably less than 1 km, in the Northern Aleutian Basin. Southward approaching the Aleutian Islands the transition zone thickens so that no major discontinuity exists between the crust and mantle.

The effect of planar dipping structures on the source and receiver for constant ray parameter

Article

Aug 1977

Charles Langston

A geometrical ray method is developed for wave calculations involving three-dimensional planar dipping interfaces. Justification for the method is based on analogy with first-motion approximations derived from generalized ray theory where frequency dependence in the reflection-transmission coefficients is related to changes in the complex ray parameter. The method is applied to finding the teleseismic response of an arbitrarily oriented dislocation source in dipping layered media and for receiver calculations which assume an impinging P or S wave be-neath a stack of dipping layers. Source results indicate that wave forms from fast azimuthally varying sources, such as strike-slip faults, are significantly dis-torted from the plane layered case for simple structures. A simple dipping Moho for dips up to 10 ° does not significantly distort vertical and radial P waves for the receiver response. However, due to azimuth anomalies introduced by inter-face dip a significant tangential P component is produced. In addition, the S-wave response becomes a function of source mechanism due to the need for specifying the incident polarization angle. Polarization studies are suggested for finding dipping structure.

The April 29, 1965, Puget Sound earthquake and the crustal and upper mantle structure of western Washington

Article

Jun 1977

Simultaneous modeling of source parameters and local layered earth structure for the April 29, 1965, Puget Sound earthquake was done using both ray and layer matrix formulations for point dislocations imbedded in layered media. The source parameters obtained are. dip 70 ° to the east, strike 344 °, rake --75 °, 63 km depth, average moment of 1.4 -1-0.6 X 10 ~6 dyne<m, and a triangular time function with a rise time of 0.5 sec and falloff of 2.5 sec. An upper mantle and crustal model for southern Puget Sound was determined from inferred reflections from interfaces above the source. The main features of the model include a distinct 15-km-thick low-velocity zone with a 2.5-km/sec P-wave-velocity contrast lower boundary situated at approximately 56-km depth. Ray calculations which allow for sources in dipping structure indicate that the inferred high contrast value can trade off significantly with interface dip provided the structure dips eastward. The ef-fective crustal model is less than 15 km thick with a substantial sediment section near the surface. A stacking technique using the instantaneous amplitude of the analytic signal is developed for interpreting short-period teleseismic observations. The in-ferred reflection from the base of the low-velocity zone is recovered from short-period P and S waves. An apparent attenuation is also observed for pP from com-parisons between the short-and long-period data sets. This correlates with the local surface structure of Puget Sound and yields an effective Q of approximately 65 for the crust and upper mantle.

Velocity Gradients in the Continental Crust from Head‐Wave Amplitudes

Article

Jan 1971

David P. Hill

Small velocity gradients in a refracting horizon have a pronounced effect on the spectral amplitudes of head waves. Negative velocity gradients and anelasticity (Q-1) result in a similar amplitude decay with distance for narrow-bandwidth data. Positive velocity gradients result in a net amplitude gain with distance compared with the head wave from a homogeneous, perfectly elastic refractor. Wave-theoretical expressions for these effects applied to published amplitude data for the major crustal refraction branches, Pg and P*, suggest that the `granitic' crust in the Basin and Range province has either negative velocity gradients of the order of 10-2 km/sec/km or an anelastic Q of the order of 400, whereas the `granitic' crust in the eastern United States and on the California coast has slightly positive velocity gradients. Similarly, the 'basaltic' intermediate layer appears to have a negative gradient of the order of 10-2 sec-1 under the Snake River plain and null or slightly positive gradients under Lake Superior and Mississippi. Velocity gradients inferred from laboratory measurements on granite and basic igneous rocks, together with published geothermal gradients, are generally consistent with the gradients inferred from amplitude data.

The Juan de Fuca plate

Article

Sep 1978
Eos Trans. AGU

R. P. Riddihough

In the history of science, the theory of plate tectonics is so recent that one might expect that the details of its development would be unequivocally documented. Strangely, they are not. A number of books, a recent special AGU session, and countless articles have tried to follow the thread that captured all those ephemeral sparks of insight in a working theory. There are still almost as many views of how it happened as there are scientists involved (see Figure 1).

The use of converted phases to infer the depth of the lithosphere-asthenosphere boundary beneath South America

Article

May 1977

Seismic waves arriving between the P and S phases have been observed at South Ameri- can seismograph stations from large local deep- focus earthquakes. The travel times, frequency content, and amplitudes for these waves are con- sistent with their being sp phases: S waves which convert to P waves upon refraction across an ap- proximately horizontal interface at a depth of 400 km. The velocity change across the inter- face is at least 5% over a transition zone of at most a few kilometers. The polarity of the sp indicates that there is a velocity reversal be- low this interface. We therefore suggest that the discontinuity may be the lithosphere- asthenosphere boundary.

Puget Sound, Washington, Earthquake and the Mantle Structure Beneath the Northwestern United States

Article

Jan 1971

A detailed study of the travel time anomalies of the Seattle earthquake supports the existence of a high-velocity slab dipping at 50°E. beneath southwestern Canada and the northwestern United States.

Small Plate Tectonics in the Northeastern Pacific

Article

Jan 1971

ELI A. SILVER

Lithospheric plate motions in the northeastern Pacific were complicated at about 2.5 m.y. B.P. by the movement along a major northeast-trending fault cutting Cascadia Basin. An estimate of the slip rate along this fault gives critical information on the relative motions of four geometrically interdependent blocks. The fault is presently inactive. Seventy km of slip along this fault during 2 m.y. or less gives an average slip rate of about 3-5 cm/yr or greater, and resulting plate motions suggest a significantly greater rate of net subduction along the continental margin off Oregon than off Washington and Vancouver Island. Subduction rate off Oregon is less sensitive to slip rate along this fault than is subduction off Washington.

Transitional Tectonics and Late Cenozoic Structure of the Continental Margin off Northernmost California

Article

Jan 1971

ELI A. SILVER

The continental margin north of the Cape Mendocino triple junction is a region of tectonic transition between structures related to underthrusting on the continental slope and those related to right-lateral strike-slip along the coast. Underthrusting of late Cenozoic rocks of the continental slope by the Gorda lithospheric plate is seen by the presence of magnetic anomaly 3, age 5 m.y., beneath the slope. Folds and faults mapped by reflection profiling on the continental slope are parallel to it, and the faults have dip separations predominantly with west side down. These structures are best explained as resulting from oblique underthrusting, with structural trends controlled by the slope direction. Northwest-trending faults occur along the coast and reported earthquake mechanism solutions show right-lateral first motions. These faults and related folds affect Mesozoic rocks and parallel the San Andreas fault system which lies to the south, most probably sharing its origin - that of shear interaction between the Pacific and North American lithospheric plates.

Magnetic survey off the west coast of North America, 40°N. latitude to 52°N. latitude

Article

Aug 1961

An extensive high-resolution magnetic survey of total field at sea level reveals unusual north-south lineations and much crustal faulting. Computations indicate that the linear pattern is due to strongly magnetized mafic rocks beneath the sediments.

Paleomagnetic evidence for tectonic rotation of Jurassic Plutons in Blue Mountains, Eastern Oregon

Article

Jul 1980

To test the hypothesis that pre-Tertiary rocks of eastern Oregon have undergone clockwise rotation, a paleomagnetic investigation was made of granitic and high-temperature rocks of the upper Jurassic-Lower Cretaceous intrusive suite of the Blue Bountains province, northeastern Oregon. At nine sampling sites in three widely separated regions the paleomagnetic direction showd a high level of intrasite and intersite consistency, with a mean direction of I=63°, D=30°, alpha95=10°. The observed inclination is consistent with that expected on the basis of paleomagnetic results for stable North America, indicating that the intrusives have not undergone northward translation since cooling. The paleomagnetic declination, on the other hand, indicates that the region sampled has undergone a clockwise rotation of 60°+/-29° relative to the stable craton. Two of the bodies sampled are intrusive into the exotic Seven devils terrane, and the remaining body is intrusive into the tectonic melange of a separate arctrench terrane, which may well have been the North American margin. The consistency in magnetic directions between the three regions (k=151) indicates that saturing of the exotic terrane took place prior to the emplacement of the intrusives. The early Tertiary Clarno formation situated to the west of the region sampled has rotated substantially less than the Blue Mountains [Beck et al., 1978], indicating that much of the rotation of the Blue Mountains occurred prior to the Eocene. to account for the observed rotations, we discuss two alternative models. In the first, rotation accompanies back arc rifting associated with the Early Cretaceous westward rifting of the Kalmath Mountains. In the second, rotation reflects Late Cretaceous regional dextral shearing in the Blue Mountain area, which is located several hundred kolometers south of major right lateral faults known to extend from Alaska down at least to northern Washington.

Geodetic evidence for aseismic subduction of the Juan de Fuca Plate

Article

Jan 1979

A down-to-the-east crustal tilt rate across western Washington disclosed by precise leveling over a 70-year period suggests that the Juan de Fuca plate is aseismically underthrusting the North American plate. According to this hypothesis, the frequent occurrence of large thrust earthquake that ordinarily accompany plate convergence need not be expected along the Juan de Fuca subduction zone. This conclusion is consistent with the fact that there have been no great earthquakes in western Washington in historical time (the past 140 years).

Crustal Structure Modeling of Earthquake Data 2. Velocity Structure of the Puget Sound Region, Washington

Article

Jun 1976

Robert S. Crosson

A nonlinear least squares modeling procedure has been developed to estimate simultaneously hypocenter parameters, station corrections, and velocity model parameters by using P wave (or S wave) arrivals from local earthquakes at a regional array. This procedure is applied to P wave data obtained from the 14station telemetered seismograph network in the Puget Sound region of western Washington. Forty selected earthquakes with a depth distribution from near the surface to over 50 km were used, providing a stable inverted model having a large step increase in velocity in the upper 10 km of the crust and exhibiting a comparatively low velocity gradient between 10- and 40-km depths. Transition to an upper mantle velocity of 7.8 km/s is indicated at approximately the 41-km depth, earlier measurements of an anomalously low upper mantle velocity west of the Cascade Range thus being confirmed. The inverted model indicates the presence of a low-velocity zone at the base of the crust, although the exact configuration of this zone is difficult to determine because of a lack of resolving power at critical depths. Relocation of a number of earthquakes using the derived model indicates a substantial reduction in average residuals and a corresponding increase in the general confidence level of hypocenter locations for the region.

Seismic Refraction Studies off Oregon and Northern California

Article

Mar 1968

Seismic refraction profiles across Juan de Fuca ridge, west of the coast of Oregon, show that the ridge is the surface expression of an upraised crust and mantle; the crustal velocity is slightly higher than normal, and the mantle velocity is low. Near the axis of the ridge the mantle velocity is below normal, and the mantle may be as shallow as 7 km below sea level. The topographic asymmetry of the ridge is caused by burial of the east flank of an essentially symmetrical ridge by the sediments of the Cascadia basin, trapped behind the ridge. The base of the sedimentary layer and the base of the oceanic crust both reach depths almost as deep as those normal for the ocean basins under the Tufts abyssal plain to the west and at the foot of the continental slope to the east. In the Gorda basin, just east of Gorda ridge, new measurements agree with older measurements by R. W. Raitt, which show very shallow depth to mantle (depths between 6 and 8 km below sea level) and low mantle velocity, both here and on the west side of Gorda ridge. Juan de Fuca ridge and Gorda ridge thus appear to have the same structure as the East Pacific rise. Profiles on the continental shelf off Washington, Oregon, and northern California show that there is a thick sedimentary section beneath the shelf. Rocks with the velocity of the oceanic crust were found at a depth of 10 km beneath the shelf off Oregon and northern California; mantle depth was determined as 17 km west of the Oregon-Washington border, where the water is 0.12 km deep, and 14 km west of central Oregon at the outer edge of the shelf, where the water is 0.7 km deep.

Cenozoic Tectonics of the Western United States

Article

Jan 1966
REV GEOPHYS

The Cenozoic structures of the western United States are interpreted here as being products mostly of horizontal motion of the crust. The distribution of strike‐slip faulting, tensional fragmentation of the brittle upper crust or rupturing of the entire continental crust, and compression define a pattern of northwestward motion increasing irregularly southwestward toward coastal California. Hans Becker, in 1934, and S. W. Carey, in 1958, are among those who have suggested such a tectonic system. The aggregate Cenozoic right‐lateral displacement of Cretaceous and older rocks and structures by the northwest‐trending strike‐slip faults of coastal California is about 500 km. The greater part of this movement has occurred along the San Andreas fault, but many other faults share in it. At least six earthquakes within the past century have been accompanied by lateral displacements at the surface along faults of the San Andreas system. Successively greater offsets of successively older geologic terranes demonstrate continuing motion throughout Cenozoic time. Late Miocene materials have been displaced at least 160 km; Oligocene, at least 260 km. The present velocity of regional shear strain, about 6 cm/yr, demonstrated by geodetic resurveying in southern and central California, is about 8 times faster than the average needed to account for the total movement within the Cenozoic. The faults are in general associated with structures formed by oblique tension south of Los Angeles and with structures due to oblique compression north of that city. The opening of the Gulf of California and the Salton Trough by the oblique rifting of Baja California and the Peninsular Ranges away from mainland Mexico is the greatest of the tensional effects. The strike‐slip faults may be confined to the crust. Earthquake foci extend no deeper than 16 km. The faults end to the south in the Gulf of California, whose crustal structure is oceanic. To the north, the San Andreas turns seaward as the north‐facing Gorda scarp, west in line of which in deeper water is the south‐facing Mendocino escarpment, produced apparently by an inactive left‐lateral oceanic fault. The continental sliver of coastal and Baja California, west of the faults of the San Andreas system, may be drifting northwestward independently over the ocean floor and the mantle, and the leading point of the sliver may have been deflected westward when it hit the Mendocino scarp on the sea floor. East of this coastal movement system is the Basin and Range province, whose obvious Cenozoic structures are dominated by block faulting. The present ranges have formed mostly since early Miocene time, similar older ranges having been destroyed by erosion and deformation. The normal faulting, which is not associated within the region with any complementary tectonic compression, requires crustal extension as its basic cause. If the faults maintain their average 60° dips at depth, extension is half the dip‐slip amount; but probably the major faults flatten downward, and the amount of extension about equals that of shallow dip‐slip. Total Cenozoic extension in northern Nevada and Utah may have been 300 km. Concurrent volcanism much augmented the thinned and fragmented crust, and the volcanic terranes in turn have been fragmented by block faulting. Right‐lateral strike‐slip faults trend northwestward in lanes between normal‐fault maintain blocks in the southwestern part of the Basin‐Range province. Cenozoic displacements reach 50 km on the Las Vegas fault and 80 km on the Death Valley‐Furnace Creek faults. Northeast of the strike‐slip faults, ranges and basins trend north‐northeastward in tension‐gash orientation. Within the belt of lateral faulting, ranges undergoing active normal faulting mostly trend north‐northwestward in oblique pull‐apart orientation. The Sierra Nevada and Klamath Mountains have moved northwestward and rotated counterclockwise, thus moving away from the continental interior more in the north than in the south, and the extension distributed behind them has formed the Basin‐Range province. The narrow block‐fault Rio Grande valley system of New Mexico and southern Colorado is structurally and topographically similar to the rift valleys of East Africa and reflects localized crustal extension. The Idaho batholith, like the Sierra Nevada batholith, is drifting northwestward as an unbroken plate. Extension east of the Idaho batholith is taken up by normal‐fault fragmentation in south‐central Idaho and southwestern Montana, whereas extension south of the batholith has produced a rift through the continental crust, the Snake River Plain, filled deeply by lava. Seismic velocities indicate granitic crust to be lacking in at least the western part of the plain. Right‐lateral faults of the Osburn system bound the batholithic plate on the north, and the motion they represent is taken up north of them by extension forming fault troughs. Integration of geologic and geophysical information shows that large regions of the Northwest are lava accumulations of continental crustal thickness, not old continental crust covered by lava. The volcanic terrane of northwestern Oregon and southwestern Washington forms new volcanic crust in a region which was oceanic before Cenozoic time. The volcanic terrane of southeastern Oregon, northeastern California, and northwestern Nevada fills an irregular tension rift through the Mesozoic continental crust. This rift resulted from the westward motion of the Klamath Mountains region, which was sundered from a position south of the Mesozoic terrane of northeastern Oregon and which was bent oroclinally as it moved westward in post‐middle Eocene time. The Mesozoic terrane of northeastern Oregon pivoted away from the Idaho batholith to form a smaller orocline and left a triangular rift since filled by lava. Independent motion of continental crust over mantle and oceanic crust seems to be indicated. Inertial forces due to redistribution of rotational momentum among crustal fragments, mantle, and core may provide the motive power.

A seismic refraction and gravity study of the earth''s crust in British Columbia

Article

Apr 1965

Explosion refraction studies have been carried out in areas adjacent to Vancouver Island, including the Strait of Georgia and Johnstone Strait and along the west coast of the Island. A refraction line has also been observed from a large explosion in Seymour Narrows, eastward through the mountains and for some distance across the plains of Alberta. Although the surficial strata complicate the interpretation, a series of short refraction profiles consistently reveal the presence of an intermediate layer with velocity of about 6.8 km/sec. in the coastal area. The depth to the upper boundary of this layer varies from about 11 km. along the west coast of the Island to less than 5 km. in the Strait of Georgia and along the east side of the Island. The longer range observations parallel to the coast indicate this layer to be more than 40 km. thick. On the profile eastward through the mountains a velocity of 7.8 km/sec. has been found for Pn, from an unreversed profile, with a crustal thickness of approximately 30 km.

Crustal Reflection of Plane P and SV Waves

Article

Nov 1962

Norman A. Haskell

Expressions in terms of layer matrices have been derived for the motion of the free surface when plane harmonic P or SV waves are incident at any given angle at the base of a horizontally layered crust. A computer program has been written to calculate normalized surface displacements as functions of horizontal phase velocity (or angle of incidence) and wave period. Numerical values have been obtained for the same single-layered model of the continental crust for which the corresponding calculations for SH waves have been presented previously. The surface motion pattern is similar to that of the SH case, but it is somewhat more complex owing to PS and SP conversions at the layer boundaries. Variations in amplitude and phase of the surface displacements are particularly large and erratic in the case of SV waves that are incident at angles such that α1<c<αn, where c is horizontal phase velocity and α1 and αn are the P wave velocities in the crustal layer and mantle, respectively. It is recommended that data from this region be avoided in studies of focal mechanisms that are based on the attitude of the plane of polarization of the S waves. For angles of incidence such that c>αn, the crustal transfer functions for incident SH and SV waves are sufficiently similar for conventional methods to be used in determining the angle of polarization with moderate accuracy.

Distribution of Stresses in the Descending Lithosphere from a Global Survey of Focal-Mechanism Solutions of Mantle Earthquakes

Article

Jan 1971
REV GEOPHYS

A region-by-region analysis of 204 reliable focal-mechanism solutions for deep and intermediate-depth earthquakes strongly supports the idea that portions of the lithosphere that descend into the mantle are slablike stress guides that align the earthquake-generating stresses parallel to the inclined seismic zones. At intermediate depths extensional stresses parallel to the dip of the zone are predominant in zones characterized either by gaps in the seismicity as a function of depth or by an absence of deep earthquakes. Compressional stresses parallel to the dip of the zone are prevalent everywhere the zone exists below about 300 km. These results indicate that the lithosphere sinks into the asthenosphere under its own weight but encounters resistance to its downward motion below about 300 km. Additional results indicate contortions and disruptions of the descending slabs; however, stresses attributable to simple bending of the plates do not seem to be important in the generation of subcrustal earthquakes. This summary, intended to be comprehensive, includes nearly all solutions obtainable from the World-Wide Standardized Seismograph Network (WWSSN) for the period 1962 through part of 1968 plus a selection of reliable solutions of pre-1962 events, and it includes data from nearly every region in the world where earthquakes occur in the mantle. The double-couple or shear dislocation model of the source mechanism is adequate for all the data.

Relations of andesites, granites, and derivative sandstones to arc-Trench tectonics

Article

William R. Dickinson

Andesitic volcanogenic sequences, granitic batholith belts, and derivative graywacke‐arkose sedimentary successions are prominent rock assemblages associated with alpinotype peridotite‐gabbro belts and other characteristic tectonic features in orogenic regions or mobile belts where repeated crustal deformation and metamorphism have occurred. Field relations in the circum‐Pacific region indicate that andesitic eruptive suites and granitic intrusive suites are commonly consanguineous and roughly contemporaneous and that they have shed voluminous detritus into coeval graywacke‐arkose belts nearby. Modern systems of oceanic trenches and parallel magmatic arcs are probable analogues of the tectonic settings in which the three related rock assemblages formed. Data on crustal geophysics, trace‐element geochemistry, and strontium‐isotope ratios preclude participation of sialic crust in the generation of andesitic magmas at shallow levels but permit alternative hypotheses of primary partial melts from the mantle, derivative melts differentiated from primary basaltic melts, or melts from oceanic lithosphere slabs descending along inclined seismic zones beneath the volcanic arcs. In Quaternary andesitic suites, areal petrologic variations, particularly in potash content, are consistent tranverse to active volcanic chains regardless of longitudinal variations in crustal thickness. Levels of potash content in different suites correlate well with depths to the inclined seismic zone beneath, although significant scatter of points is apparent. Petrologic data from older andesitic terranes can be used to plot approximate positions and inclinations of paleoseismic zones. The anatectic hypothesis for the origin of magmatic plutons in intrusive batholiths is challenged by apparent comagmatic associations with andesitic eruptives, common sequences of intrusion from mafic to felsic, doubtful presence of suitable geosynclinal roots in some areas, available strontium‐isotope ratios, difficult geothermal inferences, and unexpected episodicity or periodicity of repeated intrusive events that are correlative throughout large longitudinal segments of batholith belts. Consistent positions of batholith belts along the trends of relatively high‐temperature and low‐pressure members of paired metamorphic belts suggest that the granitic plutons were emplaced in the roots of complex volcanoplutonic arcs, and that granitic intrusive magmas may be derived from the same deep sources as andesitic eruptive magmas. Transverse petrologic asymmetry within Mesozoic batholiths of western North America is reminiscent of the similar petrologic asymmetry within Cenozoic volcanic terranes, and may be used to construct speculative paleoseismic zones for the volcanoplutonic arcs whose roots the batholiths may represent. Graywacke and arkose sequences that lie on the Pacific side of andesitic volcanogenic and granitic batholith belts are composed mainly of first‐cycle volcanic and plutonic detritus and commonly form large parts of the relatively low‐temperature and high‐pressure members of paired metamorphic belts. Detritus eroded during and between successive episodes of volcanism and plutonism in the adjacent volcano‐plutonic provenances was deposited in parallel subsiding belts, where it was progressively buried as an inverse record of the successive magmatic increments to the arc regions. The graywacke‐arkose belts commonly include two parallel divisions. Distal facies of strongly deformed trench and continental‐rise deposits were ground against and beneath the seaward flanks of the volcanoplutonic arcs. Proximal facies of more orderly strata were deposited in sediment traps between trenches and arcs in the tectonic position occupied by shelves, slopes, and troughs of varied bathymetric character in modern arc‐trench systems. The interpretations in this paper attempt to bring petrologic inferences about orogenic rock assemblages in line with current mobilist tectonic concepts that are supplanting previous stabilist views. The formation of the three rock assemblages discussed is probably the principal means by which continental crust is formed from the mantle.

Structure under Mount Rainier, Washington, Inferred from Teleseismic Body Waves

Article

Jan 1979

Charles Langston

Teleseismic long-period P waves recorded at the World-Wide Standard Seismograph Network station LON (Longmire, Washington) are shown to exhibit strong anomalous particle motion not attributable to instrument miscalibration or malfunction. In particular, a large and azimuthally smoothly varying tangential component is observed after vector rotation of horizontal P waves into the ray direction and after application of a deconvolution technique which equalizes effective source time functions and removes the instrument response. These tangential waves attain amplitudes comparable to the radial component and demonstrate wave form antisymmetry about a NNE azimuth. A model which contains a single high-contrast interface dipping toward the NNE at a depth of 15-20 km can explain most of the characteristics of the long-period P wave data, provided dips are greater than about 10° and only the interference of P and Ps generated at the interface is considered. The model breaks down for later arrivals which are presumably multiples or scattered waves. Examination of long-period S waves from several deep teleseisms shows a prominent Sp arrival 18 s before S. The timing of this phase conversion suggests an interface at about 145-km depth, and its sense of polarity suggests that the velocity contrast is from higher to lower velocities as depth decreases. This interface may correspond to the bottom of the upper mantle low-velocity zone in the area.

Seismicity and Tectonics of the Northeast Pacific Ocean

Article

Jun 1968
J GEOPHYS RES

The hypocenters of approximately 260 earthquakes that occurred during the 10-year period 1954-1963 in. the northeast Pacific and in adjacent continental areas were relocated in order to study the relationship of seismicity to large-scale tectonic features in the region between the Mendocino fracture zone and central Alaska. The major seismic zones in this region are related to a series o,f growing oceanic ridges connected by transform faults. Nearly all the earthquakes on the Blanco fracture zone were concentrated between the Gorda and the Juan de Fuca ridges. Activity on the Mendocino fracture zone was restricted almost exclusively to a 400-km segment of the fracture zone between the coast of northern California and the Gorda ridge. Evidence from the distribution and mechanism of earthquakes suggests that the San Andreas fault bends abruptly near Cape Mendocino and follows the seismically active part of the Mendocino fracture zone. The oceanic block north of the Mendocino fracture zone and east of the Gorda ridge is being deformed internally as well as along its borders. Although the Gorda ridge, which possesses a prominent median rift, exhibited considerable activity during this period, only one event was found on the Juan de Fuca ridge, which lacks a prominent median rift. A requirement for the occurrence of seismic activity along a ridge crest seems to be the presence of a prominent rift valley. The distribution of epicenters suggests that the main zone of present-day seismic activity in southeast Alaska follows the Fairweather fault, passes out to sea near 58øN, remains offshore, and continues as far south as the northern end of the Juan de Fuca ridge. This entire fault zone, which is called the Queen Charlotte Islands fault, is interpreted as a dextral transform fault that connects the Juan de Fuca ridge with the northeastern end of the Aleutian island arc system. Earthquake mechanism sol.utions for some o,f the larger events are in agreement with the above interpretations. No observable seismic activity was associated with the Rocky Mountain trench, the Tintina valley, or the Chatham Strait. Several areas in coastal Alaska that have been relatively aseismic during the past 60 years are discussed as possible sites of future eaxthquakes.

Paleomagnetic evidence for tectonic rotation of the Oregon Coast Range /

Article

Jan 1977

Robert Whitten. Simpson

Thesis (Ph. D.)--Dept. of Geophysics, Stanford University. Bibliography: leaves 99-109.

Paleomagnetic evidence for tectonic rotation of the Oregon Coast Range

Article

Jan 1977
GEOLOGY

Robert W. Simpson

Paleomagnetic directions from lower, middle, and upper Eocene sedimentary and volcanic rocks in the Oregon Coast Range point 50° to 70° east of the expected Eocene field direction. This discrepancy is attributed to the tectonic rotation of a block at least 225 km in length extending from just north of the Klamath Mountains to north of Newport, Oregon. Two models are developed for the tectonic evolution of the Pacific Northwest. In the first, the coastal block is rotated seaward about a pivot point near its southern end in order to restore it to its Eocene position. In the second, it is assumed that the coastal block extends all the way to the Olympic Peninsula. Rotating the block back about a pivot near its northern end moves the southern end and the Klamath Mountains eastward across Oregon and restores the Klamath metamorphic belts to a position against the line of Cretaceous batholiths in northwestern Nevada.

Spreading of the Ocean Floor: New Evidence

Article

Jan 1967

F. J. Vine

It is suggested that the entire history of the ocean basins, in terms of ocean-floor spreading, is contained frozen in the oceanic crust. Variations in the intensity and polarity of Earth's magnetic field are considered to be recorded in the remanent magnetism of the igneous rocks as they solidified and cooled through the Curie temperature at the crest of an oceanic ridge, and subsequently spread away from it at a steady rate. The hypothesis is supported by the extreme linearity and continuity of oceanic magnetic anomalies and their symmetry about the axes of ridges. If the proposed reversal time scale for the last 4 million years is combined with the model, computed anomaly profiles show remarkably good agreement with those observed, and one can deduce rates of spreading for all active parts of the midoceanic ridge system for which magnetic profiles or surveys are available. The rates obtained are in exact agreement with those needed to account for continental drift. An exceptionally high rate of spreading (approximately 4.5 cm/year) in the South Pacific enables one to deduce by extrapolation considerable details of the reversal time scale back to 11.5 million years ago. Again, this scale can be applied to other parts of the ridge system. Thus one is led to the suggestion that the crest of the East Pacific Rise in the northeast Pacific has been overridden and modified by the westward drift of North America, with the production of the anomalous width and unique features of the American cordillera in the western United States. The oceanic magnetic anomalies also indicate that there was a change in direction of crustal spreading in this region during Pliocene time from eastwest to southeast-northwest. A profile from the crest to the boundary of the East Pacific Rise, and the difference between axial-zone and flank anomalies over ridges, suggest increase in the frequency of reversal of Earth's magnetic field, together, possibly, with decrease in its intensity, approximately 25 million years ago. Within the framework of ocean-floor spreading, it is suggested that magnetic anomalies may indicate the nature of oceanic fracture zones and distinguish the parts of the ridge system that are actively spreading. Thus data derived during the past year lend remarkable support to the hypothesis that magnetic anomalies may reveal the history of the ocean basins.

Magnetic Anomalies over a Young Oceanic Ridge off Vancouver Island

Article

Nov 1965

The recent speculation that the magnetic anomalies observed over oceanic ridges might be explained in terms of ocean-floor spreading and periodic reversals of the earth's magnetic field may now be reexamined in the light of suggested reversals during the past 4 million years and the newly described Juan de Fuca Ridge.

Transform Faults, Oceanic Ridges, and Magnetic Anomalies Southwest of Vancouver Island

Article

Nov 1965

J. Tuzo Wilson

The San Andreas Fault and a large fault off British Columbia are interpreted as examples of the recently proposed "transform faults." They are joined by a short, isolated length of oceanic ridge striking N20°E, with an associated "window" of young crust. The displacement along these faults is estimated at 400 kilometers.

A study of the crust in Puget Sound using a fixed seismic source

Jan 1975

H Zuercher
Zuercher

A study of upper mantle structure in the Pacific Northwest using P waves from teleseisms

Jan 1974

J W Lin
Lin

Jan 1975

W E Shannon
F Lombardo
B Compton
Shannon

A study of upper mantle structure in the Pacific Northwest using P waves from teleseisms Ph.D. thesis Univ. of Wash

J W Lin

A study of the crust in Puget Sound using a fixed seismic source M.S. thesis Univ. of Wash

H Zuercher

Halliday Canadian Seismograph Operations-1974 Seismological Series Number 70 Energy Mines and Resources Canada Earth Physics Branch Ottawa Canada

W E F Shannon
B Lombardo
R J Compton

Evidence for the subducting lithosphere under Southern Vancouver Island and Western Oregon from teleseismic P wave conversions

Abstract

No full-text available

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