Article

Earth's inner core rotation, 1971 to 1974, illuminated by inner-core scattered waves

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Abstract

The solid inner core is at the center of the Earth, gravitationally held within the liquid outer core. It is one of the most dynamic parts of Earth’s interior. Since the initial claim of inner core differential rotation relative to the mantle, its existence and rate have been challenged for over two decades. Here, we re-examine the seismic records of two megaton nuclear tests in Novaya Zemlya, Russia, three years apart, from the Large Aperture Seismic Array in Montana, USA. Using an improved static time correction from an antipodal event, we refine the resolution of the beamforming of PKiKP and its inner-core scattered coda. Then, we measure the slight time shifts (tenths of seconds) between the inner-core-scattered waves from the two events with moving-time-window cross-correlation. Applying a novel back-projection method, we locate the inner-core regions that scatter the energy within the PKiKP coda based on its slowness and the lapse time. We then measure the inner core rotation, first assuming alignment with Earth’s rotation axis, then finding the best-fitting differential rotation axis with a grid search. The rotation rates here are robust and consistent across the many scattered arrivals throughout the inner core scattering wavetrain. Our results indicate 0.10°/year inner core super-rotation rate from 1971 to 1974 aligned with Earth’s rotation axis, or 0.125°/year with the rotation axis tilting about 8° from the Earth’s rotation axis, which yields a marginally better fit to the observed time shifts.

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... Initial models of fairly steady superrotation of Earth's IC relative to mantle (1) have slowed from 1° per year to 0.05 to 0.15° per year (2)(3)(4)(5) and have been challenged by models with only deformation of the IC boundary (ICB) (6,7), seismological models with strong variations in rotation rate (8,9), and geodynamic models with small equilibrium oscillations (10)(11)(12). It is fair to say that there is no current consensus. ...
... We estimate the differential rotation of the IC by measuring changes in backscattering within the IC of P waves of a pair of nuclear explosions. We did this previously for another pair of explosions (2)(3)(4), and here, we follow the same procedure. Our data are two explosions recorded by the Large Aperture Seismographic Array (LASA) (13) (see fig. ...
... We take the same approach as Wang and Vidale (2) to estimate the IC differential rotation from 1969 to 1971 from the Amchitka explosions, simplifying it because of the reduced resolution arising from lower seismogram correlation. Predicted time shifts for an IC superrotating 1° for the Amchitka and NZ geometries (Fig. 1, B and C) are nearly orthogonal because of their pole-perpendicular and pole-parallel paths, respectively (Fig. 1A). ...
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... We did make a few assumptions that the mantle influence in the ddt is small, that each local gradient is linear, and that the IC lateral shifts are from the rigid body rotation of the IC around the spin axis of the Earth, all of which seem reasonable. The new rate is also consistent with relatively low rates (0.05-0.10°/yr) in recent studies (Vidale, 2019;Yang & Song, 2020b) and remarkably close to the most recent estimate of 0.125°/yr from IC scattering waves by Wang and Vidale (2022). Such a slow rotation can also reconcile with a marginal rate of 0.13 ± 0.11°/yr from the normal-mode data (Laske & Masters, 2003). ...
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Many geothermal and volcanic regions experience remote and regional triggering following large earthquakes. The transient or permanent changes in stresses acting on faults and fractures can lead to changes in seismicity rates following either the passage of teleseismic waves or the permanent change in stresses following regional events. One such region of prevalent triggering is the Coso Geothermal Field (CGF) in eastern California, which is located roughly 30 km to the north of the 2019 Mw 7.1 Ridgecrest epicenter. Previous regional earthquakes have not only seemingly caused increase in seismicity rate surrounding the CGF, but also showed an absence of such rate increases in the CGF itself. To test whether seismicity rates in the CGF were dissimilar to the surrounding area following the Mw 7.1 Ridgecrest earthquake, I carry out seismicity rate change calculations using a catalog of seismicity compiled using a local seismic network and find that the behavior at CGF is identical to the surrounding area. Comparisons of seismicity rate changes calculated using a regional-network-derived catalog, and the local-network-derived catalog show that for a moderate, regional earthquake (2009 Mw 5.2 Olancha, California), the local network catalog reveals a change in seismicity rate whereas the regionally network catalog shows no significant changes. The differences are possibly related to incomplete sampling of seismicity using the regional network due to the existence of a shallow brittle–ductile transition centered on the CGF. The CGF, thus, is prone to triggering from both teleseismic and regional earthquakes.
Article
Temporal changes of inner-core (IC) seismic phases have been confirmed with high-quality waveform doublets. However, the nature of the temporal changes is still controversial. We investigated systematically the temporal changes of IC refracted (PKIKP) and reflected (PKiKP) waves with a large data set of waveform doublets. We used non-IC reference phase (mainly SKP), which eliminated ambiguity where the temporal changes come from. We found that the temporal changes have always started at refracted PKIKP and the travel time changes correlate better with PKIKP. Changes in reflected PKiKP can be easily contaminated by the strong and time-varying PKIKP and coda wave trains and therefore are not reliable indicators for IC boundary changes. Combining with previous observations, we conclude that the temporal changes come mostly (if not all) from the IC interior and IC surface changes as the sole source suggested previously can be ruled out. The differential rotation of the IC shifting its heterogeneous uppermost structures is the simplest and most reasonable explanation for the origin of the time-varying IC waves. A rotation rate of about 0.05-0.1° per year with possible decadal fluctuation can reconcile all temporal change observations from body waves, IC scattering, and normal mode data.
Article
Using earthquakes to find earthquakes Earthquake catalogs elucidate the behavior of faults and allow for rough estimates of when large earthquakes might occur. Cataloging small earthquakes is challenging because the small signal is often indistinguishable from noise. Ross et al. used a template-matching algorithm to find almost two million tiny earthquakes previously missed by other earthquake-logging techniques in Southern California (see the Perspective by Brodsky). This more-complete catalog can be used to better understand faults, earthquake reoccurrence, and other geophysical processes. Science , this issue p. 767 ; see also p. 736
Article
Seeking geophysical explanations for the periodic ∼six-year oscillation (SYO) previously found in the Earth's length-of-day variation (ΔLOD), we analyze the global GPS displacement and geomagnetic data using the array processing technique of OSE (Optimal Sequence Estimation), and find clear evidences of the 6-yr signals which manifest as a westward rotary propagating wave of the sectoral spherical-harmonic pattern of degree-2 order-2 (Y22). Based on the period, the spatial pattern, and the amplitudes and the estimated phases that exhibit consistent synchronicity among the three datasets, we propose the following scenario: The mantle-inner core gravitational (MICG) coupling gives rise to a 6-yr axial torsional libration of the inner core controlled by the sectoral Y22 density anomalies, or the equatorial ellipticities, in the inner core and the (lower) mantle, the angular momentum exchange between which manifests as the SYO in ΔLOD. It forces into action a pressure wave-2 cyclic in 6 yr through the fluid outer core, which in turn produces the corresponding GPS and geomagnetic variations. Our findings provide insight into the dynamics of the deep Earth, and corroborate a positive density anomaly for the lower-mantle Large Low-Shear-Velocity Provinces.
Article
Earthquake swarms represent a sudden increase in seismicity that may indicate a heterogeneous fault-zone, the involvement of crustal fluids and/or slow fault slip. Swarms sometimes precede major earthquake ruptures. An earthquake swarm occurred in October 2015 near San Ramon, California in an extensional right step-over region between the northern Calaveras Fault and the Concord–Mt. Diablo fault zone, which has hosted ten major swarms since 1970. The 2015 San Ramon swarm is examined here from 11 October through 18 November using template matching analysis. The relocated seismicity catalog contains ∼4000 events with magnitudes between −0.2<Md<3.6. The swarm illuminated three sub-parallel, southwest striking and northwest dipping fault segments of km-scale dimension and thickness of up to 200 m. The segments contain coexisting populations of different focal-mechanisms, suggesting a complex fault zone structure with several sets of en échelon fault orientations. The migration of events along the three planar structures indicates a complex fluid and faulting interaction processes. We searched for correlations between seismic activity and tidal stresses and found some suggestive features, but nothing that we can be confident is statistically significant.
Article
Stresses in the lithosphere arises from multiple natural loading sources that include both surface and body forces. The largest surface loads include near-surface water storage, snow and ice, atmosphere pressure, ocean loading, and temperature changes. The solid Earth also deforms from celestial body interactions and variations in Earth's rotation. We model the seasonal stress changes in California from 2006 through 2014 for seven different loading sources with annual periods to produce an aggregate stressing history for faults in the study area. Our modeling shows that the annual water loading, atmosphere, temperature, and Earth pole-tides are the largest loading sources and should each be evaluated to fully describe seasonal stress changes. In California we find the hydrological loads are the largest source of seasonal stresses. We explore the seasonal stresses with respect to the background principal stress orientation constrained with regional focal mechanisms and analyze the modulation of seismicity. Our results do not suggest a resolvable seasonal variation for the ambient stress orientation in the shallow crust. When projecting the seasonal stresses into the background stress orientation we find the timing of microseismicity modestly increases from a ~8 kPa seasonal mean-normal-stress perturbation. The results suggest faults in California are optimally oriented with the background stress field and respond to subsurface pressure changes, possibly due to processes we have not considered in this study. At any time a population of faults are near failure as evident from earthquakes triggered by these slight seasonal stress perturbations.
Article
We investigate the dynamics of the inner core wobble (ICW), the Euler-Liouville wobbling motion of the Earth's solid inner core, under the mantle-inner core gravitational (MICG) torques within the Earth. Chao [2016] derived the full 3-D equation of motion for the MICG dynamics in terms of the spherical-harmonic multipoles of mass density, and focused on the axial component for inner-core's torsional libration. Here, aiming for the ICW, we deduce the 2-D equatorial component of the MICG torque owing to the oblateness of the mantle and the inner core. The period of the free Eulerian wobble of a hypothetical isolated rigid inner core would be a prograde +414 days. The action of the added MICG equatorial torque is found to be (negatively) strong enough to render the wobbling motion to become retrograde (with a negative frequency), which is further but slightly modified by the elastic yielding feedback of the inner core. Imposing yet further the passive effect of the hydrostatic pressure due to the fluid outer core greatly lengthens the natural period to become decadal. The final estimate of the ICW natural period in accordance with the (seismological) PREM Earth model is a retrograde PICW ≈ −15.6 years, in contrast to a prograde +6.6 years supposed in the literature. The corresponding spring constant (per radian of wobble) is 5.5 × 1022 N m. Our results instigate likely identification of the ICW with decadal wobbles observed in the Earth's polar motion.
Article
Tidal triggering of earthquakes is hypothesized to provide quantitative information regarding the fault's stress state, poroelastic properties, and may be significant for our understanding of seismic hazard. To date, studies of regional or global earthquake catalogs have had only modest successes in identifying tidal triggering. We posit that the smallest events that may provide additional evidence of triggering go unidentified and thus we developed a technique to improve the identification of very small magnitude events. We identify events applying a method known as inter-station seismic coherence where we prioritize detection and discrimination over characterization. Here we show tidal triggering of earthquakes on the San Andreas Fault. We find the complex interaction of semi-diurnal and fortnightly tidal periods exposes both stress threshold and critical state behavior. Our findings reveal earthquake nucleation processes and pore pressure conditions – properties of faults that are difficult to measure, yet extremely important for characterizing earthquake physics and seismic hazards.
Article
Geothermal areas are long recognized to be susceptible to remote earthquake triggering, probably due to the high seismicity rates and presence of geothermal fluids. However, anthropogenic injection and extraction activity may alter the stress state and fluid flow within the geothermal fields. Here we examine the remote triggering phenomena in the Coso geothermal field and its surrounding areas to assess possible anthropogenic effects. We find that triggered earthquakes are absent within the geothermal field but occur in the surrounding areas. Similar observation is also found in the Salton Sea geothermal field. We hypothesize that continuous geothermal operation has eliminated any significant differential pore pressure between fractures inside the geothermal field through flushing geothermal precipitations and sediments out of clogged fractures. To test this hypothesis, we analyze the pore-pressure-driven earthquake swarms, and they are found to occur outside or on the periphery of the geothermal production field. Therefore, our results suggest that the geothermal operation has changed the subsurface fracture network, and differential pore pressure is the primary controlling factor of remote triggering in geothermal fields.
Article
The aims of this paper are: (i) Formulating the dynamics of the mantle-inner core gravitational (MICG) interaction in terms of the spherical-harmonic multipoles of mass density. The modeled MICG system is composed of two concentric rigid bodies (mantle and inner core) of near-spherical but otherwise heterogeneous configuration, with a fluid outer core in between playing a passive role. We derive the general equation of motion for the vector rotation, but only focus on the polar component that describes the MICG axial torsional libration. The torsion constant and hence the square of the natural frequency of the libration is proportional to the product of the equatorial ellipticities of the mantle and inner-core geoid embodied in their multipoles (of two different types) of degree 2 and order 2 (such as the Large Low-Shear-Velocity Provinces above the core-mantle boundary). (ii) Studying the geophysical implications upon equating the said MICG libration to the steady 6-year oscillation that are observed in the Earth's spin rate, or the length-of-day variation (ΔLOD). In particular the MICG torsion constant is found to be = CIC σz2 ≈ 6.5 ×1019 N m, while the inner core's (BIC −AIC) ≈1.08 ×1031 kg m2 gives the inner core triaxiality (BIC −AIC)/CIC ≈1.8 ×10-4, about 8 times the whole-Earth value. It is also asserted that the required inner-core ellipticity amounts to no more than ~140 m in geoid height, much smaller than the sensitivity required for the seismic wave travel-time to resolve the variation of the inner-core.
Article
The possibility that tidal stress can trigger earthquakes is long debated. In particular, a clear causal relationship between small earthquakes and the phase of tidal stress is elusive. However, tectonic tremors deep within subduction zones are highly sensitive to tidal stress levels, with tremor rate increasing at an exponential rate with rising tidal stress. Thus, slow deformation and the possibility of earthquakes at subduction plate boundaries may be enhanced during periods of large tidal stress. Here we calculate the tidal stress history, and specifically the amplitude of tidal stress, on a fault plane in the two weeks before large earthquakes globally, based on data from the global, Japanese, and Californian earthquake catalogues. We find that very large earthquakes, including the 2004 Sumatran, 2010 Maule earthquake in Chile and the 2011 Tohoku-Oki earthquake in Japan, tend to occur near the time of maximum tidal stress amplitude. This tendency is not obvious for small earthquakes. However, we also find that the fraction of large earthquakes increases (the b-value of the Gutenberg-Richter relation decreases) as the amplitude of tidal shear stress increases. The relationship is also reasonable, considering the well-known relationship between stress and the b-value. This suggests that the probability of a tiny rock failure expanding to a gigantic rupture increases with increasing tidal stress levels. We conclude that large earthquakes are more probable during periods of high tidal stress. © 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
Article
Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.
Article
The seismic wavefield mainly contains reflected, refracted and direct waves but energy related to elastic scattering can also be identified at frequencies of 1 Hz and higher. The scattered, high-frequency seismic wavefield contains information on the small-scale structure of the Earth's crust, mantle and core. Due to the high thermal conductivity of mantle materials causing rapid dissipation of thermal anomalies, the Earth's small-scale structure most likely reveals details of the composition of the interior, and, is therefore essential for our understanding of the dynamics and evolution of the Earth. Using specific ray configurations we can identify scattered energy originating in the lower mantle and under certain circumstances locate its point of origin in the Earth allowing further insight into the structure of the lowermost mantle. Here we present evidence, from scattered PKP waves, for a heterogeneous structure at the core-mantle boundary (CMB) beneath southern Africa. The structure rises approximately 80 km above the CMB and is located at the eastern edge of the African LLSVP. Mining-related and tectonic seismic events in South Africa, with m(b) from 3.2 to 6.0 recorded at epicentral distances of 119.3 degrees to 138.8 degrees from Yellowknife Array (YKA) (Canada), show large amplitude precursors to PKPdf arriving 3-15 s prior to the main phase. We use array processing to measure slowness and backazimuth of the scattered energy and determine the scatterer location in the deep Earth. To improve the resolution of the slowness vector at the medium aperture YKA we present a new application of the F-statistic. The high-resolution slowness and backazimuth measurements indicate scattering from a structure up to 80 km tall at the CMB with lateral dimensions of at least 1200 km by 300 km, at the edge of the African Large Low Shear Velocity Province. The forward scattering nature of the PKP probe indicates that this is velocity-type scattering resulting primarily from changes in elastic parameters. The PKP scattering data are in agreement with dynamically supported dense material related to the Large Low Shear Velocity Province.
Article
Seismic observations provide strong evidence that Earth's inner core is anisotropic, with larger velocity in the polar than in the equatorial direction. The top 60-80 km of the inner core is isotropic; evidence for an innermost inner core is less compelling. The anisotropy is most likely due to alignment of hcp (hexagonal close-packed) iron crystals, aligned either during solidification or by deformation afterward. The existence of hemispherical variations used to be controversial, but there is now strong evidence from both seismic body wave and normal mode observations, showing stronger anisotropy, less attenuation, and a lower isotropic velocity in the western hemisphere. Two mechanisms have been proposed to explain the hemispherical pattern: either (a) inner core translation, wherein one hemisphere is melting and the other is solidifying, or (b) thermochemical convection in the outer core, leading to different solidification conditions at the inner core boundary. Neither is (yet) able to explain all seismically observed features, and a combination of different mechanisms is probably required.
Article
Geothermal Development he Coso Geothermal Field, located in east central Califor-nia (Fig. 1), hosts a world-class power-generating project that has been in continuous operation for the past 15 years. The project is located on the test and evaluation ranges of the Na-val Air Weapons Station, China Lake—the Navy's premier research and development (R&D) facility for air-to-air and air-to-ground ordnance. Fully financed by private investment, the Coso geother-mal power project is a testament to creativity in business and gov-ernment relations. At its peak, the project produced more than 273 megawatts (MW) of electricity that is all sold into the local utility grid under a long-term power sales agreement. The geologic setting of the field is a releasing bend step-over in a dextral strike-slip fault system. Local crustal thinning accounts for the shallow (<2 km), very hot (200° -328°C) re-source. Given the present rate of production and reservoir pro-jections based on historical data, it is anticipated that the field will be capable of producing electricity for at least 25, and possi-bly as many as 50 more years. The overall military geothermal program is managed by the Geothermal Program Office (GPO) located at China Lake, CA. That office is located within the U.S. Navy, but has the broader mandate to oversee exploration for—and development of—geo-thermal resources wherever they occur on lands under the control of any of the nation's military services. The GPO executes two broad functions: resource development and resource management. The entire program is guided by the underlying principal that mis-sion integrity is paramount. Thus, if the mission of a candidate facility will be adversely impacted beyond mitigation, a geother-mal project will not proceed. However, it has been found that most real or perceived impediments can be successfully resolved so that viable geothermal power projects can—and do—move forward.
Article
Geothermal/volcanic regions are most susceptible to local earthquake triggering by regional and remote earthquakes. Transient stresses caused by surface waves of these earthquakes can activate critically stressed faults. Though earthquakes can be triggered in geothermal/volcanic regions, it is less understood how these regions differ in their triggering responses to distant earthquakes. We conduct a systematic survey of local earthquakes triggered by distant earthquakes in three geothermal/volcanic regions of California: Long Valley Caldera, Coso Geothermal Field, and Geysers Geothermal Field. We examine waveforms of distant earthquakes with magnitudes ≥ 5.5 occurring between 2000 and 2012 and compute β-statistics to confirm the significance of our findings. We find that Long Valley, Coso, and Geysers vary in triggering frequency – 2.0%, 3.8%, and 6.8% in the 12 year period, respectively – and when compared to the triggering of deep tectonic tremors along the Parkfield-Cholame section of San Andreas Fault (9.2% in the 12 year period). Stress triggering thresholds vary among the regions with Long Valley having the highest of ~5 kPa and ~1 kPa for the other regions. Because dynamic stresses from distant earthquakes are similar in these three regions, the varying triggering behavior likely reflects faults having a tendency to be at or near failure. This is compatible with Geysers having a higher a-value in the Gutenberg-Richter relationship and higher geothermal productivity than the other two regions. The observation of more frequency triggering of tremor than microearthquakes is consistent with recent laboratory studies on increasing triggerability with lower effective stress.
Article
We show that the Schuster test alone does not provide a sufficient condition to assert the existence of a periodicity in an earthquake catalog. Such periodicities can be detected by computing a spectrum of Schuster p-values (the probability to observe such a level of periodic variations in a catalog occurring out of a constant seismicity rate). We show that the detection level is slightly period dependent, and we provide an analytical expression relating the amplitude of seismicity-rate variations to the confidence level at which the probability that the observed variations be due to chance can be discarded. The Schuster spectrum also provides information about the deviation from a sinusoidal function of the periodicity of the seismicity rate, and identifies an eventual imperfect declustering of the catalog, making it coincidently a potential tool to determine whether or not a catalog has been properly declustered. Applying this tool to the Nepalese seismicity, we demonstrate annual variations of the seismicity rate of amplitude up to 40%, while no other periodicity appears. In particular, no variations of seismicity at any of the tidal periods are observed, indicating that the relative amplitude response of the seismicity at these periods is less than 18%.
Article
Studies of nonvolcanic tremor (NVT) have established the significant impact of small stress perturbations on NVT generation. Here we analyze the influence of the solid earth and ocean tides on a catalog of ˜550,000 low frequency earthquakes (LFEs) distributed along a 150 km section of the San Andreas Fault centered at Parkfield. LFE families are identified in the NVT data on the basis of waveform similarity and are thought to represent small, effectively co-located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km. We calculate the sensitivity of each of these 88 LFE families to the tidally induced right-lateral shear stress (RLSS), fault-normal stress (FNS), and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity. LFE occurrence is most strongly modulated by fluctuations in shear stress, with the majority of families demonstrating a correlation with RLSS at the 99% confidence level or above. Producing the observed LFE rate modulation in response to shear stress perturbations requires low effective stress in the LFE source region. There are substantial lateral and vertical variations in tidal shear stress sensitivity, which we interpret to reflect spatial variation in source region properties, such as friction and pore fluid pressure. Additionally, we find that highly episodic, shallow LFE families are generally less correlated with tidal stresses than their deeper, continuously active counterparts. The majority of families have weaker or insignificant correlation with positive (tensile) FNS. Two groups of families demonstrate a stronger correlation with fault-normal tension to the north and with compression to the south of Parkfield. The families that correlate with fault-normal clamping coincide with a releasing right bend in the surface fault trace and the LFE locations, suggesting that the San Andreas remains localized and contiguous down to near the base of the crust. The deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth. Synthesizing our observations with those of other LFE-hosting localities will help to develop a comprehensive understanding of transient fault slip below the "seismogenic zone" by providing constraints on parameters in physical models of slow slip and LFEs.
Article
A 16-station seismographic network, approximately 40 km north-south by 30 km east-west, was installed in the Coso Range, California, in September 1975 as part of a geological and geophysical assessment of the geothermal resource potential of range. During the first 2 years of network operations, 4216 local earthquakes (0.5?m?3.9) defined zones of seismicity that strike radially outward from a Pleistocene rhyolite field located near the center of the Coso Range. Most earthquakes were located in zones showing a general northwest trend across the range. Six earthquake swarms occurred within the area that includes the rhyolite field. Fault plane solutions show regional north-south compression: earthquakes located in northwest striking zones generally had right lateral strike slip focal mechanisms, those in northeast striking zones left lateral strike slip focal mechanisms, and those in north-south striking zones both normal and strike slip focal mechanisms. Earthquake depths showed little variation across the Coso Range; the depth distribution is similar to that of several carefully studied segments of the central San Andreas fault. The b value calculated for the entire range is 0.99+/-0.08. The rhyolite field has a significantly higher b value of 1.26+/-0.16 if only the shallow events (depth
Article
I report strong seismic PKP precursors recorded in the seismic stations in Tanzania for earthquakes occurred in the Fiji subduction zone. The observed PKP precursors show clear onsets and rapid variations of amplitude for different events and stations. I develop a technique to use the precursor onset times to determine the location of seismic scatterers and apply forward modeling of precursor amplitudes to place constraints on the magnitude of seismic anomalies. The travel time analysis indicates that these recorded PKP precursors are caused by the seismic scattering in the lowermost 360 km of the mantle beneath the Comoros hotspot. The amplitudes of the observed PKP precursors suggest a rather complex seismic structure with a P velocity variation of at least 8%, likely associated with partial melt.
Article
The second-order moment (cross-correlation function) of earthquakes in the U.S. Geological Survey central California catalog between 1969 and 1982 was calculated with respect to a magnitude threshold M>=4.0 over interevent distances up to 80 km and interevent times up to 320 days. The statistical procedure results in a representation of the spatial-temporal structure of the catalog associated with M>=4.0 earthquakes and is capable of revealing patterns too weak to be detected in the space-time distribution of seismicity for individual earthquake sequences. A method is introduced for identifying aftershocks based on a physical two-parameter model of the earthquake interaction process. The results show that the aftershock process dominates the second-order moment and may even obscure the statistical expression of a precursory process. A concentration of foreshocks within 15 km and 3 days of M>=4.0 main shocks exhibits an apparent migration toward the main shock loci with velocity 2.6-5.3 km/d. This concentration may be related to an observed tendency for M>=4.0 events to cluster (autocorrelate) over this interevent range. With the identified aftershocks removed, the residual catalog is Poissonion in space and time. When two M>=4.0 earthquakes occur within 80 km and 40 days of each other, aftershock productivity appears to be relatively enhanced in the earlier sequence. This suggests that aftershock populations are not solely dependent on their main shocks and that unusually productive aftershock sequences may be predictors of future moderate earthquakes.
Article
We cannot observe the magnetic field inside the earth's core directly, but there is likely to be a large toroidal part of 10-100 Gauss which, together with the dipole component, could produce a magnetic torque on the inner core that tends to rotate it. Estimates based on dynamo calculations give torques of 10 --N m which is large enough to accelerate the inner core to the westward drift velocity of 0.2 ø per year within a few days. Presumably some equilibrium has been reached in which the inner core rotates with constant angular velocity and experiences no net 'torque. This rotation should have significant consequences for dynamo calculations because it is a very effective method of stretching field lines, and its helps to drive differential rotation in the liquid outer core. The core is modeled by two solid spheres permeated by a uniform field of 5 Gauss representing the dipole. The inner sphere is free to lrtate relative to the outer sphere. When a torque of 10-N m is applied to the inner sphere it reaches equilibrium with a steady angular velocity corresponding to a rotation period of 2300 years, which is similar to the westward drift speed, and a toroidal field of about 100 Gauss is induced. The inner sphere can also undergo torøidal oscillations with a period of about 10 years which may be related to the observed secular variation.
Article
Variations in Earth's rotation (defined in terms of length of day) arise from external tidal torques, or from an exchange of angular momentum between the solid Earth and its fluid components. On short timescales (annual or shorter) the non-tidal component is dominated by the atmosphere, with small contributions from the ocean and hydrological system. On decadal timescales, the dominant contribution is from angular momentum exchange between the solid mantle and fluid outer core. Intradecadal periods have been less clear and have been characterized by signals with a wide range of periods and varying amplitudes, including a peak at about 6 years (refs 2, 3, 4). Here, by working in the time domain rather than the frequency domain, we show a clear partition of the non-atmospheric component into only three components: a decadally varying trend, a 5.9-year period oscillation, and jumps at times contemporaneous with geomagnetic jerks. The nature of the jumps in length of day leads to a fundamental change in what class of phenomena may give rise to the jerks, and provides a strong constraint on electrical conductivity of the lower mantle, which can in turn constrain its structure and composition.
Article
Well separated individual PKP precursors observed at the Yellowknife seismic array (YK) for a high quality doublet of earthquakes provide an opportunity to study the location of the corresponding scatterers and assess the stability of the location estimation. Based on the comparison of the waveforms of stacked individual precursors and those of PKIKP phases, we are able to determine that most of these precursors originate from scattering of the PKPbc (rather than the PKPab) branch above the B caustic on the receiver side. This allows a reliable location of the scatterers in the lower mantle. Their depths range from 2890 km (the CMB) to 2270 km, scattering angles range from 45.8° to 16.0°, and surface projections range from southern Ontario to northern Saskatchewan in Canada. These locations are associated with transitions from slow to fast velocities in mantle tomographic models and follow the expected general dip direction of fossil slabs under north America. This suggests that the subducted slab remnants under north America have retained their compositional signature. The fact that we can essentially treat these scatterers as reflections from plane boundaries suggests that the remnant fragments of slab may be spatially extended, which should be confirmed using broadband data. Average differences in measured slowness and back-azimuth of the doublet precursors are as small as 0.08 s/deg and 1.4 deg, respectively. Our study indicates that it may be possible to locate such scatterers using single earthquakes and small aperture seismic arrays.
Article
Both British work at A.W.R.E. and American work in the A.R.P.A. Vela Uniform Program have demonstrated that arrays of seismometers may be used to enhance the signal/noise ratio of distant seismic events over the single instrument signal/noise ratio. In addition, arrays may have the virtue of suppressing signal-generated noise arising in the crust below the receiver. Other Vela research has demonstrated that buried instruments frequently have a lower noise environment than surface instruments. Borehole emplacement may be particularly effective against wind-generated seismic noise. In an attempt to exploit these possible improvements to the maximum extent we are now constructing a large aperture seismological array of buried seismometers. This instrument will consist of over 500 seismometers distributed in 21 clusters over a 200 km aperture. It will exploit modern array signal-processing techniques. If it performs as expected it will have major implications for world-wide seismological detection systems. The status of and plans for the experimental array are described along with some of the world-wide system possibilities.
Article
We examine remotely triggered microearthquakes and tectonic tremor in central California following the 2010 Mw 8.8 Chile earthquake. Several microearthquakes near the Coso Geothermal Field were apparently triggered, with the largest earthquake (Ml 3.5) occurring during the large-amplitude Love surface waves. The Chile mainshock also triggered numerous tremor bursts near the Parkfield-Cholame section of the San Andreas Fault (SAF). The locally triggered tremor bursts are partially masked at lower frequencies by the regionally triggered earthquake signals from Coso, but can be identified by applying high-pass or matched filters. Both triggered tremor along the SAF and the Ml 3.5 earthquake in Coso are consistent with frictional failure at different depths on critically-stressed faults under the Coulomb failure criteria. The triggered tremor, however, appears to be more phase-correlated with the surface waves than the triggered earthquakes, likely reflecting differences in constitutive properties between the brittle, seismogenic crust and the underlying lower crust.