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The 1983 Goodnow earthquake in the central Adirondacks New York: rupture of a simple, circular crack
Abstract
On 7 October 1983 a magnitude 5.1 (mb) earthquake occurred in the central Adirondack Mountains, near the town of Goodnow, New York. The results of an analysis show that the earthquake was due to reverse faulting with a centroidal depth of 7.5 km, striking north-south and dipping at 60° to the west. The scalar seismic moment is 1.9 × 1023 dyn cm. For P waves, for paths from northeastern United States, plausible values of t* (attenuation) range from 0.4 to 0.7 sec, and for these values the estimates of source duration range from 0.60 to 0.35 sec. Assuming a circular crack model with a rupture velocity of 3.0 km/sec, the bounds on source duration give upper and lower limits for the fault radius of 0.9 and 0.5 km, and for the stress drop of 670 and 115 bars. The preferred value of t* = 0.6 sec yields a source duration of 0.45 sec, a radius of 0.7 km, and a stress drop of 265 bars. -from Authors
... Two relatively large earthquakes with determined focal mechanisms have occurred in the Adirondack dome region since 1980 ( Fig. 1): the 2002 M w 5.0 Au Sable Forks, New York, event (Seeber et al., 2002) and the 1983 M w 4.7 Goodnow, New York, event (Seeber and Armbruster, 1986;Nabelek and Suarez, 1989;Seeber et al., 2002). The hypocenter of the Au Sable Forks event was 11 km deep, and the Goodnow event was 8 km deep. ...
... Seeber et al., 2002) had a pure thrust focal mechanism with nodal planes that strike just east of N, parallel to nearby faults, the easternmost of which links southward to the Saratoga-McGregor and associated faults ( Fig. 1; Seeber et al., 2002). The M b 5.1 Goodnow, New York, earthquake of 7 October 1983 (Nabelek and Suarez, 1989;Seeber et al., 2002) had a thrust focal mechanism with northerly striking nodal plane orientations (Fig. 1). Although many of the prominent lineaments and proposed faults trend NNE in the Adirondack dome, N-trending lineaments (and assumed faults) occur in the immediate region of the Goodnow event (Seeber et al., 2002). ...
Five earthquake swarms occurred from 2007 to 2011 near Berne, New York. Each swarm consisted of four to twenty-four earthquakes ranging from M 1.0 to M 3.1. The network determinations of the focal depths ranged from 6 km to 24 km, 77% of which were ≥14 km. High-precision, relative location analysis showed that the events in the 2009 and 2011 swarms delineate NNE-SSW orientations, collinear with NNE trends established by the distribution of the spatially distinct swarms; the events in the 2010 swarm aligned WNW-ESE. Focal mechanisms determined from the largest event in the swarms include one nodal plane that strikes NNE, collinear with the distribution of the swarms and relative events within the swarms.
Two, possibly related explanations exist for the Berne earthquake swarms. (1) The swarms were caused by reactivations of proposed blind NE- and NW-striking rift structures associated with the NE-trending Scranton gravity high. These rift structures, of uncertain age (Proterozoic or Neoproterozoic/Iapetan opening), have been modeled at depths appropriate for the seismicity. (2) The NNE-trending swarms were caused by reactivations of NNE-striking faults mapped at the surface north-northeast of the earthquake swarms. Both models involve reactivation of rift-related faults, and the development of the NNE-striking surficial faults in the second model probably was guided by the blind rift faults in the first model. The Berne swarms may be evidence that these faults are seismically capable and, if so, could sustain a maximum event on the order of Mw 5.7–6.6, based on fault segment length.
... All events in the WQSZ have a reverse-sense focal mechanism, although some spatial variability is apparent. For example, earthquakes north of $ 45°have a NE-SW oriented P axis, whereas two events in the Adirondack region in the southeastern part of the zone have E-W oriented P axis [Nabelek and Suarez, 1989;Seeber et al., 2002;Ma and Adams, 2002]. Furthermore, as noted by Kim et al. [2006], events in Lake Ontario and south of the Great Lakes are mainly of strike-slip character but have a very similar P axis orientation to reverse-sense focal mechanisms in the WQSZ. ...
... The b value and associated uncertainties were calculated using the maximum-likelihood method of Aki [1965], yielding values in the range 0.56- Figure 7. Selected focal mechanism solutions for earthquakes in the WQSZ and surrounding region. The solutions are mainly for moderate and submoderate earthquakes; they were selected from the following sources: Dineva et al. [2007], Du et al. [2003], Herrmann [1978], Horner et al. [1978], Kim et al. [2006], Ma et al. [2002], and Nabelek and Suarez [1989]. The focal mechanism parameters used to generate the image are listed in Table 3. ...
The western Quebec seismic zone (WQSZ) is a 160-km-wide band of intraplate seismicity extending 500 km from the Adirondack Highlands (United States) to the Laurentian uplands (Canada). Previous authors have proposed that the WQSZ is localized over the Mesozoic track of the Great Meteor hot spot. Here we explore this hypothesis further by investigating regional seismicity characteristics. Focal mechanisms for WQSZ earthquakes, including a new mechanism for a moderate (mN 4.5) earthquake, reveal a pattern of reverse-sense faulting with SW trending P axes changing to E-W in the southern part of the zone. We introduce a simple box-counting method to delineate spatial clusters, based on exceedance of random seismicity density. Combining this approach with focal depths from regional depth phase analysis, we find that seismicity with shallow focus (0-7 km) is characterized by a random spatial distribution, whereas earthquakes with an intermediate focal depth (8-18 km) are strongly clustered along a diffuse linear band trending N50°W. Earthquakes deeper than 18 km are confined to a few distinct clusters. These clusters are characterized by differing b values and, for at least one cluster, repeating events. Projection of hypocenters onto a deep seismic profile and comparison with preexisting crustal structures suggest that local reactivation of Precambrian structural features may have occurred; however, the Great Meteor hot spot track remains the only compelling explanation for the overall distribution of earthquakes. Proximity of seismicity clusters to historic and prehistoric earthquakes lends support to the hypothesis that modern seismicity may represent exceptionally long-lived aftershocks of large past events.
... In contrast, in the northeastern United States, strike-slip faulting predominates (Herrmann 1979;Hasegawa, Adams & Yamazaki 1985;Talwani & Rajendran 1991;Zoback 1992). Interspersed are regions where thrust faulting has been documented (Herrmann 1979;Nabelek & Suarez 1989). ...
... Indiana), seismic and faulting activity is predicted to have occurred later, at around 7 Ka BP, but the mode of failure is again predicted to be thrust faulting. Examples of pure thrust-fault earthquakes are the Mw5.4 southern Illinois earthquake of 1968 (Herrmann 1979) and the ~1~5 . 1 Goodnow earthquake in New York State (Nabelek & Suarez 1989). Moreover, in Fig. 4 the prediction that dFSM would go negative at Indiana about 7-8 Ka BP coincides with the time frame of a very large (Mw7.5) ...
In order to understand the causal relation between postglacial rebound and earthquakes, a realistic ice and water load model is used to (1) calculate stresses induced in the lithosphere and mantle by glacial loading, melting and postglacial rebound and (2) evaluate the effect of glacial loading/rebound on the failure potential for earthquakes in the upper crust. The dependence of both the failure potential and the actual mode of failure on the ambient tectonic stress magnitude, the overburden stress, and lithospheric properties are investigated. Prominent features of this analysis are the inclusion of (1) a viscoelastic mantle and thus the migration of stress, and (2) the ambient tectonic stress and overburden stress contributions in the calculation of the total stress field.
The spatio-temporal calculations, by a finite-element technique, of upper-crustal stresses and the failure potential for earthquakes indicate that fault stability is invariably enhanced directly beneath the load. For the case where stresses induced by the overburden are such that the horizontal component (Sh) is greater than or equal to the vertical component (Sv) (ζ≥ 1, where ζ= Sh/Sv), the model predicts the onset of thrust faulting and maximum earthquake activities soon after deglaciation is complete (when rebound rates are at a maximum). Observational data support this prediction. Since that time, rebound stresses have been decreasing in magnitude, but they continue to act as a trigger mechanism for optimally oriented pre-existing faults that are otherwise on the verge of failure. If one limits the existence of such faults to lie within the pre-weakened zones of eastern Canada, then the spatial distribution of current earthquakes can also be explained.
Perturbations to the magnitude of the tectonic stress components or lithospheric properties do not affect, to any significant extent, the above conclusions.
... This last mechanism may have application in some intraplate regions where the upper crust is acting as a flawed stress guide, especially for faults that are unfavourably oriented or severely misoriented for reactivation. Possible examples include the 1982 mb 5.0-5.7 Miramichi and 1983 mb 5.1 Goodnow intraplate events which occurred on steep reverse faults (Wetmiller et al., 1984;Nabelek and Suarez, 1989. Provided Byerlee-type friction values are applicable, lithostatic fluid pressure levels are a necessary prelude to failure on such severely misoriented structures (Sibson, 1990b). ...
... Earthquake recurrence is usually impacted by tectonic shear stress and shear strength of faults (Miller et al., 1996;Shimazaki and Makata, 1980). In quantitative analysis, fault failure is agreed with the episodes of fluid pressure, especially for the reactivation of unfavorably oriented fault in intraplate regions (Miller et al., 2004;Nabelek and Suarez, 1989;Wetmiller et al., 1984). The alternation of fluid pressure is thought to be synchronized with earthquake process (Sibson, 1992). ...
The present study aims to reveal the recovering period of the postseismic fluid pressure in fault zone, offering an insight into earthquake recurrence. Numerical modeling is performed based on a 2D simple layered fault-valve model to simulate the fluid activities within the earthquake fault. In order to demonstrate the features of postseismic fluid pressure in natural state, the interference of tectonic movements is not considered. The recovering period of postseismic fluid pressure includes a suddenchanging period and a much longer fluctuating period. Modeling results show that fault permeability and porosity are sensitive parameters and reversely proportional to the recovering period of the fluid pressure in earthquake fault zone. When the permeability reduces from 10-15 to 10-18 m2, the recovering period increases from 400 to 2 000 yrs, correspondently. The upper and lower fluid pressures are separated by the valve seal, causing their fluctuations in opposite tendencies.
... These estimates agree well with values for earthquakes in the CSZ obtained by other researchers using different methods (Hasegawa and Wetmiller, 1980;Boatwright, 1994;Atkinson and Somerville, 1994). The stress drops estimated in this and other studies of earthquakes in northeastern North America show an apparent wide variation (Hasegawa and Wetmiller, 1980;Choy et al., 1983;Nabelek, 1984;Ebel et al., 1986;Somerville et al., 1987;Nabelek and Suarez, 1989;Bent, 1992;Attdnson and Somerville, 1994;Boatwright, 1994). Moreover, the results of this study suggest an apparent scaling of stress drop to earthquake size. ...
Two earthquake doublets and two multiplets recorded by the Charlevoix Telemetered Network (CLTN) in the Charlevoix Seismic Zone (CSZ) of southern Quebec, Canada, have been analyzed using an empirical Green's function (EGF) method to derive the relative source time functions (RSTF's) of seven master events with MbLg = 1.2 to 4.4. We identified the doublets and multiplets using a waveform cross-correlation and relative event location technique to verify that each earthquake pair had similar focal mechanisms and hypocentral locations. Three-component S waveforms recorded by the high dynamic range (126 dB) instrumentation of the CLTN were used to extract the RSTF's.
The RSTF's reveal that six of the seven events are simple with single-source pulses having durations of 0.05 to 0.2 sec. Another earthquake (920310-0545, M 3.3) appears to be a double event with two episodes of rupturing. Azimuthal variations of the RSTF pulse amplitudes and widths provide strong evidence for the rupture directivities of five of the earthquakes (M = 1.2 to 4.4). The azimuthal variations in the RSTF pulse amplitudes were used to estimate the rupture directions and rupture velocities. Lower-bound estimates of the rupture velocity range from 0.5 to 0.7 Vs. Estimates of the rupture direction were combined with P-wave focal mechanisms for the four largest events (M 3.3 to 4.4) to identify the fault plane for these earthquakes. Source parameters were measured for the RSTF's of the master events, including seismic moments of 3.5 × 1018 to 5.3 × 1021 dyne-cm, fault radii of 100 to 330 m, and static stress drops of 2 to 90 bars. The fault radii and stress-drop estimates for M > 3 events agree well with estimates obtained by other researchers for M ∼ 3 to 4.5 earthquakes in the CSZ. We also observed apparent scaling between the stress drop and the earthquake size, which has been reported in other studies of stress drop in northeastern North America.
... The aftershock shows a weak initial P wave compared to S, and the S waves have much higher frequency content than P across a broad frequency range (about 1 to 25 Hz) at most of the stations (Fig. 8a). Though strong S waves may be partly due to the thrust mechanism for the aftershock (Nabelek and Suarez, 1989), which generates strong S and weak P waves (see, e.g., Kim, 1987;Lilwall, 1988), most of the earthquakes in the region show S waves with higher-frequency content than P. ...
We analyze the high-frequency (1 to 50 Hz) spectra of chemical explosions and earthquakes at local and regional distances in the northeastern United States and in Norway to understand the seismic signal characteristics of single explosions, multiple-hole instantaneous explosions, ripple-fired quarry blasts, and earthquakes. Our purpose is to evaluate practical discriminants, and to obtain a physical understanding of their successes and failures. High-frequency spectra from tipple-fired blasts usually show clear time-independent frequency bands due to the repetitive nature of the source and are distinctively different from the spectra of instantaneous blasts or earthquakes. However, like other discriminators based on spectral estimates, the spectrogram method requires data with high signal-to-noise ratios at high frequencies for unambiguous discrimination. In addition, banding is not seen in spectrograms for shots with small delay times (less than 8 msec) and short total durations. We have successfully modeled the observed high-frequency spectral bands up to about 45 Hz of the regional signals from quarry blasts in New York and adjacent states. Using information on shot-hole patterns and charge distribution, we find that ripple firing results in an enrichment of high-frequency S waves and efficient excitation of the Rg phase. There is an azimuthal dependence of P-wave amplitude associated with orientation of the path with respect to local topography (ridges, benches) in which the shots are emplaced. To discriminate instantaneous explosions from earthquakes, we find the P/S spectral amplitude ratio at high frequencies is complementary to the use of spec-trogram methods. A high P/S spectral ratio above 10 Hz is a stable character-istic of instantaneous explosions.
... [5] Published studies of earthquake source parameters for ENA, using a variety of methods and data, reveal contradictory results. In comparison with earthquakes in the western United States (WUS), average stress drops for ENA have been found to be anomalously high by some authors [e.g., Nuttli, 1983;Nábělek and Suárez, 1989;Hough and Seeber, 1991] and consistent with WUS by others [e.g., Somerville et al., 1987;Hanks and Johnston, 1992;Li et al., 1995]. This may in part be related to the relatively small number of earthquakes available for study in ENA (a few tens of M4-6 earthquakes compared with hundreds in WUS). ...
We find invariant high stress drops and radiated energies for a sequence of intraplate earthquakes. We estimate source parameters of the Au Sable Forks, NY, earthquake (M5, 2002) and aftershocks. This intraplate earthquake was the largest to occur in Eastern North America since 1988 and the largest to be recorded by regional broadband networks. We use the empirical Green's function (EGF) method and define a set of qualitative and quantitative rules for the selection of EGF earthquake pairs and for the quality verification of the obtained EGF spectral ratio. We use a multitaper code that performs the complex spectral division with minimum frequency leakage and allows transformation back to the time domain to check the validity of the EGF event. We estimate source parameters for 22 earthquakes (M1-M5) in the sequence. The median stress drop of 104 MPa (using Madariaga source model, and 19 MPa for a Brune model) is significantly larger than estimates for interplate earthquakes. The lower crustal strain rates, and longer fault healing times in intraplate environments, may be responsible for this high average value. We find constant stress drop between M2 and M5, up to the bandwidth resolution limit (80 Hz) of the study, and no evidence of stress drop breakdown for M2 to M1 earthquakes. We find consistently high radiated seismic energy and apparent stress, and a median radiated energy to seismic moment ratio of 9 × 10-5. This is significantly larger than estimates for interplate earthquakes (˜2 × 10-5) and consistent with higher stress drops and stronger faults.
... For sites within the ice margin, this is consistent with most of the observations in eastern Canada, except in Baffi n Island and Baffi n Bay. For sites outside the ice margin, pure thrust-fault earthquakes have been observed in the northeastern United States (Herrmann, 1979;Nabelek and Suarez, 1989); however, the current predominant mode of failure is strike slip. Since post glacial rebound stresses diminish rapidly south of the Wisconsin ice margin (Wu and Johnston, 2000), earthquakes in the northeastern United States are dominated by tectonic stresses rather than postglacial rebound stresses. ...
At postglacial rebound time scales, the intraplate continental lithosphere typically behaves as an elastic solid. However, under exceptional conditions, the effective viscosity of the lower crust and lithospheric mantle may be as low as ∼1020 Pa s, leading to ductile behavior at postglacial rebound time scales. We studied the effects of a lithospheric ductile zone on postglacial rebound-induced seismicity and deformation in eastern Canada and the northeastern United States using three types of models: (1) a reference model with no lithospheric ductile layer; (2) a model with a uniform, 25-km-thick, ductile layer embedded in the middle of the lithospheric column; and (3) a model with a dike-like vertical ductile zone, extending from mid-crust level down to the bottom of the lithosphere, along the Precambrian rift structure of the St. Lawrence Valley. Based on geothermal and rock physics data, the viscosity of the ductile zone is set to either 1020 or 1021 Pa s. We found that a narrow ductile zone cutting vertically through the lithosphere has larger effects than the uniformly thick horizontal ductile layer. Effects of a lithospheric weak zone on uplift rates may be large enough to be detected by global positioning system (GPS) measurements, especially for low viscosities. While the effect on fault stability is also large, the impact on the onset time of instability is small for sites within the ice margin. The impact on the onset time is more significant for sites outside the ice margin. Effects of a lithospheric weak zone are also significant on present-day horizontal velocities and strain rates and are at the limit of resolution for GPS measurements.
... Recognized areas of seismicity include the Lake Ontario-Niagara-Attica seismic zone and the Lake Erie seismic zone . Events in these zones are predominantly strike-slip : Bent, 1996a: Bent, , 1996bBent et al., 2003;Dineva et al., 2004;Du et al., 2003;Hashizume and Tange, 1977;Herrmann, 1978;Kim et al., 2006;Nabelek and Suarez, 1989;Nicholson et al., 1988;www.Ideo.columbia in character. Despite their variability in depth and focal mechanism, the earthquake mechanisms throughout the region are in general indicative of horizontal compression with subhorizontal P axes, striking northeast (Du et al., 2003). ...
On 20 October 2005 at 21:16 UTC, a moderate earthquake (m(N) 4.3) occurred in an area of low seismicity within Georgian Bay, approximately 12 km north of Thombury, Ontario (44.67 degrees N, 80.46 degrees W). Despite its moderate magnitude, it was exceptionally well recorded and is of particular interest because of its location 90 km from a proposed long-term storage facility for low- and medium-level nuclear waste. No damage was reported, but ground shaking was felt to a distance of 100 km. Within 24 hours after the mainshock, four portable seismograph systems were installed in the epicentral region. In total, eight events were recorded over a 4-day period, including a foreshock and six aftershocks. The unusually rich dataset from this moderate earthquake sequence enabled robust determination of hypocentral parameters, including well-constrained focal depths for most events. For the mainshock, we estimated a seismic moment of M-0 4.5 x 10(14) N m and comer frequency of 3.7 Hz, based on a spectral fit using Brune's source model. Least-squares waveform inversion of P and S phases yielded a double-couple focal mechanism with a reversesense of slip and northwest-striking nodal planes. The reverse mechanism and midcrustal focal depths (10-12 km) are characteristic, in general, of more abundant seismicity located similar to 200 km northeast of this event in the western Quebec seismic zone. These parameters differ, however, from shallow (2-6 km) earthquakes, with predominantly strike-slip mechanisms, observed near Lake Erie similar to 200 km to the south. We attribute this north-south change in rupture mechanism to variations in crustal stress induced by postglacial isostatic rebound. Aeromagnetic data in and around the epicentral region reveal prominent northwest-striking lineations caused by Precambrian mafic dykes. Under midcrustal conditions, the dyke material is mechanically stronger than generally more felsic upper-crustal host rocks. We propose that where large dykes are favorably oriented with respect to the stress field, they may strongly influence the locations of intraplate earthquake rupture in Shield regions.
... When rebound stress and fault stability are calculated for Indiana Wu and Hasegawa 1996b , a pulse of thrust fault reactivation is predicted coinciding with the time frame of a very large M7.5 Wabash Valley Indiana paleoearthquake 3 8 :5 N , 8 7 W discovered and dated by paleo-liquefaction research Obermeier et al. 1991 to have occurred 8-1 ka ago. Although pure thrust-fault earthquakes have been observed in northeastern U.S. Herrmann 1979; Nabelek and Suarez 1989 , the current predominant mode of failure there is strike-slip since rebound stress diminishes rapidly south of the Wisconsinan ice margin, so that many earthquakes in northeastern U.S. are dominated by tectonic stress rather than rebound stress. ...
A spherical, self-gravitating viscoelastic earth model is used to
calculate the spatial and temporal evolution of the glacially induced
lithospheric stress and fault stability in North America. The predicted
onset time of a pulse of earthquake activities, the mode of failure and
the magnitude of instability are investigated for three sites with
different distance from the former ice margin. It is found that glacial
unloading is able to trigger paleo-earthquakes within the ice margin
near Charlevoix (47.5°N, 70.1°W) and in Wabash Valley
(38.5°N, 87°W) outside the ice margin. However, rebound stress
decays away from the former ice margin, thus glacial unloading is
unlikely to have triggered the large M8 earthquakes in New Madrid
(36.6°N, 89.5°W).
... The mode of failure is predicted to be thrust faulting. Although pure thrust-fault earthquakes have been observed in northeastern U.S. (Herrmann 1979;Nabelek & Suarez 1989), the current predominant mode of failure there is strike-slip since rebound stress deminishes rapidly south of the Wisconsinan ice margin, so that earthquakes in northeastern U.S. are dominated by tectonic stress rather than rebound stress. ...
Knowledge of whether earthquake activity will increase in the next few thousand years is important for the planning of nuclear waste repositories. Assuming that fault instability portends earthquake activity, the rate of change in fault instability for the next few thousand years in eastern Canada is computed for two viscosity models. It is shown that a uniform-viscosity (1 x 1021 Pa ·s) mantle predicts decreasing fault instability. However, a high-viscosity (1 x 1023 Pa ·s) lower mantle predicts a significant increase in fault instability, with an overall rate of -0.06 MPa/ka. Due to the lack of consensus on lower mantle viscosity, the case for increasing earthquake activity is definitely a possibility, so more study on mantle rheology, ice deglaciation history, and intraplate earthquakes in the planning of nuclear waste repositories is needed.
... The mode of failure is predicted to be thrust faulting. Examples of pure thrust-fault earthquakes are the M w 5.4 southern Illinois earthquake of 1968(Herrmann 1979) and the mb 5.1 Goodnow earthquake in New York State(Nabelek & Suarez 1989).For later comparison, it should be noted that the compressive stress within the ice margin generally decreases in amplitude I Similar toFig. 8except for Model L2. ...
Previous investigations of the causal relationship between postglacial rebound and earthquakes in eastern Canada have focused on the mode of failure and the observed timing of the pulse of earthquake/faulting activity following deglaciation. In this study, the observational database has been extended to include observed orientations of the contemporary stress field and the rotation of stress since deglacial times. It is shown that many of these observations can be explained by a realistic ice history and a viscoelastic earth with a uniform 1021 Pa s mantle.
The effects of viscosity structure on the above predictions are also examined. It is shown that, since most of the above observations are found within the ice margin, they are not very sensitive to lithospheric thickness. Also, the inclusion of a 25 or 50 km ductile layer within the lithosphere will not decouple the seismogenic upper crust. High viscosity (1022 Pa s) in the lower mantle is rejected by the stress orientation and rotation observations. A low-viscosity (6 times 1020Pa s) upper mantle with 1.6 times 1021 Pa s in the upper part of the lower mantle and 3 times 1021 Pa s in the lower part of the lower mantle below 1200 km depth has been found to give predictions that are in general agreement with the observations.
Compressional inversion involves reverse-slip reactivation (strike slip) of normal faults inherited from earlier crustal extension during crustal shortening. Examples of seismically active inversion provinces with damaging earthquakes M < 7:8 range from active island arc systems to formerly rifted cratonic crust. Inversion structures are characterized by a distinctive structural-stratigraphic signature shown by seismic reflection profiling to be quite widespread. Compressional inversion is largely responsible for an anomalous group of seismically active reverse faults dipping at 45°-60°, distinct from dominant Andersonian thrust dips of 30° 5°. Time-averaged slip rates on such structures are generally slow (about 1 mm=yr, or less) and total reverse displacement is often limited. Reactivation mechanics does much to explain the observed dip distribution for reverse-slip ruptures, suggesting first that low-displacement faults are characterized overall by Byerlee friction (μ s ∼ 0:6), and second that high fluid overpressures are needed for reactivation of moderate-steep reverse faults. Support for the latter comes from hydrothermal veining apparently produced by fault-valve action on reverse faults exhumed from seismo-genic depths, and from distinctive geophysical anomalies in the midcrust of areas undergoing active inversion. Nucleation of earthquake ruptures on inversion structures appears to be fluid driven (H 2 O, CO 2 , etc.), with failure triggered by locally rising fluid overpressure rather than by increasing differential stress alone. Evaluation of seismic hazard from inversion structures is problematic because of their slow slip rate and long recurrence intervals, and also because their surface expression is structurally complex and often obscured. Structural-stratigraphic complexity is compounded by competition between inversion structures and younger, more optimally oriented thrust faults, and by subsidiary strike-slip accompanying inversion.
Seismotectonics is the synthesis of earthquake,
geophysical, geodetic and geological data to deduce the
tectonic framework of a region [Scholz, 1990]. This
approach has been applied successfully to active tectonic
regions such as plate boundaries, regions of intraplate
seismicity, and active volcanoes throughout the world.
Using seismic moment (M0)-length (L) data for stable continental region (SCR) faults, augmented by data not included in previous studies, this article reassesses and confirms the bilinear scaling relation for the dip-slip faults of Leonard (2010). Furthermore, using the new data, I propose a separate bilinear relation for estimating magnitude from surface rupture length. There are now a small number of SCR strike-slip earthquakes for which the fault dimensions have been estimated. I use these to constrain a trilinear scaling relation for this class of fault. For the four fault types (interplate dip slip, interplate strike slip, SCR/intraplate dip slip, and SCR/intraplate strike slip), the scaling relations for the 10 permutations of M0, D, L, W, and A are given. © 2014, Bulletin of the Seismological Society of America. All rights reserved.
A worldwide compilation of well-constrained fault ruptures and focal depths of earthquakes reveals that the Earth's crust in many stable continental regions (SCRs) is characterized by a bimodal depth distribution with a very shallow upper crustal component. The distributions can vary in a) depth of the modes and b) strength of bimodality, probably due to intracrustal boundaries, differences in frictional and rheological properties, heat-flow densities, strain-rates, and tectonic forces or forces stemming from the surface. Overall, SCR ruptures and SCR earthquakes are confined within the upper third (0-10 km) and/or the lower third of the crust (20-35 km), while the midcrust (10-20 km) tends to be aseismic. Historical data indicate that some SCRs show very well-developed bimodal distributions of focal depths (e.g., North Alpine foreland basin in Europe, Kachchh basin in India), while others show weak to no developed bimodal distributions (e.g., Charlevoix seismic zone, New Madrid seismic zone in the central United States). Moreover, many large SCR earthquakes (Mw 4.5-8.0) nucleate on reverse faults and close to the surface (< 5 km). Almost 80% of the seismic moment density of shallow SCR ruptures is released in the uppermost 7 km of the crust. However, focal depths of instrumentally recorded major SCR earthquakes (3.5 < mb < 6.2) and their aftershocks in the northeastern United States and adjacent Canada, for example, suggest systematic over-estimates of hypocentral depths of 88 ± 30% (standard mean ± standard mean error), probably due to sparse instrumental coverage. If error estimates for shallow SCR earthquakes, in particular, are of systematic and not of statistical origin, preconceived assumptions of focal depths within the midcrust might have region-specific implications for understanding SCR seismogenesis and for earthquake hazard estimations (e.g., ground motion).
To be unbiased and uniform across a wide geographical area, seismic hazard assessments based primarily on earthquake recurrence
rates require that the same magnitude scale be used for all earthquakes evaluated. Increasingly, moment magnitude, MW, is seen as the magnitude of preference. Moment magnitude, however, was not routinely calculated in the past for earthquakes
in Canada, necessitating the conversion from other magnitude types in common use. This step is complicated by the fact that
several magnitude scales are routinely reported for Canadian earthquakes with the choice being influenced primarily by geography
and to a lesser extent by the size of the earthquake. This paper focuses on eastern Canada, where mN is the most commonly used magnitude scale. Conversions to MW are established and evaluated. The simple conversion of applying a constant is sufficient. However, the conversion is time
dependent with the constant changing from 0.41 to 0.53 in the mid-1990s.
This Part I study, in conjunction with Part II, develops a method to determine, within specified uncertainly bounds, the seismic moment, and thus moment magnitude, of all earthquakes of stable continental regions (SCR) for which instrumental or intensity data exist. Its basis is polynomial regression analysis using a database of SCR earthquakes with direct seismic moment determination. The independent variables include modern teleseismic magnitudes and regional magnitudes (Part I), and isoseismal areas or number of recording stations (Part II). Part III is an application of the methodology of Parts I and II to several major historical earthquakes. All data used in the regressions are assigned individual uncertainties estimated from the literature or from experience; formal confidence limits (68 per cent or 95 per cent) on both the regression formulas and the predicted seismic moment values are then possible via error propagation analysis. The most complete development is for the teleseismic magnitudes Ms and mb. For both, the final regression for log(Mo) is a quadratic formula that closely emulates the relationship between amplitude magnitudes and Mo expected from dislocation theory and source-scaling arguments. Regressions are also derived for the regional magnitudes mLg and ML, because there are many SCR events, mainly pre-1964, that have no teleseismic magnitudes. Prediction uncertainties from teleseismic magnitudes in moment magnitude units are in the ± 0.18. 0.28 range, and from regional magnitudes in the ±0.23–0.38 range over a wide magnitude band. Finally, the methodology developed here is generic, even though the database is specific. Application to plate-boundary, oceanic intraplate, or active continental intraplate regions should be straightforward.
A mb(Lg) 4.6 mainshock on January 16, 1994, in the Cacoosing valley, 10 km west of Reading, southeastern Pennsylvania caused modified Mercalli VI-VII intensities and the highest earthquake damage (~$2 million) in the eastern United States since 1944. Aftershock hypocenters from a temporary local seismic network are confined to the upper 2.5 km of the crust. They are clustered around the periphery of a tabular 3×3km zone that is interpreted to outline the mainshock rupture. This zone matches the nodal plane with reverse and left-lateral slip (strike 135°, dip 54° southwest, and rake 55°) of a focal mechanism obtained from aftershock first motions and from mainshock waveforms. This rupture does not correlate with any of the faults mapped in the epicentral area, but it is parallel to the most prominent fracture set, including joints and small faults. Maximum possible strike-slip accumulated on the causative fault is no more than several tenths of meters. A large carbonate rock quarry is centered above the rupture. We calculate a small, but significant (0.13 MPa) Coulomb stress increase caused by the quarry on the shallow portion of the rupture. Most of this increase was caused by pore pressure rise after the quarry was abandoned in December 1992 and flooded. Seismicity started may 1993. We conclude that the 1993-1997 Cacoosing Valley sequence is probably triggered by the quarry. About 200 km northeast of Cacoosing, another quarry in early Paleozoic carbonate rocks triggered the 1974 mb(Lg) 3.0 Wappingers Falls earthquake.
Relative source time functions (RSTF) have been estimated for four underground nuclear explosions and seven earthquakes in Central Asia using broadband P waveforms of nearby smaller events as empirical Green's functions (EGF). RSTFs of the four explosions (m(sub b) = 5.3 to 6.5) are each dominated by a simple pulse with a source duration of 0.4 to 0.8 s. RSTFs for two of the explosions show a significant secondary pulse with a pulse width similar to that of the first pulse. We conclude that the secondary phases are most likely associated with the spall slapdown phenomenon. Seismic moment releases by the spall phases are less than one third of those by the first explosion pulses. Elastic radii of the explosions are estimated to be about 0.25 to 0.5 km, and stress drops of the explosions range from 13 to 52 MPa. In contrast, RSTFs of earthquakes studied (m(sub b) = 5.5 to 6.6) typically comprise multiple source pulses with a total source duration from a few to several tens of seconds, indicating that the complex source process involves a fault dimension of several tens of kilometers. Stress drops of the earthquakes are much smaller than those of the nuclear explosions, ranging from 0.5 to 3.5 MPa. Our study demonstrates the power of the EGF method for retrieving RSTFs and reveals that differences in RSTFs and source parameters can be used to distinguish large nuclear explosions from moderate to large earthquakes (m(sub b) greater than or equal 5.5).
In this paper, two different methods to solve scattering problems in acoustic or elastic
media are coupled to enhance their usefulness. The multiple multipole (MMP) expansions
are used to solve for the scattered fields in homogeneous regions which are possibly
unbounded. The finite element (FE) method is used to calculate the scattered fields
in heterogeneous but bounded scatterers. As the MMP method requires, the different
regions and methods are coupled together in the least squares sense. For some examples,
the scattered fields are calculated and compared to the analytical solutions. Finally, the seismograms are calculated for a scattering problem with several scatterers, and complex geometries. Thus, the hybrid MMP-FEM technique is a very general and useful
tool to solve complex, two-dimensional scattering problems.
Focal mechanisms of 32 North American midplate earthquakes (mb=3.8-6.5) were evaluated to determine if slip is compatible with a broad-scale regional stress field derived from plate-driving forces and, if so, under what conditions. Using independent information on in situ stress orientations from well bore breakout and hydraulic fracturing data and assuming that the regional principal stresses are in approximately horizontal and vertical planes (±10°), the constraint that the slip vector represents the direction of maximum resolved shear stress on the fault plane was used to calculate relative stress magnitudes from the fault/stress geometry. The analysis confirmed a roughly north to south contrast in stress regime between the central eastern United States and southeastern Canada previously inferred from a contrast in focal mechanisms between the two areas: most central eastern United States earthquakes occur in response to a strike-slip stress regime, whereas the southeastern Canadian events require a thrust faulting stress regime. -from Author
Determines the source parameters for 14 earthquakes at Makran including the great (M w8.1) earthquake of 1945; determines the loci of seismic and aseismic slip along the plate boundary, and assesses the effects of the large forearc and accretionary wedge on the style of plate boundary slip. Earthquake source parameters are estimates. The earthquake of 1945 is shown to be an interplate thrust event. Nine smaller events in eastern Makran that are also located at or close to the plate interface have thrust mechanisms similar to that of the 1945 shock. Seaward of these thrust earthquakes lies the shallowest 70-80km of the plate boundary; this segment and the overlying accretionary wedge remain aseismic both during and between great earthquakes. The existence of thrust earthquakes indicates that either the sediments along the plate boundary in eastern Makran become sufficiently well consolidated and dewatered about 70km from the deformation front or older, lithified rocks are present within the forearc so that stick-slip sliding behavior becomes possible. This study shows that a large quantity of unconsolidated sediment does not necessarily indicate a low potential for great thrust earthquakes. -from Authors
Ground-motion models based on the Brune point-source approximation have an underlying ω2 spectrum, with a single corner frequency. These models over-predict observed spectral amplitudes at low to intermediate frequencies (∼0.1 to 2 Hz), for earthquakes with moment magnitudes M of 4 or greater. The empirical spectra of moderate to large events tend to sag at these frequencies, relative to the level suggested by the Brune point-source model.
A model that accounts for the finite extent of the fault plane correctly describes the observed spectral shapes. The model represents seismic radiation as a sum of contributions from several subfaults. Each subfault may be represented as a point source, and each subevent has an ω2 spectrum. When contributions to ground motion at an observation point are summed over all subfaults, the resulting spectral shape has two corner frequencies and more closely matches observed spectra. The more realistic spectral shape obtained through finite-fault modeling reflects the underlying reality that the radiation from real faults is formed by ruptures of their smaller parts, whose corner frequencies are higher than those implied by the full fault dimension. The two corners appear naturally as a result of subevent summation.
We use the stochastic finite-fault methodology to simulate the recorded ground-motion data from all significant earthquakes in eastern North America (ENA). These data include eight events of M > 4 recorded on modern digital instruments (regional seismographs and strong-motion instruments), and three historical events of M 5.8 to 7.3 recorded on analog instruments. The goodness of fit of synthetics to the data is defined as simulation bias, which is indicated by the difference between the logarithms of the observed and the simulated spectrum, averaged over all recordings of an earthquake. The finite-fault simulations provide an unbiased fit to the observational database over a broad frequency range (0.1 to 50 Hz), for all events.
A surprising conclusion of these simulations is that the subfault size that best fits the observed spectral shape increases linearly with moment magnitude, in an apparently deterministic manner. This strongly suggests that the subfault size can be unambiguously defined by the magnitude of the simulated earthquake. In this case, the radiation-strength factor(s), which is proportional to the square root of the high-frequency Fourier acceleration level, remains the only free parameter of the model. Its value is related to the maximum slip velocity on the fault. The strength factors for all modeled ENA events are within the range of 1.0 to 1.6, with the exception of the Saguenay mainshock (s = 2.2). This suggests a remarkable uniformity in earthquake slip processes.
We analysed aftershocks recorded by a temporary digital seismic network following the moderate Mw = 5.5 1993, Scotts Mills, Oregon, earthquake. A technique to retrieve source moment tensors from local waveforms was developed, tested, and applied to 41 small earthquakes (Mw ranging from 1.6 to 3.2). The derived focal mechanisms, although well resolved, are highly variable and do not share a common nodal plane. In contrast, the majority of the events, relocated with a joint hypocentre determination algorithm, collapse to a well-focused plane. The incompatibility of the nodal planes of most events with the plane defined by their locations suggests that the aftershocks did not occur on the fault plane, but tightly around it, outlining the rupture area rather than defining it. Furthermore, the moment tensors reveal stable P-axes, whereas T -axes plunges are highly dispersed. We detect a rotation of average T -axis plunge with depth, indicating a change from shallower, predominantly dip-slip mechanisms to deeper strike-slip mechanisms. These characteristics are difficult to explain by remnant stress concentrations on the main-shock rupture plane or asperity- and barrier-type models. We suggest that the aftershocks occurred under the ambient regional stress, triggered by a sudden weakening of the region surrounding the main-shock slip, rather than from a shear stress increase due to the main shock.
Data sets of mb(Pn) and mb(Lg) measurements are presented for three continental regions in order to investigate scaling relationships with moment magnitude W
, and event discrimination at small magnitudes. Compilations of published measurements are provided for eastern North American and central Asian earthquakes, and new measurements are reported for earthquakes located in western United States. Statistical tests on W
,:mb relationships show that the mb(Lg) scale of Nurru (1973) is transportable between tectonic regions, and a single, unified W
,:mb(Lg) relationship satisfies observations for M 6.5 in all regions. A unified relationship is also developed for nuclear explosions detonated at the Nevada Test Site and test sites of the former Soviet Union. Regional mb for explosions scale at higher rates than for earthquakes, and of significance is the finding that mb(Pn) for explosions scales at a higher rate than mb(Lg). A model is proposed where differences in scaling rates are related to effects of spectral overshoot and near-field Rg scattering on the generation of Pn and Lg waves by explosions. For earthquakes, mb(Pn) and mb(Lg) scale similarly, showing rates near 1.0 or 2/3 • log1oMo (seismic moment).
W
,:mb(Lg) scaling results are converted to unified Ms:mb(Lg) relationships using scaling laws between log Mo and Ms. For earthquakes with Ms greater than 3.0, the scaling rate is 0.69 • Ms, which is the same as it is for nuclear explosions if M., is proportional to 1.12 • log Mo, as determined by NTS observations. Thus, earthquake and explosion populations are parallel and separated by 0.68 mb units for large events. For small events (Ms < 3.0), populations may converge or diverge depending on the tectonic region in which earthquakes occur and the scaling rate of explosions at small yields. Earthquakes scale as 0.64 and 0.75 on M:mb(Lg) plots for stable and tectonic regions, respectively. While the scaling rate for explosions is —0.69, this value is uncertain due to paucity of Mo observations at small yields. Measurements of [mb(P) — mb(Lg)] for earthquakes in the western United States have an average value of —0.33 ±.03 mb units, in good agreement with Nuttli’s estimate of mb bias for NTS. This result suggests that Nuttli’s method for estimating test site bias can be extended to earthquakes to make estimates of bias on regional scales. In addition, a new approach for quick assessments of regional bias is proposed where Ms:mb(P) observations are compared with MM:mb(Lg) relationships. Catalog Ms:mb(P) data suggest that mb bias is significant for tectonic regions of southern Asia, averaging about —0.4 mb units.
The midcontinent United States between the Appalachian and Rocky Mountains contains 40 known faults or other potentially tectonic features for which published geologic information shows or suggests Quaternary tectonic faulting. We report results of a systematic evaluation of published and other publicly available geologic evidence of Quaternary faulting. These results benefit seismic-hazard assessments by (1) providing some constraints on the recurrence intervals and magnitudes of large, prehistoric earthquakes, and (2) identifying features that warrant additional study.For some features, suggested Quaternary tectonic faulting has been disproved, whereas, for others, the suggested faulting remains questionable. Of the 40 features, nine have clear geologic evidence of Quaternary tectonic faulting associated with prehistoric earthquakes, and another six features have evidence of nontectonic origins. An additional 12 faults, uplifts, or historical seismic zones lack reported paleoseismological evidence of large, Quaternary earthquakes. The remaining 13 features require further paleoseismological study to determine if they have had Quaternary earthquakes that were larger than any known from local historical records; seven of these 13 features are in or near urbanized areas where their study could affect urban hazard estimates. These seven are: (1) the belt of normal faults that rings the Gulf of Mexico from Florida to Texas, (2) the Northeast Ohio seismic zone, (3) the Valmont and (4) Goodpasture faults of Colorado, (5) the Champlain lowlands normal faults of New York State and Vermont, and (6) the Lexington and (7) Kentucky River fault systems of eastern Kentucky.
Displacement contour diagrams constructed using seismic reflection data and coal-mine plans are analysed to establish the factors determining the dimensions, shapes and displacement patterns of normal faults. For blind isolated normal faults in layered sequences the average aspect ratio is 2.15, with sub-horizontal major axes. Earthquake slip-surface aspect ratios range from 0.5 to 3.5 and are independent of slip orientation. The principal control on the shape of blind isolated faults is mechanical anisotropy associated with rock layering, resulting in layer-parallel elongation of fault surface ellipses. Faults that intersect the free surface and/or interact with nearby faults have aspect ratios ranging from 0.5 to 8.4, and are referred to as restricted. Restriction of fault growth has various effects including: (i) reduced curvature of the tip-line and of displacement contours; and (ii) increased displacement gradients in the restricted region. Many faults are restricted at more than one place on their tip-line loop and so have highly irregular shapes and displacement patterns. Subsequent linkage of interacting faults produces combined faults with aspect ratios within the normal range for unrestricted faults. Lateral interaction between faults does not necessarily lead to a change in the power-law exponent of the fault population.
Sibson, R.H., 1992. Implications of fault-valve behaviour for rupture nucleation and recurrence. In: T. Mikumo, K. Aki, M. Ohnaka, L.J. Ruff and P.K.P. Spudich (Editors), Earthquake Source Physics and Earthquake Precursors. Tectonophyslcs, 211: 283–293.Earthquakes in the shallow crust are generally thought to arise from the frictional instability of existing faults under shear stress. In simple recurrence models, failure on such faults is assumed to occur when tectonic shear stress (τ) rises to some constant critical level. However, frictional strength (τf) may also vary substantially through the interseismic period, in which case recurrence intervals between successive earthquakes are related to the time-dependence of τf as well as τ . In part, the manner in which τf changes with τ depends on the coupling between normal stress and shear stress on the fault, and is related to the mode of faulting and the manner of fault loading. Time-dependence of cohesive strength and static friction may also play a role. However, fluid pressure cycling as a consequence of fault-valve behaviour (where the fault transects a suprahydrostatic gradient in fluid pressure) may give rise to very large variations in fault strength. Geological evidence suggests that value-action may be especially important in the lower regions of the seismogenic zone, where large ruptures tend to nucleate, the largest fluid pressure fluctuations being associated with faults that remain active as a consequence of fluid overpressure though severely misoriented for reactivation in the prevailing stress field. Extensive hydrofracture dilatancy is likely to develop prefailure in the vicinity of such faults. Changes in frictional strength over the interseismic period as a result of fault-valve activity (Δτf) may greatly exceed the shear stress drop at failure (1 < Δτ < 10 MPa, typically). In such circumstances, recurrence intervals between successive events can be highly variable. A rich variety of recurrence behaviour becomes possible, depending on the relative magnitudes of Δτf and Δτ, and the coupling between shear stress accumulation and changing fluid pressure levels through the interseismic period.
The St. Elias, Alaska earthquake of 28 February, 1979 (Ms 7.2) is reanalyzed using broadband teleseismic body waves and long-period surface waves because of unresolved questions about its depth, focal mechanism, seismic moment, and location in a seismic gap. Teleseismic waveforms are simultaneously inverted to determine the source mechanism, seismic moment, rupture history and centroid depth. These data are well modeled with a point source propagating in the ESE direction with an average kinematic rupture velocity of 2.5 km/s. The best-fitting source mechanism indicates underthrusting on a NE-dipping plane. The mainshock depth of 24 km and the depth of aftershocks determined from inversions are consistent with locations on the gently dipping main thrust of the Pacific-North American plate boundary. -from Authors
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