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USGS2024

The 1886 Charleston, South Carolina, earthquake: Intensities and ground motions

The 1 September 1886 Charleston, South Carolina, earthquake was one of the largest preinstrumental earthquakes in eastern North America for which extensive contemporaneous observations were documented. The distribution of shaking was mapped shortly after the earthquake, and reconsidered by several authors in the late twentieth century, but has not been reconsidered with a modern appreciation for issues associated with macroseismic data interpretation. Detailed contemporary accounts have also never been used to map the distribution of numerical shaking intensities in the near field. In this study we reconsider macroseismic data from far‐field accounts as well as detailed accounts of damage in the near field, estimating modified Mercalli intensity values at 1297 locations including over 200 definite “not felt” reports that delineate the overall felt extent. We compare the results to the suite of ground‐motion models for eastern North America selected by the National Seismic Hazard Model, using a recently proposed mainshock rupture model and an average site condition for the locations at which intensities are estimated. The comparison supports the moment magnitude estimate, 7.3, from a recently proposed rupture model ( Bilham and Hough, 2023 ). A ShakeMap constrained by model predictions and estimated intensities further illustrates this consistency, which we show is insensitive to rupture model details. Given the uncertainty of calibration relations for magnitudes close to 7, the overall intensity distribution provides a good characterization of shaking but cannot improve the independent moment magnitude estimate. We also identify a previously unrecognized early large aftershock that occurred 9–10 min after the mainshock, for which we estimate magnitude ∼5.6.

South Carolina↗

USGS2015

Introduction to the special issue on the 2012 Haida Gwaii and 2013 Craig earthquakes at the Pacific–North America plate boundary (British Columbia and Alaska)

The 27 October 2012 M w 7.8 Haida Gwaii thrust earthquake and the 5 January 2013 M w 7.5 Craig strike‐slip earthquake are the focus of this special issue. They occurred along the transform boundary between the Pacific and North American plates (Fig. 1). The most identifiable feature of the plate boundary, the strike‐slip Queen Charlotte fault, might be viewed as typical of continent–ocean transform faults because it separates the continental crust of the North American plate from oceanic crust of the Pacific plate for most of its length. However, the current relative plate motion of about 5 cm/yr is highly oblique to the Queen Charlotte fault, causing a transpressive plate boundary in the region.

Alaska, British Columbia↗

USGS2007

Comparison of earthquake source spectra and attenuation in eastern North America and southeastern Australia

The paucity of ground-motion data in stable continental regions (SCRs) remains a key limitation when developing relations that seek to predict effects of strong ground shaking from large damaging earthquakes. It is desirable to combine data from more than one SCR to increase database size, but this raises questions as to whether the source and attenuation properties of the SCRs are equivalent. We compare recently compiled spectral-amplitude databases from small to moderate events (moment magnitudes, 2.0 ??? M ??? 5.0) in both southeastern Australia and eastern North America (ENA). Both are SCRs but are widely separated, spatially and in tectonic history. We statistically compare ground motions by plotting mean and standard deviations of spectral amplitudes for data grouped in magnitude and distance bins. These comparisons show that the source and attenuation properties of the two regions are very similar, in particular, at shorter hypocentral distances R (i.e., R < 70 km). At larger distances, regional attenuation differences are observed that may be attributed to differences in crustal structure. We conclude that it is valid to combine the Australian and ENA ground-motion datasets in the development of ground-motion prediction equations, with some limitations in frequency and distance ranges. These ground-motion relations may serve as generic functions for SCRs around the world.

Bulletin of the Seismological Society of America↗

USGS2013

Seismic structure of the crust and uppermost mantle of South America and surrounding oceanic basins

We present a new set of contour maps of the seismic structure of South America and the surrounding ocean basins. These maps include new data, helping to constrain crustal thickness, whole-crustal average P-wave and S-wave velocity, and the seismic velocity of the uppermost mantle ( P n and S n ). We find that: (1) The weighted average thickness of the crust under South America is 38.17 km (standard deviation, s.d. ±8.7 km), which is ∼1 km thinner than the global average of 39.2 km (s.d. ±8.5 km) for continental crust. (2) Histograms of whole-crustal P-wave velocities for the South American crust are bi-modal, with the lower peak occurring for crust that appears to be missing a high-velocity (6.9–7.3 km/s) lower crustal layer. (3) The average P-wave velocity of the crystalline crust ( P cc ) is 6.47 km/s (s.d. ±0.25 km/s). This is essentially identical to the global average of 6.45 km/s. (4) The average P n velocity beneath South America is 8.00 km/s (s.d. ±0.23 km/s), slightly lower than the global average of 8.07 km/s. (5) A region across northern Chile and northeast Argentina has anomalously low P- and S-wave velocities in the crust. Geographically, this corresponds to the shallowly-subducted portion of the Nazca plate (the Pampean flat slab first described by Isacks et al., 1968), which is also a region of crustal extension. (6) The thick crust of the Brazilian craton appears to extend into Venezuela and Colombia. (7) The crust in the Amazon basin and along the western edge of the Brazilian craton may be thinned by extension. (8) The average crustal P-wave velocity under the eastern Pacific seafloor is higher than under the western Atlantic seafloor, most likely due to the thicker sediment layer on the older Atlantic seafloor.

Bulletin of the Seismological Society of America↗

USGS2014

Why the New Madrid earthquakes are M 7–8 and the Charleston earthquake is ∼M 7

Estimates of magnitudes of large historical earthquakes are an essential input to and can seriously affect seismic‐hazard estimates. The earthquake‐intensity observations, modified Mercalli intensities (MMI), and assigned magnitudes M of the 1811–1812 New Madrid events have been reinterpreted several times in the last decade and have been a source of controversy in making seismic‐hazard estimates in the central United States. Observations support the concept that the larger the earthquake, the greater the maximum‐felt distance. For the same crustal attenuation and local soil conditions, magnitude should be the main influence on intensity values at large distances. We apply this concept by comparing the mean MMI at distances of 600–1200 km for each of the four largest New Madrid 1811–1812 earthquakes, the 1886 Charleston, South Carolina, earthquake, the 1929 M 7.2 Grand Banks earthquake, and the 2001 M 7.6 Bhuj, India, earthquake. We fit the intensity observations using the form MMI= A + C ×dist−0.8×log(dist) to better define intensity attenuation in eastern North America (ENA). The intensity attenuation in cratonic India differs from ENA and is corrected to ENA using both the above estimate and published intensity relations. We evaluate source, marine geophysical, Q , and stress‐drop information, as well as a 1929 Milne–Shaw record at Chicago to confirm that the 1929 Grand Banks earthquake occurred in ENA crust. Our direct comparison of mean intensities beyond 600 km suggests M 7.5, 7.3, 7.7, and 6.9 for the three New Madrid 1811–1812 mainshocks and the largest aftershock and M 7.0 for the 1886 Charleston, South Carolina, earthquake, with an estimated uncertainty of 0.3 units at the 95% confidence level (based on a Monte Carlo analysis). Our mean New Madrid and Charleston mainshock magnitudes are similar to those of Bakun and Hopper (2004) and are much higher than those of Hough and Page (2011) for New Madrid.

Bulletin of the Seismological Society of America↗

USGS2003

Estimating locations and magnitudes of earthquakes in eastern North America from Modified Mercalli intensities

We use 28 calibration events (3.7 < or = M < or = 7.3) from Texas to the Grand Banks, Newfoundland, to develop a Modified Mercalli intensity (MMI) model and associated site corrections for estimating source parameters of historical earthquakes in eastern North America. The model, MMI = 1.41 + 1.68 XM - 0.00345X Delta - 2.08log (Delta), where Delta is the distance in kilometers from the epicenter and M is moment magnitude, provides unbiased estimates of M and its uncertainty, and, if site corrections are used, of source location. The model can be used for the analysis of historical earthquakes with only a few MMI assignments. We use this model, MMI site corrections, and Bakun and Wentworth's (1997 technique to estimate M and the epicenter for three important historical earthquakes. The intensity magnitude M1 is 6.1 for the 18 November 1755 earthquake near Cape Ann, Massachusetts; 6.0 for the 5 January 1843 earthquake near Marked Tree, Arkansas; and 6.0 for the 31 October 1895 earthquake. The 1895 event probably occurred in southern Illinois, about 100 km north of the site of significant ground failure effects near Charleston, Missouri.

Bulletin of the Seismological Society of America↗

USGS2016

Adjusting central and eastern North America ground-motion intensity measures between sites with different reference-rock site conditions

Adjustment factors are provided for converting ground‐motion intensity measures between central and eastern North America (CENA) sites with different reference‐rock site conditions ( V S 30 =760, 2000, and 3000 m/s) for moment magnitudes ranging from 2 to 8, rupture distances ranging from 2 to 1200 km, Fourier amplitude spectra (FAS) for frequencies ranging from 0.01 to 100 Hz, response spectra for periods ranging from 0.01 to 10.0 s, peak ground acceleration, and peak ground velocity. The adjustment factors are given for a wide range of the site diminution parameters ( κ 0 ) for sites with V S 30 =760 m/s and for a κ 0 of 0.006 s for two harder rock sites. Fourteen CENA velocity profiles with V S 30 values within a factor of 1.1 of 760 m/s were used to derive average FAS amplification factors as a function of frequency, which were then used in simulations of peak ground‐motion parameters and response spectra to derive the adjustment factors. The amplification function differs from that used in western North America (e.g., Campbell and Boore, 2016 ) in having a peak near 9 Hz, due to the resonance of motions in the relatively thin low‐velocity material over hard rock that characterizes many CENA sites with V S 30 near 760 m/s. We call these B/C sites, because this velocity marks the boundary between National Earthquake Hazards Reduction Program site classes B and C ( Building Seismic Safety Council, 2004 ). The adjustments for short‐period motions are sensitive to the value of κ 0 , but there are very few if any determinations of κ 0 for CENA B/C sites. For this reason, we determined κ 0 from multiple recordings at Pinyon Flat Observatory (PFO), California, which has a velocity‐depth profile similar to those of CENA B/C sites. The PFO and other results from the literature suggest that appropriate values of κ 0 for CENA B/C sites are expected to lie between 0.01 and 0.03 s.

Bulletin of the Seismological Society of America↗

USGS2006

MMI attenuation and historical earthquakes in the basin and range province of western North America

Earthquakes in central Nevada (1932-1959) were used to develop a modified Mercalli intensity (MMI) attenuation model for estimating moment magnitude M for earthquakes in the Basin and Range province of interior western North America. M is 7.4-7.5 for the 26 March 1872 Owens Valley, California, earthquake, in agreement with Beanland and Clark's (1994) M 7.6 that was estimated from geologic field observations. M is 7.5 for the 3 May 1887 Sonora, Mexico, earthquake, in agreement with Natali and Sbar's (1982) M 7.4 and Suter's (2006) M 7.5, both estimated from geologic field observations. MMI at sites in California for earthquakes in the Nevada Basin and Range apparently are not much affected by the Sierra Nevada except at sites near the Sierra Nevada where MMI is reduced. This reduction in MMI is consistent with a shadow zone produced by the root of the Sierra Nevada. In contrast, MMI assignments for earthquakes located in the eastern Sierra Nevada near the west margin of the Basin and Range are greater than predicted at sites in California. These higher MMI values may result from critical reflections due to layering near the base of the Sierra Nevada.

Bulletin of the Seismological Society of America↗

USGS2018

Why aftershock duration matters for probabilistic seismic hazard assessment

Most hazard assessments assume that high background seismicity rates indicate a higher probability of large shocks and, therefore, of strong shaking. However, in slowly deforming regions, such as eastern North America, Australia, and inner Honshu, this assumption breaks down if the seismicity clusters are instead aftershocks of historic and prehistoric mainshocks. Here, therefore we probe the circumstances under which aftershocks can last for 100–1000 years. Basham and Adams (1983) and Ebel et al. (2000) proposed that intraplate seismicity in eastern North America could be aftershocks of mainshocks that struck hundreds of years beforehand, a view consonant with rate–state friction (Dieterich, 1994), in which aftershock duration varies inversely with fault‐stressing rate. To test these hypotheses, we estimate aftershock durations of the 2011 Mw 9 Tohoku‐Oki rupture at 12 sites up to 250 km from the source, as well as for the near‐fault aftershocks of eight large Japanese mainshocks, sampling faults slipping 0.01 to 80 mm/yr . Whereas aftershock productivity increases with mainshock magnitude, we find that aftershock duration, the time until the aftershock rate decays to the premainshock rate, does not. Instead, aftershock sequences lasted a month on the fastest‐slipping faults and are projected to persist for more than 2000 years on the slowest. Thus, long aftershock sequences can misguide and inflate hazard assessments in intraplate regions if misinterpreted as background seismicity, whereas areas between seismicity clusters may instead harbor a higher chance of large mainshocks, the opposite of what is being assumed today.

Bulletin of the Seismological Society of America↗

USGS2013

The attenuation of Fourier amplitudes for rock sites in eastern North America

We develop an empirical model of the decay of Fourier amplitudes for earthquakes of M 3–6 recorded on rock sites in eastern North America and discuss its implications for source parameters. Attenuation at distances from 10 to 500 km may be adequately described using a bilinear model with a geometric spreading of 1/R 1.3 to a transition distance of 50 km, with a geometric spreading of 1/R 0.5 at greater distances. For low frequencies and distances less than 50 km, the effective geometric spreading given by the model is perturbed using a frequency‐ and hypocentral depth‐dependent factor defined in such a way as to increase amplitudes at lower frequencies near the epicenter but leave the 1 km source amplitudes unchanged. The associated anelastic attenuation is determined for each event, with an average value being given by a regional quality factor of Q=525f 0.45 . This model provides a match, on average, between the known seismic moment of events and the inferred low‐frequency spectral amplitudes at R=1 km (obtained by correcting for the attenuation model). The inferred Brune stress parameters from the high‐frequency source terms are about 600 bars (60 MPa), on average, for events of M>4.5.

Bulletin of the Seismological Society of America↗

USGS1994

Wide-band analysis of the 3 March 1985 central Chile earthquake: Overall source process and rupture history

We apply a linear, finite-fault waveform inversion scheme to the near-source strong-motion records, the teleseismic body waves, and the long-period Rayleigh waves recorded for the 3 March 1985 Chile earthquake to recover the mainshock rupture history. The data contain periods between about 2 and 350 sec and are inverted by allowing a variable dislocation rise time at each point on the fault. The results indicate that the mainshock had a seismic moment of 1.5 × 10 28 dyne-cm ( M w 8.0) and ruptured mainly updip and to the south of the hypocenter for a distance of about 150 km along the Nazca-South America plate boundary. A smaller northward component of propagation is also evident, giving a total rupture length of about 200 km. The total source duration of the mainshock is 70 sec, with the majority of the slip occurring within the first 40 sec in a broad 100-km-wide zone in the northern half of the rupture area. Slip in this region extends from a depth of 55 km to within about 10 km of the surface and contains two areas of maximum slip (2.3 and 2.9 m) with rise times of approximately 14 sec. Slip in the southern portion of the fault reaches lower peak values (1.8 m) and extends downdip to depths no greater than 30 km. An independent variable rise-time inversion of the teleseismic body waves alone yields similar results, indicating that a significant component of slow fault motion is not required for this earthquake. The mainshock was preceded by several smaller precursors, the largest of which is an M w ∼ 6.6 thrust earthquake occurring at a depth of 22 km in the shallow 15° dipping portion of the plate interface.

Bulletin of the Seismological Society of America↗

USGS2025

Bayesian ETAS modeling for the Pacific Northwest: Uncovering effects of tectonic regimes, regional differences, and swarms on aftershock parameters

The Pacific Northwest (PNW) of North America has high seismic hazard due to numerous earthquake sources under populated areas. It hosts several tectonic regimes and subregional seismic zones that are hypothesized to have different patterns of earthquake and aftershock occurrence. It is also predisposed to earthquake swarms, which can complicate the statistical modeling of these patterns. We present the first statistical seismicity model of the PNW catalog using the epidemic‐type aftershock sequence (ETAS) framework. We develop a Bayesian inference procedure that provides a stable estimation of both ETAS parameters and their uncertainties for different sets of PNW earthquakes, even those with very sparse catalogs. The Bayesian approach allows us to investigate how parameter estimates change between the intraslab and crustal tectonic regimes, the northern and southern PNW, and when swarms are included and excluded from the catalog. We also utilize our Bayesian framework to calculate parameter estimates under different prior beliefs about PNW seismicity, as well as to propagate catalog measurement errors into ETAS parameter estimates. We discuss the implications of parameter differences across the region for aftershock forecasting for the PNW.

Pacific Northwest↗

USGS2011

Regional spectral analysis of three moderate earthquakes in Northeastern North America

We analyze Fourier spectra obtained from the horizontal components of broadband and accelerogram data from the 1997 Cap-Rouge, the 2002 Ausable Forks, and the 2005 Rivière-du-Loup earthquakes, recorded by Canadian and American stations sited on rock at hypocentral distances from 23 to 602 km. We check the recorded spectra closely for anomalies that might result from site resonance or source effects. We use Beresnev and Atkinson’s (1997) near-surface velocity structures and Boore and Joyner’s (1997) quarter-wave method to estimate site response at hard- and soft-rock sites. We revise the Street et al. (1975) model for geometrical spreading, adopting a crossover distance of r o =50 km instead of 100 km. We obtain an average attenuation of Q =410±25f 0.50±0.03 for S+L g +surface waves with ray paths in the Appalachian and southeastern Grenville Provinces. We correct the recorded spectra for attenuation and site response to estimate source spectral shape and radiated energy for these three earthquakes and the 1988 M 5.8 Saguenay earthquake. The Brune stress drops range from 130 to 419 bars, and the apparent stresses range from 39 to 63 bars. The corrected source spectral shapes of these earthquakes are somewhat variable for frequencies from 0.2 to 2 Hz, falling slightly below the fitted Brune spectra.

Bulletin of the Seismological Society of America↗

USGS2006

An assessment of the impact of the 2003 EPRI ground-motion prediction models on the USGS national seismic-hazard maps

Ground-motion attenuation relations have an important impact on seismic hazard analyses. Ground-motion modeling is particularly sensitive to assumptions about wave-propagation attenuation (crustal Q and geometrical spreading), as well as source and site conditions. Studies of path attenuation from earthquakes in eastern North America (ENA) provide insights into the appropriateness of specific attenuation relations. An Electric Power Research Institute (EPRI) (2003, 2004) study combines published ENA ground-motion attenuation relations into four model forms: single-corner, double-corner, hybrid-empirical, and finite-fault. When substituted in the U.S. Geological Survey 2002 national seismic hazard maps for the five ENA relations originally used in those hazard calculations, the EPRI (2003) relations predict similar ground motions and hazard at short periods (<0.5 sec) and significantly lower ground motions (20%-50%) at longer periods (>0.5 sec), relative to the 2002 national maps. A major reason for this difference is due to the crustal seismic-wave attenuation model assumed in a few of the ENA relations combined into the EPRI (2003, 2004) models. Although appropriate differences in geometrical spreading models among ENA relations can also be significant, a few ENA relations have 1-Hz Q-values (Q0) that are below the EPRI (1993) consensus range for Q0 when coupled with a geometrical spreading of R-0.5. The EPRI (2003, 2004) single-corner relation is strongly influenced by the inclusion of ENA relations with assumed Q0 below the EPRI (1993) range, which explains much of the discrepancy in predictions at longer periods.

Bulletin of the Seismological Society of America↗

USGS2016

Evaluation of six NEHRP B/C crustal amplification models proposed for use in western North America

We evaluate six crustal amplification models based on National Earthquake Hazards Reduction Program (NEHRP) B/C crustal profiles proposed for use in western North America (WNA) and often used in other active crustal regions where crustal properties are unknown. One of the models is based on an interpolation of generic rock velocity profiles previously proposed for WNA and central and eastern North America (CENA), in conjunction with material densities based on an updated velocity–density relationship. A second model is based on the velocity profile used to develop amplification factors for the Next Generation Attenuation (NGA)‐West2 project. A third model is based on a near‐surface velocity profile developed from the NGA‐West2 site database. A fourth model is based on velocity and density profiles originally proposed for use in CENA but recently used to represent crustal properties in California. We propose two alternatives to this latter model that more closely represent WNA crustal properties. We adopt a value of site attenuation ( κ 0 ) for each model that is either recommended by the author of the model or proposed by us. Stochastic simulation is used to evaluate the Fourier amplification factors and their impact on response spectra associated with each model. Based on this evaluation, we conclude that among the available models evaluated in this study the NEHRP B/C amplification model of Boore (2016) best represents median crustal amplification in WNA, although the amplification models based on the crustal profiles of Kamai et al. (2013 , 2016, unpublished manuscript, see Data and Resources ) and Yenier and Atkinson (2015) , the latter adjusted to WNA crustal properties, can be used to represent epistemic uncertainty.

Bulletin of the Seismological Society of America↗

USGS2011

Modifications to existing ground-motion prediction equations in light of new data

We compare our recent ground-motion prediction equations (GMPEs) for western North America (WNA; Boore and Atkinson, 2008 [BA08]) and eastern North America (ENA; Atkinson and Boore, 2006 [AB06]; Atkinson, 2008 [A08]) to newly available ground-motion data. Based on these comparisons, we suggest revisions to our GMPEs for both WNA and ENA. The revisions for WNA affect only those events with M ≤ 5:75, while those for ENA affect all magnitudes. These are simple modi[1]fications to the existing GMPEs that bring them into significantly better agreement with data. The wealth of new data clearly demonstrates that these modifications are warranted; we therefore recommend the use of the updated equations for seismic hazard analyses and other applications. More detailed studies are under way by many investigators (including ourselves) to develop a new generation of ground-motion models in both WNA and ENA from scratch, through a comprehensive reevaluation of source, path, site, and modeling issues. In time, those more complete models will replace those proposed in this study. However, as the new models will be several years in development, we recommend using the modified models proposed herein, labeled BA08′ (for WNA), AB06′ (for ENA), and A08′ (for ENA, to replace A08), as interim updates to our existing models. The proposed models are in demonstrable agreement with a rich database of ground motions for moderate-magnitude earthquakes in both WNA and ENA and are constrained at larger magnitudes by the BA08 magnitude and distance scaling.

Bulletin of the Seismological Society of America↗

USGS2001

Comparisons of ground motions from the 1999 Chi-Chi, earthquake with empirical predictions largely based on data from California

This article has the modest goal of comparing the ground motions recorded during the 1999 Chi-Chi, Taiwan, mainshock with predictions from four empirical-based equations commonly used for western North America; these empirical predictions are largely based on data from California. Comparisons are made for peak acceleration and 5%-damped response spectra at periods between 0.1 and 4 sec. The general finding is that the Chi-Chi ground motions are smaller than those predicted from the empirically based equations for periods less than about 1 sec by factors averaging about 0.4 but as small as 0.26 (depending on period, on which equation is used, and on whether the sites are assumed to be rock or soil). There is a trend for the observed motions to approach or even exceed the predicted motions for longer periods. Motions at similar distances (30-60 km) to the east and to the west of the fault differ dramatically at periods between about 2 and 20 sec: Long-duration wave trains are present on the motions to the west, and when normalized to similar amplitudes at short periods, the response spectra of the motions at the western stations are as much as five times larger than those of motions from eastern stations. The explanation for the difference is probably related to site and propagation effects; the western stations are on the Coastal Plain, whereas the eastern stations are at the foot of young and steep mountains, either in the relatively narrow Longitudinal Valley or along the eastern coast-the sediments underlying the eastern stations are probably shallower and have higher velocity than those under the western stations.

Bulletin of the Seismological Society of America↗

USGS2004

Remotely triggered seismicity on the United States west coast following the Mw 7.9 Denali fault earthquake

The M w 7.9 Denali fault earthquake in central Alaska of 3 November 2002 triggered earthquakes across western North America at epicentral distances of up to at least 3660 km. We describe the spatial and temporal development of triggered activity in California and the Pacific Northwest, focusing on Mount Rainier, the Geysers geothermal field, the Long Valley caldera, and the Coso geothermal field. The onset of triggered seismicity at each of these areas began during the Love and Raleigh waves of the M w 7.9 wave train, which had dominant periods of 15 to 40 sec, indicating that earthquakes were triggered locally by dynamic stress changes due to low-frequency surface wave arrivals. Swarms during the wave train continued for ∼4 min (Mount Rainier) to ∼40 min (the Geysers) after the surface wave arrivals and were characterized by spasmodic bursts of small ( M ≤ 2.5) earthquakes. Dynamic stresses within the surface wave train at the time of the first triggered earthquakes ranged from 0.01 MPa (Coso) to 0.09 MPa (Mount Rainier). In addition to the swarms that began during the surface wave arrivals, Long Valley caldera and Mount Rainier experienced unusually large seismic swarms hours to days after the Denali fault earthquake. These swarms seem to represent a delayed response to the Denali fault earthquake. The occurrence of spatially and temporally distinct swarms of triggered seismicity at the same site suggests that earthquakes may be triggered by more than one physical process.

North America↗