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Results for “Seismological Society of America”

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USGS1986

Crustal structure of Oaxaca, Mexico, from seismic refraction measurements

Seismic refraction and gravity data have been analyzed to obtain a model of the compressional-wave velocity structure of the ocean-to-continent transition in the State of Oaxaca in southwestern Mexico. Crustal thickness on the continent at the latitude 18°N is 45 ± 4 km, based on reflected phases from the Moho discontinuity. The crust has been modeled with three layers, with velocities of 4.3 to 4.6, 5.0 to 5.7, and 6.85 to 7.0 km/sec, each with positive velocity gradient. The crust thins to 10 km at the coast near Pinotepa Nacional, where Precambrian metamorphic rocks are exposed 45 km from the mid-America trench. Offshore, the oceanic structure consists of an 8-km-thick crust with a normal crustal velocity structure (Spudich and Orcutt, 1980). The apparent dip of the subducting plate beneath western Mexico is 10°. On the oceanic side, strong reflections suggest a minimum depth of 35 km for the lithosphere-asthenosphere boundary. The asthenosphere has a seismic velocity of 7.6 km/sec, and a thin lid in which the velocity is 8.6 km/sec.

Oaxaca↗

USGS2018

Seismic hazard, risk, and design for South America

We calculate seismic hazard, risk, and design criteria across South America using the latest data, models, and methods to support public officials, scientists, and engineers in earthquake risk mitigation efforts. Updated continental scale seismic hazard models are based on a new seismicity catalog, seismicity rate models, evaluation of earthquake sizes, fault geometry and rate parameters, and ground‐motion models. Resulting probabilistic seismic hazard maps show peak ground acceleration, modified Mercalli intensity, and spectral accelerations at 0.2 and 1 s periods for 2%, 10%, and 50% probabilities of exceedance in 50 yrs. Ground shaking soil amplification at each site is calculated by considering uniform soil that is applied in modern building codes or by applying site‐specific factors based on V S 30 "> V S 30 shear‐wave velocities determined through a simple topographic proxy technique. We use these hazard models in conjunction with the Prompt Assessment of Global Earthquakes for Response (PAGER) model to calculate economic and casualty risk. Risk is computed by incorporating the new hazard values amplified by soil, PAGER fragility/vulnerability equations, and LandScan 2012 estimates of population exposure. We also calculate building design values using the guidelines established in the building code provisions. Resulting hazard and associated risk is high along the northern and western coasts of South America, reaching damaging levels of ground shaking in Chile, western Argentina, western Bolivia, Peru, Ecuador, Colombia, Venezuela, and in localized areas distributed across the rest of the continent where historical earthquakes have occurred. Constructing buildings and other structures to account for strong shaking in these regions of high hazard and risk should mitigate losses and reduce casualties from effects of future earthquake strong ground shaking. National models should be developed by scientists and engineers in each country using the best available science.

Bulletin of the Seismological Society of America↗

USGS2003

2001 Bhuj, India, earthquake engineering seismoscope recordings and Eastern North America ground-motion attenuation relations

Engineering seismoscope data collected at distances less than 300 km for the M 7.7 Bhuj, India, mainshock are compatible with ground-motion attenuation in eastern North America (ENA). The mainshock ground-motion data have been corrected to a common geological site condition using the factors of Joyner and Boore (2000) and a classification scheme of Quaternary or Tertiary sediments or rock. We then compare these data to ENA ground-motion attenuation relations. Despite uncertainties in recording method, geological site corrections, common tectonic setting, and the amount of regional seismic attenuation, the corrected Bhuj dataset agrees with the collective predictions by ENA ground-motion attenuation relations within a factor of 2. This level of agreement is within the dataset uncertainties and the normal variance for recorded earthquake ground motions.

Bulletin of the Seismological Society of America↗

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↗

USGS1997

Site amplifications for generic rock sites

Seismic shear-wave velocity as a function of depth for generic rock sites has been estimated from borehole data and studies of crustal velocities, and these velocities have been used to compute frequency-dependent amplifications for zero attenuation for use in simulations of strong ground motion. We define a generic rock site as one whose velocity at shallow depths equals the average of those from the rock sites sampled by the borehole data. Most of the boreholes are in populated areas; for that reason, the rock sites sampled are of particular engineering significance. We consider two generic rock sites: rock, corresponding to the bulk of the borehole data, and very hard rock, such as is found in glaciated regions in large areas of eastern North America or in portions of western North America. The amplifications on rock sites can be in excess of 3.5 at high frequencies, in contrast to the amplifications of less than 1.2 on very hard rock sites. The consideration of unattenuated amplification alone is computationally convenient, but what matters for ground-motion estimation is the combined effect of amplification and attenuation. For reasonable values of the attenuation parameter κ 0 , the combined effect of attenuation and amplification for rock sites peaks between about 2 and 5 Hz with a maximum level of less than 1.8. The combined effect is about a factor of 1.5 at 1 Hz and is less than unity for frequencies in the range of 10 to 20 Hz (depending on κ 0 ).

Bulletin of the Seismological Society of America↗

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↗

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↗