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Results for “Bulletin Seismological Society America”

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At least 433 records · Page 24Linked to original sources
USGS2015

Determining generic velocity and density models for crustal amplification calculations, with an update of the Boore and Joyner (1997) Generic Site Amplification for Graphic Site Amplification

This short note contains two contributions related to deriving depth‐dependent velocity and density models for use in computing generic crustal amplifications. The first contribution is a method for interpolating two velocity profiles to obtain a third profile with a time‐averaged velocity to depth Z that is equal to a specified value (e.g., for shear‐wave velocity V S , for Z =30  m, in which the subscript S has been added to indicate that the average is for shear‐wave velocities). The second contribution is a procedure for obtaining densities from V S . The first contribution is used to extend and revise the Boore and Joyner (1997) generic rock V S model, for which , to a model with the more common . This new model is then used with the densities from the second contribution to compute crustal amplifications for a generic site with .

Bulletin of the Seismological Society of America↗
USGS2016

Site effects in Port-au-Prince (Haiti) from the analysis of spectral ratio and numerical simulations.

To provide better insight into seismic ground motion in the Port‐au‐Prince metropolitan area, we investigate site effects at 12 seismological stations by analyzing 78 earthquakes with magnitude smaller than 5 that occurred between 2010 and 2013. Horizontal‐to‐vertical spectral ratio on earthquake recordings and a standard spectral ratio were applied to the seismic data. We also propose a simplified lithostratigraphic map and use available geotechnical and geophysical data to construct representative soil columns in the vicinity of each station that allow us to compute numerical transfer functions using 1D simulations. At most of the studied sites, spectral ratios are characterized by weak‐motion amplification at frequencies above 5 Hz, in good agreement with the numerical transfer functions. A mismatch between the observed amplifications and simulated response at lower frequencies shows that the considered soil columns could be missing a deeper velocity contrast. Furthermore, strong amplification between 2 and 10 Hz linked to local topographic features is found at one station located in the south of the city, and substantial amplification below 5 Hz is detected near the coastline, which we attribute to deep and soft sediments as well as the presence of surface waves. We conclude that for most investigated sites in Port‐au‐Prince, seismic amplifications due to site effects are highly variable but seem not to be important at high frequencies. At some specific locations, however, they could strongly enhance the low‐frequency content of the seismic ground shaking. Although our analysis does not consider nonlinear effects, we thus conclude that, apart from sites close to the coast, sediment‐induced amplification probably had only a minor impact on the level of strong ground motion, and was not the main reason for the high level of damage in Port‐au‐Prince.

Port-au-Prince↗
USGS2016

Seismic envelope-based detection and location of ground-coupled airwaves from volcanoes in Alaska

Volcanic explosions and other infrasonic sources frequently produce acoustic waves that are recorded by seismometers. Here we explore multiple techniques to detect, locate, and characterize ground‐coupled airwaves (GCA) on volcano seismic networks in Alaska. GCA waveforms are typically incoherent between stations, thus we use envelope‐based techniques in our analyses. For distant sources and planar waves, we use f ‐ k beamforming to estimate back azimuth and trace velocity parameters. For spherical waves originating within the network, we use two related time difference of arrival (TDOA) methods to detect and localize the source. We investigate a modified envelope function to enhance the signal‐to‐noise ratio and emphasize both high energies and energy contrasts within a spectrogram. We apply these methods to recent eruptions from Cleveland, Veniaminof, and Pavlof Volcanoes, Alaska. Array processing of GCA from Cleveland Volcano on 4 May 2013 produces robust detection and wave characterization. Our modified envelopes substantially improve the short‐term average/long‐term average ratios, enhancing explosion detection. We detect GCA within both the Veniaminof and Pavlof networks from the 2007 and 2013–2014 activity, indicating repeated volcanic explosions. Event clustering and forward modeling suggests that high‐resolution localization is possible for GCA on typical volcano seismic networks. These results indicate that GCA can be used to help detect, locate, characterize, and monitor volcanic eruptions, particularly in difficult‐to‐monitor regions. We have implemented these GCA detection algorithms into our operational volcano‐monitoring algorithms at the Alaska Volcano Observatory.

Alaska↗
USGS2016

Developments in new fluid rotational seismometers: Instrument performance and future directions

In this article we describe prototype designs and tests for low-cost rota- tional medium- and strong-motion seismometers using three types of proof mass (two liquid and one solid) and a number of transducer configurations. This article describes the third set of designs and tests in our development program. The details of our results for most of these are in the E electronic supplement to this article, whereas here we concentrate on three of the most promising and representative design combinations. Most of our results pertain to sensors with water or silicon oil as the proof mass, though we also tested a torsion-bar design with a solid proof mass. We find that most mass–transducer combinations lead to output proportional to rotational acceleration, with varying degrees of fidelity. Most combinations we tested can be dismissed from further development for reasons of performance or inconvenience during analysis of acceleration response (compare with E electronic supplement). In this article, we describe three of the more promising combinations, one each for the three types of response functions we measured. Of these three, one mass–transducer combination in particular (a hinged sensing element and capacitive transduction) has output voltage closely proportional to rotational displacement (angle) over a wide frequency range; such displacement proportionality obviates two of the integration steps normally re- quired to solve for continuum single-point motions or correct for tilt-induced errors in horizontal translational sensors. Thus, although we illustrate two other designs of some promise, we propose a new design that follows this displacement-proportional path while increasing the device’s sensitivity to on-axis rotations, improving its manu- facturing ease and lowering its sensitivity to translational motions.

Bulletin of the Seismological Society of America↗
USGS2016

Seismic site characterization of an urban dedimentary basin, Livermore Valley, California: Site tesponse, basin-edge-induced surface waves, and 3D simulations

Thirty‐two accelerometers were deployed in the Livermore Valley, California, for approximately one year to study sedimentary basin effects. Many local and near‐regional earthquakes were recorded, including the 24 August 2014 M w 6.0 Napa, California, earthquake. The resulting ground‐motion data set is used to quantify the seismic response of the Livermore basin, a major structural depression in the California Coast Range Province bounded by active faults. Site response is calculated by two methods: the reference‐site spectral ratio method and a source‐site spectral inversion method. Longer‐period (≥1  s) amplification factors follow the same general pattern as Bouguer gravity anomaly contours. Site response spectra are inverted for shallow shear‐wave velocity profiles, which are consistent with independent information. Frequency–wavenumber analysis is used to analyze plane‐wave propagation across the Livermore Valley and to identify basin‐edge‐induced surface waves with back azimuths different from the source back azimuth. Finite‐element simulations in a 3D velocity model of the region illustrate the generation of basin‐edge‐induced surface waves and point out strips of elevated ground velocities along the margins of the basin.

Bulletin of the Seismological Society of America↗
USGS2016

Erratum to Surface‐wave green’s tensors in the near field

Haney and Nakahara (2014) derived expressions for surface‐wave Green’s tensors that included near‐field behavior. Building on the result for a force source, Haney and Nakahara (2014) further derived expressions for a general point moment tensor source using the exact Green’s tensors. However, it has come to our attention that, although the Green’s tensors were correct, the resulting expressions for a general point moment tensor source were missing some terms. In this erratum, we provide updated expressions with these missing terms. The inclusion of the missing terms changes the example given in Haney and Nakahara (2014).

Bulletin of the Seismological Society of America↗
USGS2017

A fault‐based model for crustal deformation in the western United States based on a combined inversion of GPS and geologic inputs

We develop a crustal deformation model to determine fault‐slip rates for the western United States (WUS) using the Zeng and Shen (2014) method that is based on a combined inversion of Global Positioning System (GPS) velocities and geological slip‐rate constraints. The model consists of six blocks with boundaries aligned along major faults in California and the Cascadia subduction zone, which are represented as buried dislocations in the Earth. Faults distributed within blocks have their geometrical structure and locking depths specified by the Uniform California Earthquake Rupture Forecast, version 3 (UCERF3) and the 2008 U.S. Geological Survey National Seismic Hazard Map Project model. Faults slip beneath a predefined locking depth, except for a few segments where shallow creep is allowed. The slip rates are estimated using a least‐squares inversion. The model resolution analysis shows that the resulting model is influenced heavily by geologic input, which fits the UCERF3 geologic bounds on California B faults and ±one‐half of the geologic slip rates for most other WUS faults. The modeled slip rates for the WUS faults are consistent with the observed GPS velocity field. Our fit to these velocities is measured in terms of a normalized chi‐square, which is 6.5. This updated model fits the data better than most other geodetic‐based inversion models. Major discrepancies between well‐resolved GPS inversion rates and geologic‐consensus rates occur along some of the northern California A faults, the Mojave to San Bernardino segments of the San Andreas fault, the western Garlock fault, the southern segment of the Wasatch fault, and other faults. Off‐fault strain‐rate distributions are consistent with regional tectonics, with a total off‐fault moment rate of 7.2 × 10 18 "> 7.2 × 10 18 and 8.5 × 10 18    N · m / year "> 8.5 × 10 18 N ⋅ m / year for California and the WUS outside California, respectively.

Bulletin of the Seismological Society of America↗
USGS2016

Simulation of earthquake ground motions in the eastern United States using deterministic physics‐based and site‐based stochastic approaches

Earthquake ground‐motion recordings are scarce in the central and eastern United States (CEUS) for large‐magnitude events and at close distances. We use two different simulation approaches, a deterministic physics‐based method and a site‐based stochastic method, to simulate ground motions over a wide range of magnitudes. Drawing on previous results for the modeling of recordings from the 2011 M w 5.8 Mineral, Virginia, earthquake and using the 2001 M w 7.6 Bhuj, India, earthquake as a tectonic analog for a large magnitude CEUS event, we are able to calibrate the two simulation methods over this magnitude range. Both models show a good fit to the Mineral and Bhuj observations from 0.1 to 10 Hz. Model parameters are then adjusted to obtain simulations for M w 6.5, 7.0, and 7.6 events in the CEUS. Our simulations are compared with the 2014 U.S. Geological Survey weighted combination of existing ground‐motion prediction equations in the CEUS. The physics‐based simulations show comparable response spectral amplitudes and a fairly similar attenuation with distance. The site‐based stochastic simulations suggest a slightly faster attenuation of the response spectral amplitudes with distance for larger magnitude events and, as a result, slightly lower amplitudes at distances greater than 200 km. Both models are plausible alternatives and, given the few available data points in the CEUS, can be used to represent the epistemic uncertainty in modeling of postulated CEUS large‐magnitude events.

Bulletin of the Seismological Society of America↗
USGS2016

Subsurface fault damage zone of the 2014 M w 6.0 South Napa, California, earthquake viewed from fault‐zone trapped waves

The aftershocks of the 24 August 2014 M w 6.0 South Napa earthquake generated prominent fault‐zone trapped waves (FZTWs) that were recorded on two 1.9‐km‐long seismic arrays deployed across the northern projection (array 1, A1) and the southern part (A2) of the surface rupture of the West Napa fault zone (WNFZ). We also observed FZTWs on an array (A3) deployed across the intersection of the Franklin and Southampton faults, which appear to be the southward continuations of the WNFZ. A1, A2, and A3 consisted of 20, 20, and 10 L28 (4.5 Hz) three‐component seismographs. We analyzed waveforms of FZTWs from 55 aftershocks in both time and frequency to characterize the fault damage zone associated with this M w 6.0 earthquake. Post‐ S coda durations of FZTWs increase with epicentral distances and focal depths from the recording arrays, suggesting a low‐velocity waveguide along the WNFZ to depths in excess of 5–7 km. Locations of the aftershocks showing FZTWs, combined with 3D finite‐difference simulations, suggest the subsurface rupture zone having an S ‐wave speed reduction of ∼40%–50% between A1 and A2, coincident with the ∼14‐km‐long mapped surface rupture zone and at least an ∼500‐m‐wide deformation zone. The low‐velocity waveguide along the WNFZ extends further southward to at least A3, but with a more moderate‐velocity reduction of 30%–35% at ray depth. This last FZTW observation suggests continuity between the WNFZ and Franklin fault. The waveguide effect may have localized and amplified ground shaking along the WNFZ and the faults farther to the south (see a companion paper by Catchings et al. , 2016 ).

California↗
USGS2016

Maximum magnitude ( M max ) in the central and eastern United States for the 2014 U.S. Geological Survey Hazard Model

Probabilistic seismic‐hazard assessment (PSHA) requires an estimate of M max , the moment magnitude M of the largest earthquake that could occur within a specified area. Sparse seismicity hinders M max estimation in the central and eastern United States (CEUS) and tectonically similar regions worldwide (stable continental regions [SCRs]). A new global catalog of moderate‐to‐large SCR earthquakes is analyzed with minimal assumptions about enigmatic geologic controls on SCR M max . An earlier observation that SCR earthquakes of M 7.0 and larger occur in young (250–23 Ma) passive continental margins and associated rifts but not in cratons is not strongly supported by the new catalog. SCR earthquakes of M 7.5 and larger are slightly more numerous and reach slightly higher M in young passive margins and rifts than in cratons. However, overall histograms of M from young margins and rifts and from cratons are statistically indistinguishable. This conclusion is robust under uncertainties in M , the locations of SCR boundaries, and which of two available global SCR catalogs is used. The conclusion stems largely from recent findings that (1) large southeast Asian earthquakes once thought to be SCR were in actively deforming crust and (2) long escarpments in cratonic Australia were formed by prehistoric faulting. The 2014 seismic‐hazard model of the U.S. Geological Survey represents CEUS M max as four‐point probability distributions. The distributions have weighted averages of M 7.0 in cratons and M 7.4 in passive margins and rifts. These weighted averages are consistent with M max estimates of other SCR PSHAs of the CEUS, southeastern Canada, Australia, and India.

Bulletin of the Seismological Society of America↗
USGS2017

System identification based on deconvolution and cross correlation: An application to a 20‐story instrumented building in Anchorage, Alaska

Deconvolution and cross‐correlation techniques are used for system identification of a 20‐story steel, moment‐resisting frame building in downtown Anchorage, Alaska. This regular‐plan midrise structure is instrumented with a 32‐channel accelerometer array at 10 levels. The impulse response functions (IRFs) and correlation functions (CFs) are computed based on waveforms recorded from ambient vibrations and five local and regional earthquakes. The earthquakes occurred from 2005 to 2014 with moment magnitudes between 4.7 and 6.2 over a range of azimuths at epicenter distances of 13.3–183 km. The building’s fundamental frequencies and mode shapes are determined using a complex mode indicator function based on singular value decomposition of multiple reference frequency‐response functions. The traveling waves, identified in IRFs with a virtual source at the roof, and CFs are used to estimate the intrinsic attenuation associated with the fundamental modes and shear‐wave velocity in the building. Although the cross correlation of the waveforms at various levels with the corresponding waveform at the first floor provides more complicated wave propagation than that from the deconvolution with virtual source at the roof, the shear‐wave velocities identified by both techniques are consistent—the largest difference in average values is within 8%. The median shear‐wave velocity from the IRFs of five earthquakes is 191 m/s for the east–west (E‐W), 205 m/s for the north–south (N‐S), and 176 m/s for the torsional responses. The building’s average intrinsic‐damping ratio is estimated to be 3.7% and 3.4% in the 0.2–1 Hz frequency band for the E‐W and N‐S directions, respectively. These results are intended to serve as reference for the undamaged condition of the building, which may be used for tracking changes in structural integrity during and after future earthquakes.

Alaska↗
USGS2016

Three ingredients for Improved global aftershock forecasts: Tectonic region, time-dependent catalog incompleteness, and inter-sequence variability

Following a large earthquake, seismic hazard can be orders of magnitude higher than the long‐term average as a result of aftershock triggering. Because of this heightened hazard, emergency managers and the public demand rapid, authoritative, and reliable aftershock forecasts. In the past, U.S. Geological Survey (USGS) aftershock forecasts following large global earthquakes have been released on an ad hoc basis with inconsistent methods, and in some cases aftershock parameters adapted from California. To remedy this, the USGS is currently developing an automated aftershock product based on the Reasenberg and Jones (1989) method that will generate more accurate forecasts. To better capture spatial variations in aftershock productivity and decay, we estimate regional aftershock parameters for sequences within the García et al. (2012) tectonic regions. We find that regional variations for mean aftershock productivity reach almost a factor of 10. We also develop a method to account for the time‐dependent magnitude of completeness following large events in the catalog. In addition to estimating average sequence parameters within regions, we develop an inverse method to estimate the intersequence parameter variability. This allows for a more complete quantification of the forecast uncertainties and Bayesian updating of the forecast as sequence‐specific information becomes available.

California↗
USGS2016

Kinematic ground motion simulations on rough faults including effects of 3D stochastic velocity perturbations

We describe a methodology for generating kinematic earthquake ruptures for use in 3D ground‐motion simulations over the 0–5 Hz frequency band. Our approach begins by specifying a spatially random slip distribution that has a roughly wavenumber‐squared fall‐off. Given a hypocenter, the rupture speed is specified to average about 75%–80% of the local shear wavespeed and the prescribed slip‐rate function has a Kostrov‐like shape with a fault‐averaged rise time that scales self‐similarly with the seismic moment. Both the rupture time and rise time include significant local perturbations across the fault surface specified by spatially random fields that are partially correlated with the underlying slip distribution. We represent velocity‐strengthening fault zones in the shallow (<5 km) and deep (>15 km) crust by decreasing rupture speed and increasing rise time in these regions. Additional refinements to this approach include the incorporation of geometric perturbations to the fault surface, 3D stochastic correlated perturbations to the P ‐ and S ‐wave velocity structure, and a damage zone surrounding the shallow fault surface characterized by a 30% reduction in seismic velocity. We demonstrate the approach using a suite of simulations for a hypothetical M w 6.45 strike‐slip earthquake embedded in a generalized hard‐rock velocity structure. The simulation results are compared with the median predictions from the 2014 Next Generation Attenuation‐West2 Project ground‐motion prediction equations and show very good agreement over the frequency band 0.1–5 Hz for distances out to 25 km from the fault. Additionally, the newly added features act to reduce the coherency of the radiated higher frequency ( f >1 Hz) ground motions, and homogenize radiation‐pattern effects in this same bandwidth, which move the simulations closer to the statistical characteristics of observed motions as illustrated by comparison with recordings from the 1979 Imperial Valley earthquake.

Bulletin of the Seismological Society of America↗
USGS2018

Review: Past and future fault rupture lengths in seismic source characterization—The long and short of it

The potential rupture length of an active fault is primary input, and an uncertainty, in source characterization for seismic hazard analysis. The past several decades have seen source models in which faults are divided into potential rupture segments based on fault‐specific behavioral and geometric observations. These models (prescribed segmentation) produced single and multisegment ruptures for use in earthquake probability and regional ground‐motion estimates. The 2013 Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3), relaxed segmentation through modeling of fault‐to‐fault jumps that result in ruptures along the San Andreas fault system as long as 1200 km. Principle controls of UCERF3 rupture length are the stepover distance between different faults or fault sections, and their orientation relative to modeled coulomb stress changes. Observations of surface faulting worldwide since the mid‐1800s in shallow continental crust show that only 30 of the reported ruptures, about 12% of the reported examples, have been longer than 100 km; only 6 have been longer than 300 km and none has exceeded 500 km. A review of characteristics of long ruptures shows that, in general, they occur along essentially continuous traces with limited stepping or branching from one fault to another as opposed to rupture propagation on linkages of different faults with highly variable physical characteristics. These commonly occupy from 10% to 70% of the total host fault length. Neither segmented nor relaxed models of rupture length have been tested by the occurrence of actual ruptures. There is no reason why segmentation models incorporating timing, dynamic, and rheological factors cannot provide reasonable approximations of future rupture length. For many faults of concern worldwide, this may be the most effective approach.

California↗
USGS2016

Noise reduction in long‐period seismograms by way of array summing

Long‐period (>100 s period) seismic data can often be dominated by instrumental noise as well as local site noise. When multiple collocated sensors are installed at a single site, it is possible to improve the overall station noise levels by applying stacking methods to their traces. We look at the noise reduction in long‐period seismic data by applying the time–frequency phase‐weighted stacking method of Schimmel and Gallart (2007) as well as the phase‐weighted stacking (PWS) method of Schimmel and Paulssen (1997) to four collocated broadband sensors installed in the quiet Albuquerque Seismological Laboratory underground vault. We show that such stacking methods can improve vertical noise levels by as much as 10 dB over the mean background noise levels at 400 s period, suggesting that greater improvements could be achieved with an array involving multiple sensors. We also apply this method to reduce local incoherent noise on horizontal seismic records of the 2 March 2016 M w 7.8 Sumatra earthquake, where the incoherent noise levels at very long periods are similar in amplitude to the earthquake signal. To maximize the coherency, we apply the PWS method to horizontal data where relative azimuths between collocated sensors are estimated and compared with a simpler linear stack with no azimuthal rotation. Such methods could help reduce noise levels at various seismic stations where multiple high‐quality sensors have been deployed. Such small arrays may also provide a solution to improving long‐period noise levels at Global Seismographic Network stations.

Bulletin of the Seismological Society of America↗
USGS2016

Continuity of the West Napa–Franklin fault zone inferred from guided waves generated by earthquakes following the 24 August 2014 Mw 6.0 South Napa Earthquake

We measure peak ground velocities from fault‐zone guided waves (FZGWs), generated by on‐fault earthquakes associated with the 24 August 2014 M w 6.0 South Napa earthquake. The data were recorded on three arrays deployed across north and south of the 2014 surface rupture. The observed FZGWs indicate that the West Napa fault zone (WNFZ) and the Franklin fault (FF) are continuous in the subsurface for at least 75 km. Previously published potential‐field data indicate that the WNFZ extends northward to the Maacama fault (MF), and previous geologic mapping indicates that the FF extends southward to the Calaveras fault (CF); this suggests a total length of at least 110 km for the WNFZ–FF. Because the WNFZ–FF appears contiguous with the MF and CF, these faults apparently form a continuous Calaveras–Franklin–WNFZ–Maacama (CFWM) fault that is second only in length (∼300 km) to the San Andreas fault in the San Francisco Bay area. The long distances over which we observe FZGWs, coupled with their high amplitudes (2–10 times the S waves) suggest that strong shaking from large earthquakes on any part of the CFWM fault may cause far‐field amplified fault‐zone shaking. We interpret guided waves and seismicity cross sections to indicate multiple upper crustal splays of the WNFZ–FF, including a northward extension of the Southhampton fault, which may cause strong shaking in the Napa Valley and the Vallejo area. Based on travel times from each earthquake to each recording array, we estimate average P ‐, S ‐, and guided‐wave velocities within the WNFZ–FF (4.8–5.7, 2.2–3.2, and 1.1–2.8 km/s, respectively), with FZGW velocities ranging from 58% to 93% of the average S ‐wave velocities.

California↗
USGS2016

Potentially induced earthquakes during the early twentieth century in the Los Angeles Basin

Recent studies have presented evidence that early to mid‐twentieth‐century earthquakes in Oklahoma and Texas were likely induced by fossil fuel production and/or injection of wastewater ( Hough and Page, 2015 ; Frohlich et al. , 2016 ). Considering seismicity from 1935 onward, Hauksson et al. (2015) concluded that there is no evidence for significant induced activity in the greater Los Angeles region between 1935 and the present. To explore a possible association between earthquakes prior to 1935 and oil and gas production, we first revisit the historical catalog and then review contemporary oil industry activities. Although early industry activities did not induce large numbers of earthquakes, we present evidence for an association between the initial oil boom in the greater Los Angeles area and earthquakes between 1915 and 1932, including the damaging 22 June 1920 Inglewood and 8 July 1929 Whittier earthquakes. We further consider whether the 1933 M w 6.4 Long Beach earthquake might have been induced, and show some evidence that points to a causative relationship between the earthquake and activities in the Huntington Beach oil field. The hypothesis that the Long Beach earthquake was either induced or triggered by an foreshock cannot be ruled out. Our results suggest that significant earthquakes in southern California during the early twentieth century might have been associated with industry practices that are no longer employed (i.e., production without water reinjection), and do not necessarily imply a high likelihood of induced earthquakes at the present time.

California↗
USGS2016

Ground motion in the presence of complex Topography II: Earthquake sources and 3D simulations

Eight seismic stations were placed in a linear array with a topographic relief of 222 m over Mission Peak in the east San Francisco Bay region for a period of one year to study topographic effects. Seventy‐two well‐recorded local earthquakes are used to calculate spectral amplitude ratios relative to a reference site. A well‐defined fundamental resonance peak is observed with individual station amplitudes following the theoretically predicted progression of larger amplitudes in the upslope direction. Favored directions of vibration are also seen that are related to the trapping of shear waves within the primary ridge dimensions. Spectral peaks above the fundamental one are also related to topographic effects but follow a more complex pattern. Theoretical predictions using a 3D velocity model and accurate topography reproduce many of the general frequency and time‐domain features of the data. Shifts in spectral frequencies and amplitude differences, however, are related to deficiencies of the model and point out the importance of contributing factors, including the shear‐wave velocity under the topographic feature, near‐surface velocity gradients, and source parameters.

California↗