ABSTRACT We invert the shear‐wave displacement spectra obtained from 30 three‐component, broadband waveforms recorded within 300 km of the 6 November 2011 Mw 5.7 Prague, Oklahoma, earthquake to recover the site‐response contribution using an inversion method that simultaneously inverts for source, path, and site effects. Site‐response functions identify resonant frequencies within a range of 0.1–10 Hz that generally coincide with spectral peaks in horizontal‐to‐vertical ratio curves derived from the recorded waveforms. S‐wave velocity profiles available for several sites were also used to calculate theoretical SH transfer functions that predict the site amplification due to the near‐surface soil structure down to depths of 30–50 m. The transfer functions do not provide resonance information below about 5–8 Hz, indicating that the spectral peaks in the site response obtained from the waveform analysis result from deeper velocity variations. A 0.3 Hz spectral peak observed at several stations, for example, coincides with the strong, surface‐wave amplitudes observed at 3 s periods for induced M≥3 earthquakes in Oklahoma and Kansas, suggesting that this resonant peak may be due to surface waves trapped in the upper ∼2 km sedimentary layer of the crust. Both shallow and deep contributions to the site response are important for the characterization of ground motion from central and eastern North America (CENA) earthquakes. We obtain a corner frequency of 0.229, consistent with independent observations of the size of the event. A frequency‐dependent attenuation relation of Q(f)=1107f0.398 consistent with prior CENA path measurements is also derived.
Ground motion from local earthquakes and the SHIPS (Seismic Hazards Investigation in Puget Sound) experiment is used to estimate site amplification factors in Seattle. Earthquake and SHIPS records are analyzed by two methods: (1) spectral ratios relative to a nearby site on Tertiary sandstone, and (2) a source/site spectral inversion technique. Our results show site amplifications between 3 and 4 below 5 Hz for West Seattle relative to Tertiary rock. These values are approximately 30% lower than amplification in the Duwamish Valley on artificial fill, but significantly higher than the calculated range of 2 to 2.5 below 5 Hz for the till-covered hills east of downtown Seattle. Although spectral amplitudes are only 30% higher in the Duwamish Valley compared to West Seattle, the duration of long-period ground motion is significantly greater on the artificial fill sites. Using a three-dimensional displacement response spectrum measure that includes the effects of ground-motion duration, values in the Duwamish Valley are 2 to 3 times greater than West Seattle. These calculations and estimates of site response as a function of receiver azimuth point out the importance of trapped surface-wave energy within the shallow, low-velocity, sedimentary layers of the Duwamish Valley. One-dimensional velocity models yield spectral amplification factors close to the observations for till sites east of downtown Seattle and the Duwamish Valley, but underpredict amplifications by a factor of 2 in West Seattle. A two-dimensional finite-difference model does equally well for the till sites and the Duwamish Valley and also yields duration estimates consistent with the observations for the Duwamish Valley. The two-dimensional model, however, still underpredicts amplification in West Seattle by up to a factor of 2. This discrepancy is attributed to 3D effects, including basin-edge–induced surface waves and basin-geometry–focusing effects, caused by the proximity of the Seattle thrust fault and the sediment-filled Seattle basin.
We apply a kinematic finite-fault inversion scheme to Pnl displacement waveforms recorded at 14 regional stations (Δ < 2°) to recover the distribution of coseismic slip for the 2004 Parkfield earthquake using both synthetic Green's func- tions (SGFs) calculated for one-dimensional (1D) crustal-velocity models and empiri- cal Green's functions (EGFs) based on the recordings of a single Mw 5.0 aftershock. Slip is modeled on a rectangular fault subdivided into 2 × 2 km subfaults assuming a constant rupture velocity and a 0.5 sec rise time. A passband filter of 0.1-0.5 Hz is applied to both data and subfault responses prior to waveform inversion. The SGF inversions are performed such that the final seismic moment is consistent with the known magnitude (Mw 6.0) of the earthquake. For these runs, it is difficult to repro- duce the entire Pnl waveform due to inaccuracies in the assumed crustal structure. Also, the misfit between observed and predicted vertical waveforms is similar in char- acter for different rupture velocities, indicating that neither the rupture velocity nor the exact position of slip sources along the fault can be uniquely identified. The pattern of coseismic slip, however, compares well with independent source models derived using other data types, indicating that the SGF inversion procedure provides a general first- order estimate of the 2004 Parkfield rupture using the vertical Pnl records. The best- constrained slip model is obtained using the single-aftershock EGF approach. In this case, the waveforms are very well reproduced for both vertical and horizontal com- ponents, suggesting that the method provides a powerful tool for estimating the dis- tribution of coseismic slip using the regional Pnl waveforms. The inferred slip model shows a localized patch of high slip (55 cm peak) near the hypocenter and a larger slip area (∼50 cm peak) extending between 6 and 20 km to the northwest.
We compute ground motions for the Beroza (1991) and Wald et al. (1991) source models of the 1989 magnitude 6.9 Loma Prieta earthquake using four different wave-propagation codes and recently developed 3D geologic and seismic velocity models. In preparation for modeling the 1906 San Francisco earthquake, we use this well-recorded earthquake to characterize how well our ground-motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the San Francisco Bay Area. All of the simulations generate ground motions consistent with the large-scale spatial variations in shaking associated with rupture directivity and the geologic structure. We attribute the small variations among the synthetics to the minimum shear-wave speed permitted in the simulations and how they accommodate topography. Our long-period simulations, on average, under predict shaking intensities by about one-half modified Mercalli inten- sity (MMI) units (25%-35% in peak velocity), while our broadband simulations, on average, under predict the shaking intensities by one-fourth MMI units (16% in peak velocity). Discrepancies with observations arise due to errors in the source models and geologic structure. The consistency in the synthetic waveforms across the wave- propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure. In agree- ment with earlier studies, we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the Beroza and Wald source models. Although the new 3D seismic velocity model improves upon previous velocity models, we identify two areas needing improvement. Nevertheless, we find that the seismic velocity model and the wave-propagation codes are suitable for modeling the 1906 earthquake and scenario events in the San Francisco Bay Area. Online Material: Modified Mercalli intensities and velocity waveforms, and a movie of simulated wave propagation.
Abstract Aftershocks of the 1989 Loma Prieta, California, earthquake are used to estimate site response along the San Francisco Peninsula. A total of 215 shear-wave records from 24 sources and 21 sites are used in a linear inversion for source and site response spectra. The methodology makes no assumptions about the shape of the source spectrum. However, to obtain a stable, unique inverse a Q model and geometrical spreading factor are assumed, as well as a constraint on site response that sets the site response averaged over two specific stations to 1.0. Site responses calculated by this formulation of the problem are compared with other studies in the same region that use different methodologies and / or data. The shear-wave site responses compare favorably with estimates based on an ω2-constrained source model. Comparison with coda amplification factors is not as close, but still favorable considering that the coda values were determined for nearby locations with similar geology, and not the same sites. The degree of agreement between the three methods is encouraging considering the very different assumptions and data used.
Abstract We compare teleseismic P-wave records for earthquakes in the magnitude range from 6.0 to 9.5 with synthetics for a self-similar, ω2 source model and conclude that the energy radiated by very large earthquakes (Mw > 814) is not self-similar to that radiated from smaller earthquakes (Mw < 814). Furthermore, in the period band from 2 sec to several tens of seconds, we conclude that large subduction earthquakes have an average spectral decay rate of ω-1.5. This spectral decay rate is consistent with a previously noted tendency of the ω2 model to overestimate Ms for large earthquakes.
Research Article| September 25, 2017 Seismic Response of Soft Deposits due to Landslide: The Mission Peak, California, Landslide Stephen Hartzell; Stephen Hartzell aU.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, shartzell@usgs.gov Search for other works by this author on: GSW Google Scholar Alena L. Leeds; Alena L. Leeds aU.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, shartzell@usgs.gov Search for other works by this author on: GSW Google Scholar Randall W. Jibson Randall W. Jibson aU.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, shartzell@usgs.gov Search for other works by this author on: GSW Google Scholar Author and Article Information Stephen Hartzell aU.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, shartzell@usgs.gov Alena L. Leeds aU.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, shartzell@usgs.gov Randall W. Jibson aU.S. Geological Survey, Denver Federal Center, Box 25046, MS 966, Denver, Colorado 80225, shartzell@usgs.gov Publisher: Seismological Society of America First Online: 27 Sep 2017 Online Issn: 1943-3573 Print Issn: 0037-1106 Bulletin of the Seismological Society of America (2017) 107 (5): 2008–2020. https://doi.org/10.1785/0120170033 Article history First Online: 27 Sep 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Stephen Hartzell, Alena L. Leeds, Randall W. Jibson; Seismic Response of Soft Deposits due to Landslide: The Mission Peak, California, Landslide. Bulletin of the Seismological Society of America 2017;; 107 (5): 2008–2020. doi: https://doi.org/10.1785/0120170033 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract The seismic response of active and intermittently active landslides is an important issue to resolve to determine if such landslides present an elevated hazard in future earthquakes. To study the response of landslide deposits, seismographs were placed on the Mission Peak landslide in the eastern San Francisco Bay region for a period of one year. Numerous local and near‐regional earthquakes were recorded that reveal a complexity of seismic response phenomena using the horizontal‐to‐vertical spectral ratio method. At lower frequencies, a clear spectral peak is observed at 0.5 Hz common to all four stations in the array and is attributed to a surface topographic effect. At higher frequencies, other spectral peaks occur that are interpreted in terms of local deposits and structures. Site amplification from the standard reference site method shows the minimum amplification with a factor of 2, comparing a site on and off the landslide. A site located on relatively homogeneous deposits of loose soils shows a clear spectral peak associated with the thickness of the deposit. Another site on a talus‐filled graben near the headscarp shows possible 2D or 3D effects from subsurface topography or scattering within and between buried sandstone blocks. A third site on a massive partially detached block below the crown of the headscarp shows indications of resonance caused by the reverberation of shear waves within the block. The varied seismic response of different parts of this complex landslide is consistent with other studies which found that, although landslide response is commonly enhanced in the downslope direction of landslide movement, such a response does not occur uniformly or consistently. When it does occur, enhanced site response parallel to the direction of landslide movement would contribute to landslide reactivation during significant earthquakes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Ground-motion records from aftershocks of the 1994 Northridge earthquake and mainshock records from the 1971 San Fernando, 1987 Whittier Narrows, 1991 Sierra Madre, and 1994 Northridge earthquakes are used to estimate site response relative to a rock site for the urban Los Angeles area. Site response is estimated at 232 mainshock and 201 aftershock sites relative to a low-amplitude site in the Santa Monica Mountains. Average amplification values are calculated for the frequency bands: 1 to 3, 3 to 5, and 5 to 7 Hz. These bands are chosen based on limitations in aftershock recording equipment at lower frequencies and reduced significance to the building inventory at higher frequencies. Site amplification factors determined at the instrumented locations are grouped by the surficial geology and contoured to produce a continuous spatial estimation of amplification. The maps in this article represent the first attempt to produce estimates of site amplification based on observations of ground motion for such a large areal extent of the Los Angeles region. These maps are expected to evolve as more data become available and more analysis is done.
