Seismic Wave Amplification in Las Vegas: Site Response and Empirical Estimates of Ground Motion (original) (raw)
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Seismic Wave Amplification in Las Vegas: Site Characterization Measurements and Response Models
2004
As part of a multidisciplinary effort to understand seismic wave amplification in Las Vegas Valley, we conducted geotechnical and seismic refraction field studies, geologic and lithologic interpretation, and geophysical model building. Frequency-dependent amplifications (site response) and peak ground motions strongly correlate with site conditions as characterized by the models. The models include basin depths and velocities, which also correlate against ground motions. Preliminary geologic models were constructed from detailed geologic and fault mapping, logs of over 500 wells penetrating greater than 200 m depth, gravity-inversion results from the USGS, and USDA soil maps. Valley-wide refraction studies we conducted in 2002 and 2003 were inverted for constraints on basin geometry, and deep basin and basement P velocities with some 3-d control to depths of 5 km. Surface-wave studies during 2002-2004 characterized more than 75 sites within the Valley for shear velocity to depths ex...
Site Response in Las Vegas Valley, Nevada from NTS Explosions and Earthquake Data
Pure and Applied Geophysics, 2006
We report site response in Las Vegas Valley (LVV) from historical recordings of Nevada Test Site (NTS) nuclear explosions and earthquake recordings from permanent and temporary seismic stations. Our data set significantly improves the spatial coverage of LVV over previous studies, especially in the northern, deeper parts of the basin. Site response at stations in LVV was measured for frequencies in the range 0.2-5.0 Hz using Standard Spectral Ratios (SSR) and Horizontal-Vertical Spectral Ratios (HVR). For the SSR measurements we used a reference site (approximately NEHRP B ''rock'' classification) located on Frenchman Mountain outside the basin. Site response at sedimentary sites is variable in LVV with average amplifications approaching a factor of 10 at some frequencies. We observed peaks in the site response curves at frequencies clustered near 0.6, 1.2 and 2.0 Hz, with some sites showing additional lower amplitude peaks at higher frequencies. The spatial pattern of site response is strongly correlated with the reported depth to basement for frequencies between 0.2 and 3.0 Hz, although the frequency of peak amplification does not show a similar correlation. For a few sites where we have geotechnical shear velocities, the amplification shows a correlation with the average upper 30-meter shear velocities, V 30 . We performed two-dimensional finite difference simulations and reproduced the observed peak site amplifications at 0.6 and 1.2 Hz with a low velocity near-surface layer with shear velocities 600-750 m/s and a thickness of 100-200 m. These modeling results indicate that the amplitude and frequencies of site response peaks in LVV are strongly controlled by shallow velocity structure.
Understanding Ground Motion in Las Vegas: Insights from Data Analysis and Two-Dimensional Modeling
2004
Seismic ground motions are amplified in low velocity sedimentary basins relative to adjacent sites on high velocity hard rock. We used historical recordings of NTS nuclear explosions and earthquake recordings in Las Vegas Valley to quantify frequencydependent basin amplification using Standard Spectral Ratios. We show that amplifications, referred to as site response, can reach a factor of 10 in the frequency band 0.4-2.0 Hz. Band-averaged site response between 0.4-2.0 Hz is strongly correlated with basin depth. However, it is also well known that site response is related to shallow shearwave velocity structure. We simulated low frequency (f<1Hz) ground motion and site response with two-dimensional elastic finite difference simulations. We demonstrate that physically plausible models of the shallow subsurface, including low velocity sedimentary structure, can predict relative amplification as well as some of the complexity in the observed waveforms. This study demonstrates that site response can be 2 modeled without invoking complex and computationally expensive three-dimensional structural models.
1976
An empirical model for scaling Fourier Amplitude Spectra of strong earthquake ground acceleration in terms of magnitude, M, epicentral distance, R, and recording site conditions has been presented. The analysis based on this model implies that: (a) the Fourier amplitude spectra of strong-motion accelerations are characterized by greater energy content and relatively larger amplitudes for long-period waves corresponding to larger magnitudes M, (b) the shape of Fourier amplitude spectra does not vary appreciably for the distance range between about 10 and 100 km, and (c) long-period spectral amplitudes (T > 1 sec) recorded on alluvium are on the average 2.5 times greater than amplitudes recorded on basement rocks, whereas short-period (T < 0.2 sec) spectral amplitudes tend to be larger on basement rocks. It has been shown that the uncertainties which are associated with the forecasting of Fourier amplitude spectra in terms of magnitude, epicentral distance, site conditions, and component direction are considerable and lead to the rhnge of spectral amplitudes which for an 80 per cent confidence interval exceed one order of magnitude. A model has been presented which empirically approximates the distribution of Fourier spectrum amplitudes and enables one to estimate the spectral shapes which are not exceeded by the presently available data more than 100 (1-p) per cent of time where p represents the desired confidence level (0 < p <1). No. of Accelerograms Earthquake Used in No.* this Study Magnitude Caltech Report No.
First-generation site-response maps for the Los Angeles region based on earthquake ground motions
Bulletin of the Seismological Society of America
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 gr...
Soil Dynamics and Earthquake Engineering, 2000
Three studies of site amplification factors, based on the recorded aftershocks, and one study based on strong motion data, are compared one with another and with the observed distribution of damage from the Northridge, CA, earthquake of 17 January 1994 M L 6:4: In the epicentral area, when the peak ground velocities are larger than v m Ϸ 15 cm=s; nonlinear response of soil begins to distort the amplification factors determined from small amplitude (linear) wave motion. Moving into the area of near-field and strong ground motion v m Ͼ 30 cm=s; the site response becomes progressively more affected by the nonlinear soil response. Based on the published results, it is concluded that site amplification factors determined from small amplitude waves (aftershocks, small earthquakes, coda waves) and their transfer-function representation may be useful for small and distant earthquake motions, where soils and structures respond to earthquake waves in a linear manner. However in San Fernando Valley, during the Northridge earthquake, the observed distribution of damage did not correlate with site amplification determined from spectra of recorded weak motions. Mapping geographical distribution of site amplification using other than very strong motion data, therefore appears to be of little use for seismic hazard analyses. ᭧
This paper reports on studies investigating source and propagation effects of seismic waves emitted by small chemical explosions conducted in the granites of Climax Stock on the Nevada National Security Site (formerly the Nevada Test Site) as part of the Source Physics Experiments (SPE). To date, two such explosions (SPE-1 and SPE-2) have been executed with chemical yields of 100 and 1000 kg and burial depths of 55 and 46 m, respectively. A third experiment conducted in the summer of 2012 is not reported on in this paper. As many as four follow-on experiments in this SPE series are planned in subsequent years. All experiments are designed to be detonated in the same emplacement hole and recorded by the same free-field, free-surface, close-in, and local networks of ground- motion sensors, including some infrasound sensors. The focus of this paper is on the analysis and modeling of waveforms recorded close-in on five linear geophone lines emanating on a spread of azimuths from ground ...
The Carquinez Bridge geotechnical arrays are operated by the California Strong Motion Instrumentation Program (CSMIP) and recorded a peak ground acceleration (PGA) of approximately 1.0g at ground surface during the 2014 South Napa earthquake. The recorded PGA was significantly larger than those at the nearby surface sites. This study considers surface and downhole recordings from the additional 28 earthquakes recorded at the same arrays to understand the effects of wave propagation and site response at these arrays. Several site response analyses are performed to understand soil nonlinearity using the observed ground accelerations during the 2014 South Napa sequence. Apparent shear wave velocities are calculated from downhole records, which show clear reduction as ground motion intensity increases. Empirical transfer functions (EFTs) are also calculated in which the resonance frequencies became lower during strong shaking during the 2014 South Napa main shock. The in-situ critical damping ratio appears to be frequency dependent in the soft clay deposits. Lower damping at frequencies greater than about 5 Hz may have contributed to the observed PGA at Array #1 during the main shock.