A comparison of coda and S-wave spectral ratios as estimates of site response in the southern San Francisco Bay area
49
Citation
37
Reference
20
Related Paper
Citation Trend
Abstract:
Abstract We calculate spectral ratios for S waves and codas to evaluate the amplification of 14 sites in the southern San Francisco Bay area relative to a nearby bedrock site in the Coyote Hills. Our data are seismograms written by Loma Prieta aftershocks: the epicentral distances and azimuths to the stations are effectively the same. All the sites in the study are amplified with respect to the reference site at frequencies from 0.5 to 7 Hz. The shapes of the S -wave and coda spectral ratios are similar, but the coda ratios are greater than the S -wave ratios by as much as a factor of 4. The difference is larger for sites on alluvial and bay mud deposits, particularly at frequencies around 1 Hz, suggesting the presence of waves trapped in the alluvial basin. In general, the length of the analysis window affects the S -wave spectral ratios for alluvial sites. Longer windows give ratios similar to coda ratios, apparently because these windows include more of the phases that contribute to the coda. We classify the sites according to their geological characteristics and surficial shear-wave velocities. For the S -wave ratios, the differences between the classes showed no systematic trend; the softest and hardest soil classes we consider have practically identical S -wave amplifications. The average coda ratios for the site classes clearly increase as the soil classes include slower and “softer” materials. After correction for differences in reference sites, the coda amplifications are very similar to the relative amplifications for these site classes estimated by Borcherdt and Glassmoyer (1992) from the strong-motion recordings of the 1989 Loma Prieta earthquake.Keywords:
Coda
Seismogram
Bedrock
S-wave
Bay mud
The omega-squared spectral model with two independent parameters (stress drop, moment) provides a simple parameterization of ground motion spectra for statistical analysis. Accurate determination of source parameters of small earthquakes requires accounting for distortion of the source spectrum by each site response spectrum. Record spectra are inverted here to find separate station and event spectra. Source parameters are found by an automated objective method using integrals of each event spectrum. Quantitative estimates of error are carried through the entire analysis. These methods are applied to digital records of aftershocks of the 1980 Mammoth Lakes California earthquake sequence. Stress drop is found to be independent of source radius for the 90 events with best-determined source parameters. The ratio of Hanks to Brune stress drop is remarkably constant and independent of source radius, showing that the spectra, on average, have a constant shape that is scaled by two parameters, corner frequency and low-frequency level. Stress drops have a log-normal distribution with the standard deviation being about a factor of 2.
Seismic moment
Source model
Cite
Citations (478)
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.
Coda
Cite
Citations (111)
Abstract Ground-motion records from aftershocks of the 1994 Northridge earthquake are used to estimate site response in the urban Los Angeles area. Over 1300 shear-wave records from 61 sources and 90 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. To obtain a stable unique inverse, a Q model and geometrical spreading factor are assumed. In addition, the site response at a hard-rock site is constrained to be approximately 1.0 with a kappa of 0.02. The site-response spectra compare favorably with the results of previous and on-going investigations in Los Angeles. A couple of first-order effects are lower site response in the surrounding mountains, dominated by Mesozoic and Tertiary rocks, and higher values in the San Fernando and Los Angeles Basins, containing surficial Pleistocene and Holocene alluvial deposits. Results show good correlation of high site-response spectral values with localized areas of severe damage (Interstate 10 collapse, Sherman Oaks, Northridge, Interstate 5/14 collapse). However, widespread trends in site response across the sedimentary basins are not obvious. The data suggest that site responses are lower near the southern margin of the San Fernando Valley for sources to the north, due to north-dipping sedimentary structures. But the general pattern of site response is characterized by high variability on length scales less than a kilometer. Variations of a factor of 2 in site response are observed over the length scale of 200 m and for the same surficial geologic unit. For some of the alluvial basin sites, surface-wave generation is a significant contributor to elevated site response at lower frequencies, below 2 Hz. The total damage pattern for the Northridge earthquake is influenced by strong source directivity to the north and strong local site effects. The correlation of weak-motion site-response estimates with areas of significant damage demonstrates the value of these field measurements in future urban planning and in the reduction of seismic risk in urban areas.
Response spectrum
Strong ground motion
Cite
Citations (114)
Abstract The spectral ratio technique is a common useful way to estimate empirical transfer function to evaluates site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique, originally proposed by Nakamura to analyze Rayleigh waves in the microtremor records, is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. These cities are very different by their geological and tectonic contexts and also by the very different epicentral distances to the main seismogenic zones affecting each city. Each time we compare the results of Nakamura's technique with standard spectral ratios. In all three cases the results are very encouraging. We conclude that, if site effects are caused by simple geology, a first estimate of dominant period and local amplification level can be obtained using records of only one station.
Microtremor
Rayleigh Wave
Spectral Analysis
Cite
Citations (1,038)
Numerical simulation of noise is used to investigate the characteristics of the spectral ratio between horizontal and vertical components (H/V ratio) and its sensitivity to various parameters in order to better appreciate the reliability of the technique proposed by Nakamura (1989) to estimate site amplification effects from single station noise recordings. Noise is simulated as the signal produced at a single site by a set of superficial sources (unidirectional forces or dipoles) disposed all around with random amplitude and time delay. Individual signals from a single source are computed by the discrete wave number technique. Synthetic calculations for 15 soil profiles show that this ratio exhibits a single, clear peak, the location of which is independent of the source excitation function, but strongly correlated with the local geological structure: its frequency is very close to the S wave resonance frequency. This peak appears to be mainly controlled by the polarization curve of the fundamental Rayleigh waves, which in turn exhibits a sharp peak around the fundamental resonance mode of the sedimentary structure. A similar result is found for the H/V ratio obtained for incident plane SV waves. In contrast, the amplitude of this peak exhibits a poor correlation with the ground motion amplification of S waves at resonance frequency. It is shown to be related with a high sensitivity on the value of the Poisson's ratio in the uppermost layer presumed to be the noise source layer, and, though to a much lesser extent, on the mean distance between site and noise sources. It is concluded that Nakamura's method can clearly allow the resonance frequency of a given sedimentary site to be measured very efficiently and very cheaply, but that its use for deriving the amplification at resonance frequency seems still premature from a theoretical point of view.
Rayleigh Wave
Cite
Citations (657)
Abstract Site amplifications of direct S waves and coda waves are studied and compared using high-quality, three-component digital data from aftershocks of the Little Skull Mountain, Neveda, earthquake. We use data from 12 stations installed on a variety of geological and topographic site conditions, distributed widely in space with different azimuths and epicentral distances. S-wave site amplifications are obtained in the frequency range from about 0.5 to over 30 Hz, while coda-wave site amplifications are obtained in a frequency range from about 1.5 to over 30 Hz. A thorough statistical analysis of these results was performed. We find that (1) for S waves, all three components at a station follow a similar frequency-dependent trend. The amplitudes of the two horizontal components match more closely, while the vertical component shows consistently lower amplification than the horizontal components at low frequencies. (2) For coda waves, all three components share both similar frequency-dependent trends and amplification level. (3) S-wave and coda-wave site amplification are consistent for stations with epicentral distance greater than about 10 km (which is about the average focal depth of the earthquakes we used). Within the epicentral distance of 10 km, however, some stations show discordant S-wave and coda-wave site amplifications. Possible factors are that the direct S waves are affected by particular wave propagation paths and that at short distance SV is partitioned with more energy on the horizontal components and less on the vertical components than at larger distances.
Coda
S-wave
Cite
Citations (36)
abstract Measurements of ground motion generated by nuclear explosions in Nevada were made for 37 locations near San Francisco Bay, California. The results were compared with the San Francisco 1906 earthquake intensities and the strong-motion recordings of the San Francisco earthquake of March 22, 1957. The recordings show marked amplitude variations which are related consistently to the geologic setting of the recording site. For sites underlain by a layer of younger bay mud or artificial fill, maximum horizontal ground velocities generally increased with thickness of the layer and were as much as ten times greater than those recorded on nearby bedrock. The maximum vertical velocities for these sites were between 1 and 3.5 times greater. Spectral amplification curves clearly define a “dominant ground period” of about 1 second for sites underlain by younger bay mud. For sites underlain by older, more consolidated sediments, no clearly defined “dominant ground period” was found. Maximum ground velocities for the older bay sediment sites were about twice those recorded on bedrock. Consistent correlations of the results from the nuclear recordings with the 1906 earthquake intensities and the spectral amplification curves for the 1957 earthquake suggest that areas of high amplification determined from small ground motions may also be areas of high intensity in future earthquakes.
Bedrock
Bay mud
Cite
Citations (991)
Abstract Various site-response estimates are presented for a linear array deployed in the Coachella Valley, California, during the 1992 Landers/Big Bear aftershock sequence. This systematic comparison is unique in that the response of the site is clearly dominated by basin-edge-induced waves. Average sediment to bedrock spectral ratios for long S-wave windows, which include the basin-edge-induced waves, exhibit amplification factors as high as ∼ 18, below 7 Hz. The deep basin structure, which gives rise to the obvious multi-dimensional effects, produces a fundamental resonant peak that shifts between basin sites (from 0.23 to 0.48 Hz) as the depth to bedrock changes. Above 0.6 Hz, where the largest amplifications occur, the response is remarkably similar between sites and appears to be dominated by a near-surface layer that is relatively uniform across the valley. The apparent fundamental resonant frequency of this layer is between 0.8 and 1 Hz. Sediment to bedrock spectral ratios computed using shorter windows that exclude the basin-edge-induced waves imply that the multi-dimensional effects are significant only below ∼ 4 Hz, where they increase amplifications by an approximate factor of 2. Spectral ratios computed using coda windows, taken at twice the S-wave travel time, exhibit amplifications that are an average factor of 1.7 greater, between 1 and 4 Hz, than those of the S-wave estimates. This discrepancy does not improve by taking coda windows later at four times the S-wave travel time. Horizontal- to vertical-component S-wave spectral ratios do not agree with the sediment to bedrock ratios. However, they do exhibit a clear peak at the fundamental resonant frequency of the deep basin structure. Sediment to bedrock spectral ratios of ambient seismic noise are also inconsistent with the S-wave estimates. However, horizontal to vertical noise ratios exhibit clear peaks near the fundamental resonant frequencies of both the deep basin structure (below 0.6 Hz) and the suspected near-surface layer (between 0.8 and 1 Hz). Therefore, ambient-noise data appear to provide valuable constraints on the basin structure. Ongoing efforts involve multi-dimensional modeling of the observed basin-edge-induced phases and resonant frequencies.
Bedrock
Coda
Amplification factor
Cite
Citations (183)
Abstract The goals of this study are two-fold: (1) to examine the site effect of coda waves, believed to be the average site effect of shear waves, in order to understand its spatial and frequency-dependent behavior, and (2) to learn what we can about the processes that generate the coda itself. We use digital data from over 90 earthquakes, each recorded by a subset of some 150 stations in the Coast Ranges of central California between San Francisco and San Luis Obispo. Results from the band 1.5 to 24 Hz indicate that site amplification depends strongly on surface geology and frequency. At low frequencies (1.5 to 3.0 Hz), changes in amplification up to a factor of 20 are observed; amplification generally varies inversely with deposit age at sediment sites and is lowest at granite and Franciscan (basement) sites. At high frequencies (6 to 24 Hz), the pattern changes. Many granite sites, the majority in the Gabilan range, exhibit increasing amplification with frequency relative to the average station. This behavior differs strikingly from that observed at sediment and Franciscan sites which roll-off at an intermediate rate, and many sediment sites adjacent to the San Andreas fault zone which decay away even more rapidly with frequency. These results can be explained qualitatively by appealing to variable near-site impedance and attenuation, however, complicating phenomena such as mode excitation in valley sediments must exist at low frequencies. Peaked behavior at a few hard rock sites at high frequency (12 Hz) currently lacks explanation. At a few sediment sites, the low-frequency excitation is so strong that the coda envelopes begin to take on different shapes. This contradicts the assumption based on previous observations that the coda shape is independent of source-receiver location, thus, some site measurements may be artificially high. The working model of coda waves as body-wave energy backscattered from randomly distributed inhomogeneities must be modified to include the possibility of efficient near-site resonance excited by incident direct and coda wave energy.
Coda
Basement
Frequency dependence
Cite
Citations (330)
For over thirty years, attempts have been made to gain information about sediment amplification during earthquakes from observations of ambient seismic noise. While the results of several feasibility studies have been encouraging, theoretical support for the technique is scant. We present a model for the response of sedimentary layers to ambient seismic noise. The noise sources are modeled as a random distribution (in time and space) of point forces located on the Earth's free surface. This model is applied to a site where observed noise spectral ratios, relative to a rock site, have previously been shown to reveal the fundamental resonant frequency of a soft clay layer. Approximating the sediment site as a single layer over a half‐space, the horizontal noise spectrum predicted by our model reveals the fundamental resonance and first harmonic of the layer. We also examine an estimate of site response proposed by Nakamura (1989), which is formed by dividing the horizontal‐component noise spectrum by that of the vertical component. Nakamura's estimate applied to both observed and predicted noise‐spectra was also successful in identifying the fundamental resonance, with a slight (<10%) shift toward lower frequencies. Future work is needed to determine the generality of our results, and to elucidate the influence of the simplifying assumptions.
Seismic Noise
Ambient noise level
Environmental Noise
Cite
Citations (334)