Spectral amplification in a sediment-filled Valley exhibiting clear basin-edge-induced waves
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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.Keywords:
Bedrock
Coda
Amplification factor
The single isotropic scattering model proposed for the origin of seismic coda predicts a regularly decaying coda wave envelope. In real data however, this regular decay is often interrupted by non‐random arrivals (e.g. reflections) coming later in the coda. This additional energy may be due to clustered scatterers and represents a departure from the inherent assumption in the theory that the scatterers are uniformly randomly distributed. We have simulated the effect of clustered scatterers in synthetic seismograms in order to investigate their influence upon the measured coda Q ( Q c ). We show that the position and amplitude of such arrivals within the otherwise regularly decaying coda give rise to elevated Q c estimates. Often this additional energy in the coda is difficult to detect on a single isolated seismogram as evidenced by an example from the 1994 Northridge California earthquake. We therefore recommended that where possible, the use of seismic arrays (commonly reserved for P‐wave coda studies) be extended to S‐wave coda studies to aid in the detection of clustered scatterers, prior to the calculation of Q c .
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Envelope (radar)
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The input earthquake ground motion is going to be expressed as time histories considering source, path and site amplification effects especially due to the deep subsurface profile. Strong motion record and surface soil profile to the engineering bedrock is necessary for the evaluation of the site amplification factor. In this paper, we firstly show the method for evaluating the site amplification factor for sites with strong motion record and without surface soil profiles to the engineering bedrock by using microtremors. We next show the modification method of site amplification factor for sites that is a bit far away from the strong motion record site.
Bedrock
Amplification factor
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Bedrock
Response analysis
Amplification factor
Site analysis
Soil horizon
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Abstract During the months that followed the 17 January 1994 M 6.7 Northridge, California, earthquake, portable digital seismic stations were deployed in the San Fernando basin to record aftershock data and estimate site-amplification factors. This study analyzes data, recorded on 31 three-component stations, from 38 aftershocks ranging from M 3.0 to M 5.1, and depths from 0.2 to 19 km. Site responses from the 31 stations are estimated from coda waves, S waves, and ratios of horizontal to vertical (H/V) recordings. For the coda and the S waves, site response is estimated using both direct spectral ratios and a generalized inversion scheme. Results from the inversions indicate that the effect of Qs can be significant, especially at high frequencies. Site amplifications estimated from the coda of the vertical and horizontal components can be significantly different from each other, depending on the choice of the reference site. The difference is reduced when an average of six rock sites is used as a reference site. In addition, when using this multi-reference site, the coda amplification from rock sites is usually within a factor of 2 of the amplification determined from the direct spectral ratios and the inversion of the S waves. However, for nonrock sites, the coda amplification can be larger by a factor of 2 or more when compared with the amplification estimated from the direct spectral ratios and the inversion of the S waves. The H/V method for estimating site response is found to extract the same predominant peaks as the direct spectral ratio and the inversion methods. The amplifications determined from the H/V method are, however, different from the amplifications determined from the other methods. Finally, the stations were grouped into classes based on two different classifications, general geology and a more detailed classification using a quaternary geology map for the Los Angeles and San Fernando areas. Average site-response estimates using the site characterization based on the detailed geology show better correlation between amplification and surface geology than the general geology classification.
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Ranging
S-wave
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The well established separability of source, path and site effects on coda waves of local seismic events offers the most effective means for determining the site amplification factor empirically. Many studies have confirmed that the coda amplification factor represents those of S waves averaged over various directions of wave approaches. A systematic relation has been found between the coda amplification factor and the age of surface geology at the recording site in California, and has been used to interpolate coda amplification factors observed at selected sites to construct maps of amplification factors for various frequencies in California. We found that, in general, the coda amplification factor is applicable to strong ground motion at rock sites. It is also applicable to soil sites, if the basement acceleration level is lower than 0.1 g. We found, however, the coda amplification factor overestimates the observed strong motion amplification at soil sites when the basement acceleration level exceeds 0.1-0.2 g. These discrepancies can be explained by the increased absorption due to the non-linear histeresis in soil response. We present here encouraging results of successfully testing a preliminary method by M. Mahdyiar and others (1993) for correcting the coda amplification factor for the non-linear effect using the strong motion data from the Landers earthquake of 1992.
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Amplification factor
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