Site Amplification Factors Research Articles

Site amplification factors in central and southern Californi determined from coda waves : Feng Su & K. Aki, Bulletin — Seismological Society of America, 85(2), 1995, pp 452–466

Site amplification factors in central and southern Californi determined from coda waves : Feng Su & K. Aki, Bulletin — Seismological Society of America, 85(2), 1995, pp 452–466

中小規模地震の観測記録から得られる地盤増幅特性に基づく大規模地震の応答スペクトルの簡易評価法

A primitive procedure was proposed for evaluating earthquake response spectra of large-event motions to make use of records from smaller events. The result of the regression analysis of the response spectra was utilized to obtain the site amplification factors in the proposed procedure, and the formulation of the seismic-source term in the regression analysis was examined. A linear form of the moment magnitude, Mω, is good for scaling the source term of moderate earthquakes with Mω of 5.5 to 7.0, while a quadratic form of Mω and the ω-square source-spectrum model is appropriate for scaling the source term of smaller and greater earthquakes, respectively.

A comparison and test of various site-response estimation techniques, including three that are not reference-site dependent

Abstract We compare various site-response estimation techniques using after-shock data of the 1989 Loma Prieta earthquake collected in Oakland, California. Because of recent interest in comparing results from weak and strong motion (to infer any nonlinearity) and between direct S and coda waves, we pay particular attention to the uncertainties. First, sediment to bedrock spectral-ratio estimates between pairs of sites are compared with those obtained from various generalized-inversion approaches where the source and site effects of multiply recorded events are solved for simultaneously. We find that the site amplification factors are very similar among these approaches, but that the uncertainties can be significantly different depending on how the data are weighted. We also examine and test three site-response estimation techniques that do not rely on a reference site to estimate source and path effects. The first involves a parameterized source- and path-effects inversion. Even when the bedrock data are excluded from consideration, this approach is found to reveal the frequency dependence of site response at each of the sediment sites. The second technique involves taking horizontal- to vertical-component spectral ratios (receiver-function-type estimates) of shear-wave aftershock data. These are also found to reveal the frequency dependence of site response at the sediment sites, and the results for the bedrock site are relatively flat and near unity. the third estimate is formed by taking horizontal-to vertical-component ratios of ambient seismic noise, and these are shown to reveal the fundamental resonant frequency of the sediment sites. Unfortunately, discrepancies exist among all of the site-response estimates (and with one-dimensional predictions) with respect to a frequency-independent scaling factor. Nevertheless, the highly frequency-dependent character of site response is well constrained, and the fact that non-reference-site-dependent methods are capable of revealing this is promising for site-specific hazard assessments in regions that lack adequate reference sites.

Site amplification from coda waves: Validation and application to S-wave site response

Abstract The subject of this article is to see if the response to S waves at a specific site can be determined from coda waves. We used aftershock records of the 1992 Landers and Big Bear earthquakes observed at four TERRAscope stations (PAS, PFO, SVD, and GSC) in southern California. The wide dynamic range of the TERRAscope instrument provided a broadband seismogram for S waves, and, at the same time, coda waves over a long lapse-time window. We analyzed a total of 708 three-component waveforms from 65 events whose magnitudes ranged from 3.3 to 5.4. The station SVD is located on Quaternary alluvium, and the remaining stations are on rocks of Cretaceous age. We first evaluated the coda site-amplification factor using a spectral ratio method (May-eda et al. , 1991; Koyanagi et al. , 1992) for frequency bands centered at 0.75, 1.0, 1.5, 2.0, 3.0, 4.0, and 6.0 Hz for lapse-time windows after 70 sec from the origin time. We found that the site-amplification factor for the radial component is similar to that for the transverse component. This similarity was expected because codas consist of backscattered waves coming from all directions into the station, resulting in a directionally averaged site response. We then evaluated the S -wave site-amplification factor using an inversion method (Andrews, 1986) on the direct S -wave portion of the same data set. We found that the coda site response agrees with the S -wave site response within a factor of about 1.5, supporting the earlier findings by Tsujiura (1978) and justifying the use of coda waves in determining site-specific amplification for S waves. This agreement between S -wave site response and coda site response is understandable if we assume that coda waves are dominated by S waves, as theoretically predicted by Aki (1992) and Zeng (1993).

Inferred attenuation from site effect-correctedT phases recorded on the island of Hawaii

Coda site amplification factors are used to eliminate the site effect from records of three circum-PacificT phases recorded by the Hawaiian Volcano Observatory (HVO) network on the island of Hawaii. ObservedT-phase amplitudes across the island generally decrease with increasing distance from the conversion point where acoustic waves in the SOFAR channel become seismic waves propagating through the crust. However, the decay of the observedT-phase signal across the island is not regular in regions of dense station coverage, in particular, the Kilauea caldera region. We divide the maximum observedT-phase amplitudes at a given station by the coda site amplification factor obtained for the same station and frequency band (3.0Hz); the distribution of these amplitudes reveals a smooth pattern over the entire island. The distance over which the site effect-correctedT-phase amplitude decreases by one-half, combined with the apparent velocity of propagation ofT phases across the island, allows for an approximate determination of near-surfaceQ over much of the island of Hawaii. We found a region of lowQ in the Kilauea summit area (Q≈30) and east rift zone (Q≈60) with considerably higherQ in the Kaoiki and northern portions of the island (Q≈150 to 200). The lowQ values obtained in the Kilauea summit region and east rift zone are significantly lower than estimates of codaQ in the same region, suggestingT phases may be sampling the earth's near-surface properties.

Strong Motion Seismology in Japan.

This paper reviews strong motion seismology in Japan. It describes advances in strong motion seismology in the last decade, the current state of the art, and unresolved issues in the following areas: (1) Observation and data processing of strong ground motion, (2) Local site effects, and (3) Simulation and prediction of strong ground motion. In the first area, it reviews the current state of strong motion observation and data processing: fundamental subjects in strong motion seismology. In the next area, it describes empirical evaluations of site amplification factors, effects of soil nonlinearity, effects of topographical and subsurface irregularity, and application of microtremors. In the last area, it summarizes the results of empirical, semi-empirical, and numerical techniques for the simulation and the prediction of strong ground motions due to actual earthquakes. Finally, future directions of research are suggested, based on the current state of the art in strong motion seismology.

表面波が卓越する周期約2〜20秒のやや長周期地震動の距離減衰特性に関する基礎的研究 : 距離減衰式の整理と理論地震動を用いた検討

We study attenuation characteristics of peak ground motions and relative velocity response spectra of ground motions with predominant surface waves in a period range of about 2 to 20 sec. First, we point out that it is important to know how dispersion of surface waves is in order to understand attenuation relations of surface waves. Then, we calculate attenuation relations of theoretical seismograms due to a shallow point source buried in the velocity structure used in the study by Mamula, et al. (1984) on site amplification factors of many Meteorological observatories in a period range of 3 to 15 sec. As a result, we point out that the idea that attenuation of surface waves is much smaller than that of body waves, which is wide-spreaded in studies on attenuation relations of earthquake ground motions in Japan, is not always appropriate.

Seismic site amplification factors in Campi Flegrei, Southern Italy

The method of spectral ratios of S waves from local small earthquakes recorded at various sites in the Campi Flegrei volcanic area (Southern Italy) has been used to evaluate the seismic site response in the frequency range 1–15 Hz. The spectra of the S-wave trains recorded at the short-period seismic stations are corrected for attenuation using attenuation parameters (quality factor Q) previously determined for the area using the coda method. A reference station has been used, selected on the basis of the highest number of recorded earthquakes and not on geological considerations. The relative uniformity of the geological conditions and the absence of hard-rock sites conditioned such choice. The method has been checked using the coda-wave method to estimate the spectral site response, and similar results were obtained. The results show a relative peak of the spectral ratio pattern centred at low frequency (2–3 Hz) and a minimum of 0.2 at 8 Hz for two stations located on yellow tuff. Such results are interpreted as being caused by a filtering effect produced by selective attenuation. Such a pattern could be of interest for seismic engineering purposes, in view of the abundance of tuff over a wide area in this densely inhabited region.

Frequency dependence of site amplification factor in the Messina Strait area

The spectral ratio method is used here for evaluating the frequency dependence of a site amplification factor in the Messina Strait area. All stations exhibit the same features of spectral ratios evidencing two peaks at about 10 Hz and 16 Hz. We relate this observation to the same geological structures. In fact, all the stations (except the referenced one) are situated over pleistocenic sediments with a similar grain size. This causes the disappearing of any site effect when the average spectrum is used as a reference. MES station, situated in the city of Messina, presents a clear site effect at a different frequency (6 Hz) which cannot be related to any geological structure because no substantial difference is observed between the sediments on which this station is situated and the sediments on which the other stations are situated. MT1 station exhibits a spectral peak at about 2 Hz which can be explained with oscillations of a little sedimentary basin.

Frequency-dependent site amplification factors using the S-wave coda for the island of Hawaii

Abstract The coda method has been utilized to determine the seismic response at various sites on the island of Hawaii using data from the Hawaiian Volcano Observatory (HVO) seismic network. Site amplification factors are calculated using spectral ratios of S -wave coda amplitude measured for many events and lapse times between 33 and 59 sec for frequencies between 1.5 and 15.0 Hz. We find that these site amplification factors vary by factors of as much as 12 for a given frequency due to differences in absorption and/or impedance properties beneath the site. Similar to results from Japan, central California, and Long Valley, the coda site amplification factors showed strong correlation with station residuals for magnitude determined by Johnson (unpublished results) for the HVO network between 1.5 and 6.0 Hz. Johnson (unpublished results) made an independent determination of event magnitudes and station site corrections by measuring averaged windowed amplitudes of the S wave and its coda using a method previously tested with data from the southern California seismic network (Johnson, 1979). Station calibrations were simultaneously determined and included the effects of instrument gain and site effect. After correcting Johnson9s (unpublished results) results for the instrument gain provided by HVO, we obtain correlations similar to the previous works, with the best correlation appearing at 3.0 Hz. Despite the complex subsurface structures on the island of Hawaii, the frequency dependence of our coda site amplification factors displays a remarkable regional trend. In some cases, plausible causes related to the underlying geologic structures can explain the similar frequency dependence; in other cases, the similarity cannot be explained or may be coincidental. Similar frequency dependences among stations accompanied by a large variability in the absolute level of the site effect may be due to inaccurate instrument calibrations.

The relation between site amplification factor and surficial geology in central California

Abstract Site amplification factors at frequencies of 1.5, 3, 6, and 12 Hz were determined for 132 stations of the USGS seismic network in central California from coda waves of 185 local earthquakes in this area using a recursive stochastic inversion method. We found that the site amplification at a station is systematically related to the geology underlying that station. The site amplification is high for young, Quaternary sediments and decreases with increasing geologic age at all frequencies between 1.5 and 12 Hz. The rate of decrease varies with frequency where site amplification at low frequencies shows a faster rate of decrease with age than at higher frequencies. To quantify the relation between site amplification factor and site geology, the surface geology of station sites was classified into five groups, namely, (1) Quaternary sediments, (2) Pliocene sediments, (3) Miocene through Cretaceous sediments, (4) Franciscan formation and Mesozoic granitic rocks, and (5) Pre-Cretaceous metamorphic rocks. The station site amplification factors for each group were logarithmically averaged and the mean value was assigned to the median geologic age of that group. A smooth power law relation was found between the mean site amplification and the median geologic age. This relation provides a simple way of estimating site effect at a specific site with known surficial geology. Our site amplification factors were compared with magnitude station residuals determined by Eaton (1990) for the same network. A remarkable linear correlation was found between the logarithmic coda amplification factor and the magnitude station residual value, confirming that the coda method provides an effective means of estimating site amplification factor. Comparison of our site amplification factor with strong-motion results obtained from the Loma Prieta earthquake (Boore et al. , 1989; Maley et al. , 1989; Shakal et al. , 1989; Chin and Aki, 1991) suggests that weak and strong-motion site amplification factors correlate well in the region outside the epicentral source region beyond epicentral distances of about 50 km. Within the epicentral source region, however, the weak motion amplification factors estimated from coda waves do not agree with the observed for strong ground motion, suggesting a nonlinear behavior at sediment sites. These results together with studies on the correlation between weak-motion site amplification and earthquake intensity (Borcherdt, 1970; Borcherdt and Gibbs, 1976; Chavez-Garcia et al. , 1990; King et al. , 1990) demonstrate the importance of in situ determination of weak-motion site amplification for seismic zonation.

Site amplification fromS-wave coda in the Long Valley caldera region, California

AbstractFollowing the work of Phillips (1985), we have computed site amplification factors for coda waves at many sites in the Long Valley region in the eastern Sierra Nevada. We computed ratios of coda amplitudes measured at 15 stations in and around Long Valley caldera relative to a granitic site, MMPM, for six frequency bands centered at 1.5, 3.0, 6.0, 9.0, 12.0, and 15.0 Hz. All station sites located within the caldera experienced large ground motion amplification at 1.5 and 3.0 Hz, ranging between five and 17 times that of the reference site. However, at higher frequencies, these same sites exhibited significantly less amplification than the reference granite site. This is attributed to the competing effects of an impedance contrast between the basement rock and caldera fill and very high absorption in the caldera fill at high frequencies. Station MMLM, located on top of a volcanic plug, displayed the largest amplitudes of all the sites studied for frequencies between 9.0 and 15.0 Hz. A dike structure attached to the plug couples the basement rock to the surface. At high frequencies, the resulting large amplitudes at MMLM are not due to amplification resulting from a strong impedance contrast; rather, the absorption under this site is very low, perhaps lower than at the reference site, MMPM. Outside the caldera, another hard-rock site located at Devil's Postpile, MDPM, generally behaved like the reference site for all frequencies. The lowest amplifications observed came from a site outside the caldera, MDCM, located on thin pyroclastic ash deposits overlying granitic basement. This can be attributed to a dominance of absorption over the amplification caused by lower impedance of this layer. Variations among sites on similar surface geology may be due to small local variations in impedance and absorption under and adjacent to the site. The range in the spectral decay parameter, κ, between caldera and rock sites are comparable to results of Anderson and Hough (1984) for sites on alluvium and rock in the San Fernando region. These surprisingly different amplifications support the need for additional site-specific studies. Amplifications determined in this study for the frequency range 1.5 and 3.0 Hz correlate remarkably well with Eaton's (1990) residuals for duration magnitude, FMAG, and amplitude magnitude, XMAG, for the USGS northern California seismic array, further supporting the use of coda waves in determining site-specific amplification.

Site Amplification of S-Waves from Strong Motion Records in Special Relation to Surface Geology.

Source, propagation path, and local site effects in observed strong motion records are separated by an inversion method to examine the relation between local site effects and surface geology. S-wave portions of accelerograms in horizontal components observed at 19 stations for 22 events along the Pacific coast of Southern Tohoku and Kanto districts are analyzed. Magnitudes and focal depths of the events range from 4.0 to 7.0, and from 30 to 60 km, respectively. The hypocentral distances from 43 to 243 km. The observation sites are classified into 5 groups based on their surface geologies. There are obvious differences among the site amplification factors Gj(f) of these groups in the frequency range from 1 to 5 Hz. Sites on granite, whose shear-wave velocity VS is 2, 000 to 2, 700 m/s, show a Gj(f) of 2 to 3. Those on Cretaceous to Paleogene sedimentary rock, with a Vs of 1, 000 to 1, 600 m/s, show a Gj(f) of 2 to 6. The Neogene (VS= 500 to 800), the diluvial (VS = 450 m/s), and alluvial (VS = 200 m/s) sites show Gj(f) values of 5 to 20, 8 to 30, and about 40, respectively. The strong dependence of Gj(f) on shear-wave velocity of the surface stratum of the sites indicates that variations in impedance are responsible for the differences in amplification factors. The degree of the dependence on the shear-wave velocity for most sites on the sedimentary strata grows weak as frequency increases from 5 to 10 Hz. This tendency may be due to the effect of the near-site attenuation expressed by the Q-value of the ground.

エアガン実験で観測されたコーダ波の振幅特性

We have carried out a seismic refraction experiment around Bandai volcanic area, the southern part of Tohoku, by using a large capacity marine airgun. Analysis of observed seismograms from the airgun shots shows that the coda wave amplitude at each observation site is independent of hypocentral distance. This indicates that the coda wave energy is homogeneously distributed in the medium beneath the refraction profile. The coda wave amplitude correlates well to the amplitude of the direct P wave from distant earthquakes and inversely correlates to the Bouguer anomaly. The relative site amplification of the coda wave is nearly the same as that of the direct P wave, and is considered to be related to the average density or the thickness of sedimentary layer just beneath the observation site. By using the coda wave amplitude at each observation site, we can evaluate the relative site amplification. Correction of the site amplification factor gives a smooth and less scattered distribution of the P wave amplitude from the airgun shots along the refraction profile.