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Estimation of Local Site Effects on Strong Ground Motion

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Estimation of Local Site Effects on Strong Ground Motion CUREe - Kajima Research Project Final Project Report Estimation of Local Site Effects on Strong Ground Motion By Dr. Masanori Niwa Prof. Keiiti Aki Dr. Masayuki Takemura Prof. Ta-Liang Teng Mr. Kenichi Kato Mr. Tomonori Ikeura Mr. Kenji Urao Dr. Masamitu Miyamura Prof. Etsuzo Shima Dr. Tokiharu Ohta Report No. CK 91 - 01 February 1991 California Universities for Research Kajima Corporation in Earthquake Engineering ( CUREe) CUREe (California Universities for Research in Earthquake Engineering) • California Institute of Technology • Stanford University • University of California, Berkeley • University of California, Davis • University of California, Irvine • University of California, Los Angeles • University of California, San Diego • University of Southern California Kajima Corporation • Kajima Institute of Construction Technology • Information Processing Center • Structural Department, Architectural Division • Civil Engineering Design Division • Kobori Research Complex CUREe-KAJIMA RESEARCH PROJECT ESTIMATION OF LOCAL SITE EFFECTS . ON STRONG GROUND MOTION Keiiti Aki and Ta-Liang Teng Department of Geological Sciences University of Southern California January 15, 1990- January 14, 1991. Table of contents Part I. Relation between the weak-motion amplification factor and the site geo- logical age for the frequency range from 1.5 Hz to 12 Hz. ............................... l. Part II. Applicability of the weak-motion amplification factor to strong ground motion during the Loma Prieta earthquake of 1989 ........................................ 21. List of publications. ·····························································••o••······················· 65. Plan. for future publications. ........................................................................... 66. Acknowledgment. .. ., .......... ~·eo• •••••••• ··········••••••••••••••••••••••••••• ••••o••••••••• ••••••••••••••• 67 . .. Part I. Relation between the weak-motion amplification factor and the site geological age for the frequency range from 1.5 Hz to 12 Hz. Remarkably consistent results have changed from empirical studies of strong ground motion data about the effect of local geology on peak ground acceleration, peak ground velocity and response spectra, both in U.S. and Japan. Aki (1988) summarized these results in the following observations: the site amplification factor on response spectra depends on the frequency of ground motion. Soil sites show higher amplification than rock sites by a factor of 2 to 3 for periods longer than about 0.2 seconds, which the relation is reversed for periods shorter than about 0.2 seconds. This frequency dependence is reflected in the site dependence of peak ground motions. Peak ground velocity and displacement as well as the Arias intensity show high amplifications for soil sites than rock sites, which peak ground acceleration is independent of the site classification. Aki (1988) also recognized a similar reversal in the weak motion amplifica­ tion factor determined by Phillips and Aki (1986) between the granite site and the fault-zone sediment site, but not between the Franciscan rock site and non-fault­ zone sediment. In order to establish more firmly the dependence of amplification on site geology at various frequencies, we extend the work of Phillips and Aki to a greater amount of better calibrated data from the USGS central California network using a new efficient inversion method. We inverted for the relative site amplification factors in central California from coda waves of local earthquakes with magnitudes ranging between 1.8 and 3.5 recorded by the USGS seismographic network. A 90 percent variance reduction was achieved after inversion. We found that the site amplification of a station is controlled by the surface geology under­ lying that station. In general, the site amplification is high for young, Quarterary 1. and Tertiary Pliocene sediments. The amplification decreases with increasing ge- ology age. The decreasing rate is different for different frequency bands. The low frequencies show sharper decreasing rates than those of high frequencies. Surprisingly, we did not find any reversal at least up to 12 Hz for the weak motion amplification factor. On the average, younger soil sites show greater am- plification at all frequencies up to 12 Hz. This is clearly different from the trend observed for strong ground motion as mentioned earlier. In the following, we shall briefly describe the method used in the present study and summarize the result obtained. The Method Generally, the observed coda wave decay rate is very stable, independent of source-receiver location, whereas the amplitude of the coda waves is source and site dependent (Aki, 1969; Aki and Chouet, 1975). The power spectrum of the coda wave P(wlt) can be considered as a product of three factors: P(wlt) = source(w) · site(w) · path(wlt) (1) where w is the circular frequency and tis the lapse time measured from the event origin time. Take a natural log to both side of (1), we have (2) where ri(w,) is the site term, Sj(w,) is the source term, and c(w,, tk) is the coda propagation term which has been shown to be independent of source and receiver locations. Index i, j, k and l represent the station, source, lapse time and frequen­ cy, respectively. By taking an average of dijkl over all the available stations with 2. fixed indices j, k and l and then subtracting it from the original dijkl values, we arrive at the following equatioll. (3) We can further write ri = ~m Dimrm and fi = 1/Njkl ~m Imrm, where N;1c1 is the total number of usable stations for fixed indices j, k and l and Im = 1 if station m is used, or Im = 0 otherwise. Substituting these expression into (3), we obtain (4) m m where G(K) =Dim- Im/N;kl and K is a function ofindex j,k and l. For a given frequency (fixed index 1), equation (4) gives us a system oflinear equations for different stations, sources and lapse time points which could be used to determine the relative site amplification factors. In practice, the matrix provided by the linear equations above could be very sparse. A singular value decomposition and generalized inverse technique is not only expensive but also inefficient for solving such linear equations. In this work, we used a recursive inversion method (Zeng, 1990) to determine the site amplification factors, which leads to tremendous computer time saving in our inverse process. In addition, with new data points we can always revise the solution according to the recursive inverse process and avoid the redundant computation once again over the whole data set. The Results A total of 175 earthquakes recorded from 1984 to March, 1990 by the short­ period seismic system of the USGS Menlo Park seismographic network were col­ lected for this study. The magnitude of these earthquakes ranges form 1.8 to 3. 3.5. The distribution of the earthquakes and the stations are plotted in Figure 1. Figure 2a through d are geographical distribution of the natural log of the site amplification with respect to the average station for the frequency bands centered at 1.5, 3.0, 6.0, 12.0 Hz. In these figures, we used six different symbol represent_. ing six different site amplification ranges. For frequency 1.5 Hz, we can see that the highest amplification site are in the area of Watsonville and fault zone near Hollister. The surface geology of these area are composed mostly by Quaternary sediments. The Diablo range covered by the Franciscan formation has low am­ plification. It shows low site amplification for all frequency bands we studied as can see from figure 2a to 2d. The Gabilan range and the southwest area of th«? N aciminento faults have the lowest site amplification for frequency 1.5 Hz, but their site amplification slowly increased with increasing frequency. These areas are mostly composed of Mesozoic granitic rocks and Pre-Cretaceous metamorphic rocks. The site amplification along the San Andress fault are more complicated, since the geology condition along the fault zone changes dramatically. In gener­ al, we found that the site amplification of a station is strongly controlled by its underlying surface geology To quantify how different geology condition affects the station site amplifi­ cation, we divided all the stations into five groups according to their underlying geological age. These five groups are Quaternary sediments, Tertiary Pliocene, Tertiary Miocene up to Cretaceous marine, Franciscan formation and Mesozoic granite rocks and Pre-Cretaceous metamorphic rocks. The station site amplifica­ tion data in each group were averaged and the mean value was assigned to the middle age of that group. Figure 3 plots these mean values as well as their stan­ dard error of the mean. From figure 3 we can see that the site amplification is 4. high for young, Quarterary and Tertiary Pliocene sediments. The amplification decreases with increasing geology age. The decreasing rate is different for different frequency b~ds. The low frequencies show sharper decreasing rates than those of high frequencies. Table 1lists all the station site amplification values we obtained. The typical standard errors are about 0.065 for frequency 1.5 Hz, 0.056 for 3 Hz, 0.051 for 6 Hz, and 0.050 for 12 Hz. The surface geology for each station were obtained from CDMG 1:250,000 scale geology map. From the table, we can see that although the general trends of the site amplification decreasing with increasing geology age is clear, some variation within the same geology condition group also can be seen. These could be due to both the complex geological structure and the surface topography. The surface topography may sometimes play an even important role in these variations. Our inverted site amplification factors were compared with FMAG (duration magnitude) and XMAG (amplitude magnitude) site corrections determined by Eaton (1990) for the USGS Menlo Park network. A remarkable linear correlation was found between our results and the magnitude site correction values confirming the applicability of coda amplification factor to the waves on which magnitude is based. As shown in figure 4, the correlation coefficient between our results and FMAG site correction is 0.9 for frequency 1.5 Hz and 0.89 for 3 Hz.
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