A new method recently developed by Hoshiba et al. [1991] was used to separate the effects of scattering Q-1 and intrinsic Q-1 from an analysis of the S wave and its coda in Hawaii, Long Valley, and central California. Unlike the method of Wu [ 1985], which involves integration of the entire S wave energy, the new method relies on the integration of the S wave energy for three successive time windows as a function of hypocentral distance. Using the fundamental separability of source, site, and path effects for coda waves, we normalized the energy in each window for many events recorded at many stations to a common site and source. We plotted the geometric spreading-corrected normalized energy as a function of hypocentral distance. The data for all three time windows were then simultaneously fit to Monte Carlo simulations assuming isotropic body wave scattering in a medium of randomly and uniformly distributed scatterers and uniform intrinsic Q-1. In general, for frequencies less than or equal to 6.0 Hz, scattering Q-1 was greater than intrinsic Q-l, whereas above 6.0 Hz the opposite was true. Model fitting was quite good for frequencies greater than or equal to 6.0 Hz at all distances, despite the model's simplicity. The small range in energy values for any particular time window demonstrates that the site effect can be effectively stripped away using the coda method. Though the model fitting generally worked for 1.5 and 3.0 Hz, the model has difficulty in fitting the whole distance range simultaneously, especially at short distances. Despite the poor fit at low frequency, the results generally support that in all three regions the scattering Q-1 is strongly frequency dependent, decreasing proportional to frequency or faster, whereas intrinsic Q-1 is considerably less frequency dependent. This suggests that the scale length of heterogeneity responsible for scattering is at least comparable to the wavelength for the lowest frequencies studied, of the order of a few kilometers. The lithosphere studied in all three regions can be characterized as a random medium with velocity fluetuarion characterized by exponential or Gaussian autocorrelation functions which predict scattering Paper number 91JB03094. 0148-0227/92/91 JB-03094505.00 Q-1 decreasing proportional to frequency or faster. For all frequencies the observed coda Q-1 is intermediate between the total Q-1 and expected coda Q-1 in contrast with theoretical results for an idealized case of uniform distribution of scatterers and homogeneous absorption which predict that coda Q-1 should be close to the intrinsic Q-1. We will discuss possible causes for this discrepancy. Soc., 82, 57-80, 1985. Wu, R. S., and K. Aki, Multiple scattering and energy transfer of seismic waves --Separation of scattering effect from intrinsic attenuation, II, Application of the theory to Hindu Kush region, Pure At)t)l. Geophys., 128, 49-80, 1988. Zeng, Y., F. Su, andS[. Aki, Scattering wave energy propagation in a medium with randomly distributed isotropic scatterers, 1, Theory, J. Geoph...
Following 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.
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