Publication Date:
2004
Description:
We use previously determined direct-wave attenuation functions as well as
stable, coda-derived source excitation spectra to isolate the absolute S-wave site
effect for the horizontal and vertical components of weak ground motion. We use selected
stations in the seismic network of the eastern Alps. A detailed regional attenuation
function derived by Malagnini et al. (2002) for the region is used to correct the
vertical and horizontal S-wave spectra. These corrections account for the gross path
effects (i.e., all distance-dependent effects), although the source and site effects are
still present in the distance-corrected spectra. The main goal of this study is to
isolate the absolute site effect (as a function of frequency) by removing the source
spectrum (moment-rate spectrum) from the distance-corrected S-wave spectra. Typically,
removing the S-wave source spectrum is difficult because of inadequate corrections for
the source radiation pattern, directivity, and random interference. In addition to
complexities near the source, 2D and 3D structure beneath the recording site will result
in an azimuth-dependent site effect. Since the direct wave only samples a narrow range
in takeoff and backazimuth angles, multistation averaging is needed to minimize the
inherent scatter. Because of these complicating effects, we apply the coda methodology
outlined by Mayeda et al. (2003) to obtain stable moment-rate spectra. This methodology
provides source amplitude and derived source spectra that are a factor of 3-4 times more
stable than those derived from direct waves. Since the coda is commonly thought of as
scattered energy that samples all ray parameters and backazimuths, it is not very
sensitive to the source radiation pattern and 3D structure. This property makes it an
excellent choice for use in obtaining average parameters to describe the source, site,
and path effects in a region. Due to the characteristics of the techniques used in this
study, all the inverted quantities are azimuthally averaged, since the azimuthal
information is lost in the processing. Our results show that (1) all rock sites
exhibited deamplification phenomena due to absorption at frequencies ranging between 0.5
and 12 Hz (the available bandwidth), on both the horizontal and vertical components; (2)
rock-site transfer functions showed large variability at high-frequency; (3)
vertical-motion site transfer functions show strong frequency dependence; (4)
horizontal-to-vertical (H/V) spectral ratios do not reproduce the charactersitics of the
true horizontal site transfer functions; and (5) traditional, relative site terms
obtained by using reference rock sites can be misleading in inferring the behaviors of
true site transfer functions, since most rock sites have nonflat responses due to
shallow heterogeneities resulting from varying degrees of weathering. Our stable source
spectra are used to estimate the total radiated seismic energy and to compare against
similar results obtained for different regions of the world. We find that the
earthquakes in this region exhibit nonconstant dynamic stress drop scaling, which gives
further support for a fundamental difference in rupture dynamics between small and large
earthquakes.
Keywords:
Horizontal to vertical spectral ratio
;
Nakamura
;
Spectrum
;
NOISE
;
Site amplification
;
Shear waves
;
Radiation pattern
;
Deconvolution
;
BSSA
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