Estimating Absolute Site Effects

by Luca Malagnini, Kevin Mayeda, Aybige Akinci, and Pier Luigi Bragato

Abstract

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.

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