Sediment non-linearity and attenuation of seismic waves: A study of accelorograms from Lefkas, western greece

Abstract

We investigate the dependence of the S-wave high-frequency spectral-decay parameter, K ("kappa") - a measure of wave attenuation - on ground-motion amplitude, 21 three-component accelerograms from two adjacent sediment sites in the town of Lefkas, western Greece, are used, representing 17 earthquakes with magnitudes Mw 4.7-7.0 and hypocentral distances 12-93 km. Recorded peak horizontal ground accelerations (PGA) and velocities (PGV) are 22-540 cm/s2 and 1.3-54.5 cm/s. Fourier amplitude spectra are computed for S-wave windows, and the frequency range is visually determined where the high-frequency spectral decay can be approximated by a straight line on the linear-log plot; its slope (and hence K) is computed by linear regression. K is found to depend on hypocentral distance as K = 0.108+0.058R (r = 0.518). As PGV increases from 1.3 to 54.5 cm/s, K0 (K at 0 km, characteristing inelastic attenuation in the site's subsurface geology) varies between 0.060 and 0.160 s. K0 is found to correlate very strongly with log MGA (r = 0.645) (MGA - mean horizontal acceleration in the S-wave window) but also with log PGA (r = 0.447) and log PGV (r = 0.627). We attribute this behaviour to sediment non-linearity (shear-modulus degradation), resulting in the decrease of the site's dominant-resonance frequency (from about 3.5 to 2.4 Hz) and leading to the increase of K0. Our results imply that at sediment sites, an important contribution to K comes from wave attenuation (damping) in the softest sediments and show that KO is amplitude dependent, thus being a measure of sediment non-linearity. © 2001 Elsevier Science Ltd. All rights reserved.