Site Amplification of S-Waves from Strong Motion Records in Special Relation to Surface Geology.

Abstract

Source, propagation path, and local site effects in observed strong motion records are separated by an inversion method to examine the relation between local site effects and surface geology. S-wave portions of accelerograms in horizontal components observed at 19 stations for 22 events along the Pacific coast of Southern Tohoku and Kanto districts are analyzed. Magnitudes and focal depths of the events range from 4.0 to 7.0, and from 30 to 60 km, respectively. The hypocentral distances from 43 to 243 km. The observation sites are classified into 5 groups based on their surface geologies. There are obvious differences among the site amplification factors Gj(f) of these groups in the frequency range from 1 to 5 Hz. Sites on granite, whose shear-wave velocity VS is 2,000 to 2,700 m/s, show a Gj(f) of 2 to 3. Those on Cretaceous to Paleogene sedimentary rock, with a Vs of 1,000 to 1,600 m/s, show a Gj(f) of 2 to 6. The Neogene (VS= 500 to 800), the diluvial (VS = 450 m/s), and alluvial (VS = 200 m/s) sites show Gj(f) values of 5 to 20, 8 to 30, and about 40, respectively. The strong dependence of Gj(f) on shear-wave velocity of the surface stratum of the sites indicates that variations in impedance are responsible for the differences in amplification factors. The degree of the dependence on the shear-wave velocity for most sites on the sedimentary strata grows weak as frequency increases from 5 to 10 Hz. This tendency may be due to the effect of the near-site attenuation expressed by the Q-value of the ground.

Frequency-dependent Attenuation of High-frequency P and S Waves in the Upper Crust in Western Nagano, Japan

“…For this purpose, it is necessary to investigate the frequency-dependent characteristics of both Q −1 P and Q −1 S in various regions and depths. Historically, the study on the crustal seismic attenuation of S waves has been more advanced than that on P waves (CASTRO et al, 1990;TAKEMURA et al, 1991;KATO et al, 1992;KI-NOSHITA, 1994). It is mostly due to the requirement of the engineering seismology.…”

High-Frequency Site Effect of Hard Rocks at Ashio, Central Japan.

“…For sediment sites in particular, many efforts have been paid from the engineering point of view to clarify their amplification behaviors, and large site amplification at low frequencies (around a few Mertz and below) were revealed as compared with that at hard-rock sites (e.g., Aki, 1989;Boatwright et al, 1991;Takemura et al, 1991). On the other hand, there have been few studies on the hard-rock site effect (e.g., Tucker et al, 1984;Cranswick, 1988), and it is generally considered that hard-rock site effects are much smaller than those of sediment sites. However, quantitative estimation of high-frequency source characteristics such as the corner frequency and spectral fall-off is much affected by small distortion of seismograms due to the site effect as well as propagation path effect (e.g., Yoshimoto et al, 1993).…”

A Hybrid Inversion Technique for the Evaluation of Source, Path, and Site Effects Employing S-Wave Spectra for Subduction and Upper-Crustal Earthquakes in El Salvador

“…They also suggested that this trade-off does not significantly affect the source and site terms in the inversion scheme. In the case of the subduction earthquakes, Takemura et al (1991) successfully inverted the slab data in Japan constraining to ‫0.1מ‬ the geometrical attenuation term, matching the source spectra with the x 2 model and the site effects with available geotechnical data at the strong-motion stations.…”