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.