SEISMOLOGICAL EVIDENCE FOR NONLINEAR ELASTIC GROUND BEHAVIOR DURING LARGE EARTHQUAKES

Amplification of earthquake-induced seismic waves by soft superficial deposits often causes significant damages in the urban areas. In predi cting this effect for large future earthquakes, the linear elastic res ponse of soils is customarily assumed. To check this assumption, we ha ve analyzed surface and downhole acceleration data from the SMART1 and SMART2 strong motion arrays in Taiwan, covering peak accelerations of up to 0.3 g. First, frequency-dependent amplification induced by the alluvial deposits at the SMART1 array was estimated using spectral rat io technique, where the records at rock site were taken as a reference motion. Statistically validated reduction in soil amplification in th e strong motion relative to the weak motion in the frequency range bet ween approximately 1 and 9 Hz was detected. Secondly, relative site re sponses between the Pleistocene and recent sedimentary deposits at the SMART2 array were studied. Relative amplification was shown to be cle arly dependent on the excitation level. Thirdly, we compared experimen tally recorded uphole/downhole spectral ratios on weak and strong grou nd motion with the theoretical response yielded by the geotechnical co de DESRA2 which assumes hysteretic constitutive relationship of soil. Major symptoms of nonlinear ground behavior predicted by the model wer e found in the observed data. Back-calculation of the shear wave veloc ities to the depth of 47 m shows nearly 50% decrease in the strongest quakes, also accounted for by the nonlinear soil behavior.