INVESTIGATION OF NONLINEAR SITE AMPLIFICATION AT 2 DOWNHOLE STRONG GROUND MOTION ARRAYS IN TAIWAN

Non-linear seismic response of soil is studied by comparing the spectr al ratios of surface to downhole horizontal accelerations on weak and strong motion. Data from two boreholes are analysed. One is drilled in the alluvial deposits in the south-west quadrant of the SMART1 array. The second one penetrates Pleistocene terrace deposits in the norther n part of the SMART2 array. Observed weak and strong motion spectral r atios are compared with the theoretical ones predicted by the geotechn ical soil model which postulates a hysteretic constitutive law. A sign ificant non-linear response is found at the first site for the events with surface peak acceleration exceeding roughly 0.15g. Deamplificatio n of the strong motion occurred in the frequency range from approximat ely 1 to 10 Hz. The maximum observed difference between the average we ak and strong motion amplification functions of an 11 m-thick near-sur face stratum is a factor of 2.3 Nonlinear response characteristics are in qualitative agreement with the model. An additional corollary is t hat the amplification function calculated from the shear wave coda is equivalent to the average amplification calculated over the ensemble o f small earthquakes. No statistically significant non-linear response is detected on the second array, that is tentatively accounted for by the stiffer soil conditions and weaker accelerations achieved at the S MART2 site. The results indicate that the non-linear amplification can be detectable at certain soil conditions above a threshold accelerati on level.