SOURCE, PATH AND SITE EFFECTS ON DOMINANT FREQUENCY AND SPATIAL VARIATION OF STRONG GROUND MOTION RECORDED BY SMART1 AND SMART2 ARRAYS IN TAIWAN

The reduction in spatial variance of strong ground motion with increas ing earthquake magnitude has been reported recently. However, we show that the observed dependence of spatial variance on magnitude is its i mplicit dependence on the frequency content (dominant frequency) of th e wave field. Time-domain cross-correlations of pairs of accelerograms are used to quantify the spatial variations in this paper. Magnitude is one of the factors contributing to the dominant frequency. We attem pt to study separately the effects of magnitude, hypocentral distance, peak ground acceleration and focal depth on the dominant frequency in order to find the most significant one. The data base consists of 196 5 records of horizontal acceleration from 148 local earthquakes in Tai wan. The analysis shows the overwhelming effect of the source magnitud e on the formation of the dominant frequency with an empirical relatio nship: f(D)(Hz) = 19.9 - 2.84M(L) +/- 1.93 for 3.1 less-than-or-equal- to M(L) less-than-or-equal-to 7.0 No significant effect of hypocentral distance, local acceleration amplitude or depth is detected for all t heir values available (up to 170 km, 250 cm/s2, and 100 km, respective ly). The prevailing effect of magnitude on the dominant frequency is a real cause of the consistently observed reduction of spatial variance of ground motion with increasing magnitude of earthquakes.