CONTINUOUS MEASUREMENTS OF MICROTREMORS ON SEDIMENTS AND BASEMENT IN LOS-ANGELES, CALIFORNIA

Continuous measurement of microtremors at two sites on basement rock a nd sediments was carried out in Los Angeles, California, in order to u nderstand the fundamental characteristics of microtremors. A predomina nt peak with a period of about 6.5 sec was found in the microtremor sp ectra in both media. The spectral amplitude of the peaks varied gradua lly with time in a similar manner at the two sites. Their time-variant characteristics are in agreement with change in oceanic swell height observed at an oceanic buoy in the southwest of Los Angeles. This sugg ests that they originate from an oceanic disturbance. On the other han d, a clear dally variation of spectral amplitudes at a period of 0.3 s ec indicates that short-period microtremors are caused by cultural noi ses. It was found that the spectral ratio of long-period microtremors between the basement and the sediments was repeatable, although the sp ectral amplitudes at the two sites were time-variant. The spectral rat io of the long-period microtremors was similar to that derived from st rong motion records. This suggests the applicability of spectral ratio s of microtremors to assess the effects of deep sediments on long-peri od earthquake ground motion.