NORTHRIDGE AFTERSHOCKS, A SOURCE STUDY WITH TERRASCOPE DATA

Broadband and long-period displacement waveforms from a selection of N orthridge aftershocks recorded by The TERRAscope array are modeled to study source characteristics. Source mechanisms and moments are determ ined with long-period data using an algorithm developed by Zhao and He lmberger (1994), These results are compared with those by Hauksson et al, (1995) and Thio and Kanamori (1994). The width of the direct pulse s al the nearest stations PAS and CALB are measured as indications of the source duration, Another measurement of the source-lime functions of these earthquakes is obtained by comparing the short-period to long -period energy ratio in the data to that in the synthetics. These meas urements are used to estimate the relative stress drop using a formula given by Cohn et al. (1982). The depth distribution of tile relative stress drops indicates that the largest stress drops are in the depth range of 5 to 15 km for an after-shock population of 24 events. A corr elation of extended surface wave train with source depth is demonstrat ed for paths crossing the San Fernando basin.