SOURCE CHARACTERISTICS OF THE 17 JANUARY 1994 NORTHRIDGE, CALIFORNIA,EARTHQUAKE FROM REGIONAL BROAD-BAND MODELING

Broadband regional records are modeled to determine source mechanism, seismic moment, fault dimension, and rupture directivity for the 17 Ja nuary 1994 Northridge earthquake. Modeling is done using both theoreti cal Green's functions (tGf) and empirical Green's functions (eGf). Fro m the theoretical modeling, we obtain a source mechanism with strike 1 28 degrees, dip 33 degrees, and rake 106 degrees for the mainshock, us ing a source estimation algorithm by Zhao and Helmberger (1994). While the fault orientation seems resolvable from regional data, the moment estimation is less reliable due to inadequate synthetic waveform fits to the observed surface waves. This appears to be caused by the combi nation of propagational effects and fault complexities. Further invest igation of the source characteristics is carried out with a new method of using eGf's. As an eGf, we select the 17 January 1994 17:56 GMT af tershock, which occurred near the onset of the mainshock and had a sim ilar source mechanism. The source duration of the mainshock, as seen f rom the regional surface waves observed at various stations, is obtain ed by searching for the trapezoidal far-field source-time function for each station that, when convolved with the aftershock data, best simu lates the mainshock data. Stations to the north record shorter source durations than stations to the south. Modeling these with theoretical predictions of rupture on a square fault, we constrain the effective f ault dimension to be 14 km with rupture along the direction of the ave rage rake vector. A moment of (1.4 +/- 0.9) X 10(26) dyne-cm with a st ress drop of similar to 120 bars is obtained for the mainshock from ou r eGf study.