Abstract

Broadband regional records are modeled to determine source mechanism, seismic moment, fault dimension, and rupture directivity for the 17 January 1994 Northridge earthquake. Modeling is done using both theoretical Green's functions (tGf) and empirical Green's functions (eGf). From the theoretical modeling, we obtain a source mechanism with strike 128°, dip 33°, and rake 106° for the mainshock, using 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 combination of propagational effects and fault complexities. Further investigation 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 aftershock, which occurred near the onset of the mainshock and had a similar source mechanism. The source duration of the mainshock, as seen from the regional surface waves observed at various stations, is obtained by searching for the trapezodial far-field source-time function for each station that, when convolved with the aftershock data, best simulates 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 fault dimension to be 14 km with rupture along the direction of the average rake vector. A moment of (1.4 ± 0.9) × 1026 dyne-cm with a stress drop of ∼120 bars is obtained for the mainshock from our eGf study.

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