Rupture History of September 30, 1999 Intraplate Earthquake of Oaxaca, Mexico (Mw=7.5) from Inversion of Strong-Motion Data in the Frequency Domain
N.M. Shapiro, S.K. Singh, J. Pacheco, A. Iglesias, V. Cruz, J.M. Gómez
Abstract. We have studied the rupture history of the intraplate, normal-faulting Oaxaca earthquake of Sep 30, 1999 (Mw=7.5) using near-source strong-motion data and a frequency domain inversion scheme. We assume an elastic layered half-space model for the medium and two focal mechanisms corresponding to the Harvard and the NEIC CMT solutions. The hypocenter and the origin time of the earthquake (16.000N, 97.020W, H=39.7 km; 16h31min12.7s) were determined from local and regional data. For each focal mechanism we estimated the slip distribution on the fault and the associated rupture front propagation, as well as the rise time. Several inversions were carried out with different starting rupture parameters to test the uniqueness of the solution and to estimate the reliability of each parameter. The results of our inversion show that the rupture mainly propagated from ESE to WNW but also slightly downdip, with an average rupture velocity close to 3 km/s. The rise time (local duration of the rupture) ranges between 1 and 2s. The slip distribution on the fault is concentrated in two interconnected patches with a maximum slip of 2.5m located at about 20 km and 40 km WNW of the hypocenter. Most of the slip is occurs at an average depth of 45 km. A small asperity with a maximum slip of 2m is also observed close to the hypocenter. The total seismic moment is 1.8x10^20 Nm.