Rupture history of September 30, 1999 intraplate earthquake of Oaxaca, Mexico (Mw=7.5) from inversion of strong-motion data

Near-source strong motions are inverted to estimate the rupture history of intraslab, normal-faulting September 30, 1999, Oaxaca, Mexico earthquake. Two focal mechanisms (Harvard and NEIC CMT solutions) are tested for the source geometry. The inversion with the NE dipping fault plane of the Harvard solution best matches the data (strike= 295 degrees, dip= 50 degrees, rake= -82 degrees). We estimated the slip distribution on the fault and the associated rupture front propagation, as well as the rise time. The inversion results show that the rupture mainly propagated from ESE to WNW and slightly downdip, with an average rupture velocity of about 3 km/s. The rise time ranges between 1 and 2 s. The slip distribution on the fault is mainly concentrated in two interconnected patches with a maximum slip of 2.5 m located about 20 km and 40 km WNW of the hypocenter. Most of the slip is released at an average depth of 45 km. A smaller area with a maximum slip of 1.5 m is also observed close to the hypocenter. The total co-seismic moment released is equal to 1.8 x 10(20) Nm.