SLIP DISTRIBUTION OF THE 2010 APRIL 4th BAJA CALIFORNIA, MEXCO EARTHQUAKE CONSTRAINED USING TELESEISMIC BODY AND SURFACE WAVES AND HIGH RATE GPS

We study the complex rupture history of the 2010 April 4 Mw 7.3 Baja California earthquake by simultaneously inverting local high rate GPS waveforms, teleseismic body and long period surface waves. The GPS displacements are included as well. We have first constructed a 1D velocity structure from the 3D SCEC CVM-H model to represent the structure of the source region and have corrected SH and surface wave propagation effects using nearby 2009 Dec 30 M5.8 foreshock. To compress the basin reverberation effects which our velocity model could not reproduce yet, all high rate GPS waveforms were lowpass filtered to less than 0.1 Hz before the inversions. Our result reveals a complex rupture process of the 2010 event. It had a weak initiation. The energetic rupture started 6 s after the initiation as a normal fault sub-event near the hypocenter and then ruptured bilaterally to both southeast and northwest directions. The rupture to the northwest is well resolved by the extensive high rate GPS waveforms. The preferred fault plane has a strike of 313^o and a dip angle of 80^o. The rupture is very heterogeneous, changing from pure strike-slip motion to oblique motion from epicenter to the northwest, consistent with surface observations. The rupture stopped sharply 50 km northwest of epicenter. The inverted peak slip is 5 m and seismic moment of this segment is 5x10²⁶ dyne.cm. The rupture to the southeast is still under investigation. Our preliminary result suggests that this rupture extended as far as 50 km to the N133^oE with two large asperities. The first one is close to epicenter and the second one is 45 km southeast, with a peak slip of 4 m. Our inversion also requires a large normal motion subevent (Mw 6.9) on a separating fault plane with a lower dip angle (65^o) to match the long period surface waves. The total seismic moment is 1.2x10²⁷ dyne.cm, with most of slip occurring 18 to 40 s after the rupture initiation.