Characteristics of Large Earthquakes Occurring on the Shallowest Portion of the Mexican Subduction Megathrust

The large magnitude and enormous tsunami of the 2011 Tohoku earthquake came as a surprise to the seismological community. An event of this magnitude had not been documented in the zone for more than 1000 years, and the events in the previous 100 years were of Mw 7.8 or smaller. In particular, the very large slip of up to 60 m near the trench [Ito et al., 2011; Kido et al., 2011; Sato et al., 2011], was unexpected. Similarly, the Mexican subduction interface has repeatedly ruptured in magnitude Mw 7-8 earthquakes in the last 100 years [e.g. Singh et al., 1981, Kostoglodov and Pacheco, 1999]. Most of the events have rupture areas centered on the coast, breaking a depth range from 10-30 km and not the shallowest, near trench portion. However, in 1787, an M 8.6 earthquake, broke the Oaxaca segment of the subduction zone, causing a tsunami that reached more than 5 km inland [Suarez and Albini, 2009], suggesting that the near trench area broke in this event. In order to learn more about the seismic behavior of the shallowest part of subduction interface, we have studied the four largest events that have been suggested to break that portion of the fault in Mexico; the Mw 8.0, 1995 Jalisco earthquake [Hjorleifsdottir et al, to be submitted], the Mw 6.7, 2002 Guerrero [Flores et al., to be submitted], and the Mw 7.2 and Mw 6.7, 1996 and 1997 Offshore Oaxaca earthquakes. We confirm that these events do break the shallowest part of the fault interface and observe that they do not have a smooth source time function as observed for tsunami earthquakes [Kanamori and Kikuchi, 1993; Polet and Thio, 2003], but a rugged one, suggesting a rupture of multiple separate asperities. The Jalisco event ruptured updip, with an average rupture velocity of 2.5 km/s, with the shallow rupture breaking much slower. The Guerrero event on the other hand, broke a 60 km long and 10-20 km wide section, in at least 2 asperities, with a rupture velocity around 1 km/s. The Mw 6.7 Offshore Oaxaca earthquake is remarkable for the very strong water layer reverberations, which suggests a rupture occurring within a few kilometers from the trench. Our results show that the near trench area in the Mexican subduction zone is seismogenic, at least in several areas, but that the distribution of frictional properties may be more heterogeneous than in subduction zones where tsunami earthquakes have been observed.