Comparison of slip distribution of large slow slip events in Guerrero subduction zone

Aseismic slow slip events (SSEs) have been reported in most of the well geodetically instrumented subduction zones worldwide (Japan, Cascadia, Mexico, New Zealand, Costa Rica, Alaska). For most of the observed SSEs, the slip distribution on the subduction interface was inferred from the surface GPS displacements to be located at the downdip extension of the seismogenic zone, in the conditionally stable frictional regime. Hence, the stress transfer due to the SSEs on the seismogenic zone should advance the occurrence time of the next strong subduction earthquake. Since the installation of the GPS network, the Guerrero gap area, Mexico, has been affected by large SSEs in 1998, 2002, 2006. We have analyzed the Mexican permanent GPS data describing the 2002 and 2006 SSEs, so that to determine and compare their slip distribution. The resolution analysis made on those data shows that the slip distribution is dependent on the number and location of the GPS stations. The slip distribution inferred for the 2006 SSE, for which the GPS network was denser, is thus much better resolved than that for the 2002 event. The inversion result is not unique. However, for each event, all possible solutions draw a similar, overall slip distribution pattern. Both events are similar in terms of equivalent moment magnitude (7.4 to 7.6) and of maximum slip amplitude on the interface (up to 20 cm). By contrast, the size and location of the slip zone differ from one event to the next: In 2002, the zone sustaining slow slip spreads through the entire Guerrero subduction segment plus the western part of the Oaxaca subduction segment (~400 x 150 km); the slip is quite evenly distributed over the entire zone, though maximum slip concentrates both at the down dip limit of the seismogenic zone and in the stably slipping region. In 2006, slip occurs only in the Guerrero gap thus over an area twice smaller (~230 x 150 km) than before; in the west, the maximum slip is located both in the seismogenic zone and at its downdip extension, whereas in the east, the maximum slip localizes in the transition and stably slipping zone. Our results thus show that slip distribution and location of maximum slip markedly differ from one large SSE to the next. This suggests that the 2002 and 2006 events are parts of a complex multi-events sequence. Furthermore, in both events, slow slip occurred both in the seismogenic zone and below it. This demonstrates that SSEs play a role in the seismic cycle that is more complex than previously thought. This makes them a crucial element to be taken into account in seismic hazard assessment.