New Large Aseismic Slow Slip Event in Guerrero, Mexico

Thrust type aseismic slip events (transient slips or "aseisms") have recently been reported in various subduction zones. The observed aseisms have equivalent Mw of 6-7 and their duration varies from several days to a few years. An important feature of these slow events is that they usually occur near or just downdip of the seismogenic portion of the megathrust. The aseisms may be an important part of the seismic cycle of large thrust subduction zone earthquakes. In Guerrero, Mexico, we detected several such events of different magnitude in 1972, 1979, 1995, 1998, and 1999-2000 using tide gauge, leveling, and GPS data. In 2001-2002 the network of 7 continuous GPS stations recorded a new large aseismic slow event (equivalent Mw ~7.3) in Guerrero, which apparently started in the beginning of October 2001 and its peak slip phase lasted from the end of December 2001 until the end of April 2002. The surface deformation associated with this slip was observed on an area of more than ~350x200 km². The maximum velocity of the recorded crustal motion, Vh = 14.6 cm/yr, Az = 213.3°, Vz = 6.5 cm/yr, is on the CAYA GPS station, located on the coast in the central part of the Guerrero seismic gap. The steady interseismic rate at this station was Vh = 2.3 cm/yr, Az = 45.5°, Vz = -1.2 cm/yr before the aseism. There are at least two models that can fit the observed deformations during the new Guerrero aseismic event. The first model predicts an average thrust slip of ~13 cm and a temporary regional extension of the continental block. The aseismic slip occurred on nearly the entire plate interface, approximately within 30 to 220 km from the trench. The slip zone includes the shallow seismogenic interface and the deeper subhorizontal interface, which is usually considered as aseismic. Elastic half space dislocation models of the deformation observed several years before (interseismic locking) and during the last aseism reveal three distinct segments on the plate interface, which are distinguished by a coherent change of coupling and aseismic slip values. The second possible model concentrates the larger aseismic slip on the shorter interface, from 90 to 180 km from the trench, and leaves the upper shallow seismogenic interface totally or partially locked. This would imply that while the new aseism could release a significant amount of the entire elastic strain energy previously accumulated in the subduction zone below Guerrero, the seismic hazard on the Guerrero gap has not been reduced by this event.