Slow slip transients along the Oaxaca subduction segment from 1993 to 2007

We use data from 12 continuous GPS stations in southern Mexico, including eight new stations, to better characterize transient slip episodes along the Mexican subduction zone. Continuous GPS recording in Oaxaca that began 14 years ago, constituting the longest continuous GPS record in southern Mexico, defines nine distinct episodes of transient slip from 1993 to 2007, including previously unreported transient slip episodes in early 1995 and 2006. All transient slip episodes recorded in Oaxaca City were also recorded at a GPS site ?400 km away in the state of Guerrero after measurements began there in early 1997, demonstrating that transient slip affects widespread areas of southern Mexico. Well-recorded transients in 2004 and 2006 appear to have originated along the Oaxaca trench segment, hundreds of kilometres from the potentially hazardous Guerrero seismic gap. During the 2004 slip transient, displacements of more than 20 mm were recorded at stations in the Oaxaca GPS array, a factor of 2-3 larger than at sites in Guerrero. Elastic half-space modelling of the 2004 displacements at sites throughout southern Mexico indicates that most slip was focused beneath Oaxaca in deeper areas of the subduction interface immediately downdip from previous megathrust rupture zones, but that slip also may have extended significantly updip into a seismic gap along the Oaxaca coast. The best-fitting model is also able to explain transient vertical offsets of up to 30 mm observed during this event. Shortly before the onset of transient slip in 2004, a pair of moderate (Mw 5.1, 5.5) earthquakes ruptured the downdip end of the seismogenic zone immediately east of the Oaxaca seismic gap, suggesting a possible relationship between the two. In 2006, transient slip began 1-3 months earlier in Oaxaca than in other areas, offering the clearest evidence to date for the existence of one or more source regions for transient slip outside the Guerrero seismic gap, where large amplitude transient slip originated in 2002. The still-limited data indicate that transient slip that originates elsewhere in Mexico may trigger aseismic slip in Guerrero, and further indicate that transient slip beneath Oaxaca is limited to areas of the subduction interface that surround the rupture zones of previous large shallow-thrust earthquakes. " 2007 The Author Journal compilation " 2007 RAS