GPS constraints on the 2011-2012 Oaxaca slow slip event that preceded the 2012 March 20 Ometepec earthquake, southern Mexico

Abstract

We model measurements from 19 continuous GPS stations to determine the location and magnitude of a slow slip event (SSE) below southern Mexico that began in late 2011 and remained active up to the 2012 March 20 Mw = 7.4 Ometepec earthquake. Modelling of the space–time evolution of the SSE indicates that it initiated in 2011 November, migrated westward ∼2.6 km per day along the subduction interface, and reached the eventual earthquake source region ∼1 month before the 2012 March 20 earthquake occurred, in the waning stage of the SSE. The maximum slip for the SSE, ∼100 mm, occurred ∼100 km east of the earthquake rupture zone, in contrast to slip of 10–20 mm proximal to the Ometepec rupture zone. The SSE was focused downdip from the seismogenic zone everywhere along its ∼300-km-wide slip region and had a cumulative moment release of 3.0 × 1019 N•m (Mw = 6.9), similar to SSEs in 2004 and 2006 along this same area of the subduction interface. We calculate Coulomb stress changes as a result of slip during the SSE and find small but positive stress changes for the source region of the Ometepec earthquake. Our results are consistent with the hypothesis that the SSE triggered the Ometepec earthquake, although they are insufficient to demonstrate causality.