GPS Constraints on the Mw=7.5 Ometepec Earthquake Sequence, Southern Mexico: Coseismic and Postseismic Deformation
Friday, 19 December 2014: 9:30 AM
We use continuous GPS measurements from 31 stations in southern Mexico to model coseismic slip and postseismic deformation from the 20 March 2012 Mw=7.5 Ometepec earthquake, the first large thrust earthquake to occur below central Mexico during the modern GPS era. Coseismic offsets ranging from ~280 mm near the epicenter to 5 mm or less at sites far from the epicenter are fit best by a rupture focused between ~15 km and 35 km depth, consistent with an independent seismological estimate. Transient postseismic motion recorded by GPS sites as far as 300 km from the rupture has a different horizontal deformation gradient and opposite sense of vertical motion than do the coseismic offsets. A forward model of viscoelastic relaxation as a result of our new coseismic slip solution incorrectly predicts uplift in areas where postseismic subsidence was recorded and indicates that viscoelastic deformation was no more than a few percent of the measured postseismic deformation. The deformation within six months of the earthquake was thus strongly dominated by fault afterslip. The postseismic GPS time series are well fit as logarithmically decaying fault afterslip on an area of the subduction interface up to 10 times larger than the earthquake rupture zone, extending as far as 220 km inland. Afterslip had a cumulative geodetic moment of 2.0×1020 N·m, ~40% larger than the Ometepec earthquake. Tests for the shallow and deep limits for the afterslip require that it included much of the earthquake rupture zone as well as regions of the subduction interface where slow slip events and non-volcanic tremor have been recorded and areas even farther downdip on the flat interface. We examine whether aftershocks accommodated a significant fraction of the shallow postseismic slip, but find that the energy released by aftershocks accounted for no more than 10% of the postseismic moment release at any depth. Widespread afterslip below much of central Mexico suggests that most of the nearly flat subduction interface in this region is conditionally stable and thus contributes measurable transient deformation to large areas of Mexico south of and in the volcanic belt.