El Mayor-Cucapah (Mw 7.2) earthquake: Early near-field postseismic deformation from InSAR and GPS observations

Abstract:

El Mayor-Cucapah earthquake occurred on 4 April 2010 in northeastern Baja California just

south of the U.S.-Mexico border. The earthquake ruptured several previously mapped faults, as well as

some unidentified ones, including the Pescadores, Borrego, Paso Inferior and Paso Superior faults in the

Sierra Cucapah, and the Indiviso fault in the Mexicali Valley and Colorado River Delta.We conducted several

Global Positioning System (GPS) campaign surveys of preexisting and newly established benchmarks

within 30 km of the earthquake rupture. Most of the benchmarks were occupied within days after the

earthquake, allowing us to capture the very early postseismic transient motions. The GPS data show

postseismic displacements in the same direction as the coseismic displacements; time series indicate a

gradual decay in postseismic velocities with characteristic time scales of 66 ± 9 days and 20 ± 3 days,

assuming exponential and logarithmic decay, respectively. We also analyzed interferometric synthetic

aperture radar (InSAR) data from the Envisat and ALOS satellites. The main deformation features seen in

the line-of-sight displacement maps indicate subsidence concentrated in the southern and northern parts

of the main rupture, in particular at the Indiviso fault, at the Laguna Salada basin, and at the Paso Superior

fault. We show that the near-field GPS and InSAR observations over a time period of 5 months after the

earthquake can be explained by a combination of afterslip, fault zone contraction, and a possible

minor contribution of poroelastic rebound. Far-field data require an additional mechanism, most likely

viscoelastic relaxation in the ductile substrate.