InSAR AND GPS MEASUREMENTS OF CRUSTAL DEFORMATION FROM THE EL MAJOR EARTHQUAKE: LIQUEFACTION AND TRIGGERED SLIP

Our work is focused on recovering the co-seismic and postseismic deformation of the rupture using a combination of radar interferometry and GPS. Most of the surface rupture and damage occurred south of the US - Mexico Border where there are no continuously operating GPS stations and few stable monuments for measuring coseismic and postseismic deformation. Beginning on April 5 we installed four continuously operating GPS stations on the east, south, and west sides of the main rupture. In addition we began a GPS field campaign to collect post-earthquake deformation measurements on the 11 monuments within 50 km of the main rupture. The vector deformation measurements will be used to calibrate the co- and post-seismic interferograms being collected by ALOS and ENVISAT. The combined data will ultimately be used to invert for subsurface slip.

Initial InSAR results show three main features: 1) The main rupture in the Sierra Cucapah and El Major consists of at least two concentrations of right-lateral and east-side down normal faulting. Interferograms are decorrelated in the steep mountain areas where the combination of ground acceleration and steep slope produces major surface slides. 2) The southwest extension of the rupture goes beneath the agricultural area of the Colorado River Delta. This area sustained major damage. More important the ALOS interferogram reveals the boundaries of the liquefaction zone in the agricultural areas southeast of the fault zone. This zone of liquefaction is bounded on the east by the Cerro Prieto fault and on the west by the Laguna Salada fault. The zone is 18 kilometers wide and 60 km long. Field observations show that the roads have undulations with vertical amplitudes of 20-50 cm and most of the concrete lined aqueducts are fractured. Interferograms are decorrelated over much of the area although azimuth offsets help to delineate the zone of deep slip. 3) The ENVISAT interferograms show triggered slip on several strands of the northern Cerro Prieto Fault near the border, the Superstition Hills Fault, the Imperial Fault north of Interstate 8, the Coyote Creek Fault and three sections of the San Andreas Fault. One linear deformation zone southeast of Bombay Beach is either slip on a southern strand of the San Andreas Fault or subsidence along the Lake Cahuilla shoreline. We hope to establish the style and amount of triggered slip in each case using interferograms with multiple look directions.

Also co-authored on this abstract is Z. Ma.