Growth of a Structure Connecting the 2010 M 7.2 El Mayor – Cucapah Rupture with the Elsinore Faul

Abstract:

The M 7.2 El Mayor – Cucapah earthquake occurred on 4 April 2010 in the northern part of Baja, Mexico. The rupture extended about 120 km from near the northern tip of the Gulf of California to the US – Mexican border south of the Elsinore fault zone. Most of the aftershocks occurred within days of the main event. On 14 June 2010 a M 5.7 late aftershock occurred 8 km southeast of Ocotillo, CA and is the largest aftershock in the sequence. The right-lateral event occurred in a cluster of aftershocks and was followed by its own aftershock sequence. UAVSAR data were collected for a swath covering the aftershock on 13 April, 2010 just after the El Mayor – Cucapah earthquake and before the earthquake on 21 October 2009. The line was reflown 1 July 2010 after the M 5.7 14 June 2010 aftershock. Data have been continued to be collected semi yearly to yearly since then. Repeat Pass Interferomety (RPI) products spanning the aftershock show the growth of a lineament that with an azimuth of 121.5° or a strike of -58.5°. The interferograms suggest that a stepover develops following the earthquake. The epicenter of the M 5.7 aftershock is proximal to the linear discontinuity in the postseismic interferogram and the mechanism of the event is consistent with slip on this stepover. Inversions for slip on the northeast linear structure that steps west of the mainshock rupture yield a moment magnitude ranging from 5.5 – 5.8, which is consistent with the magnitude of the aftershock. Slip occurs at a depth of 2-10 km on a steeply dipping fault.