Paper No. 3
Presentation Time: 1:30 PM
BREAKING THE OCEANIC LITHOSPHERE OF A SUBDUCTING SLAB: THE 2013 MW7.7 KHASH, IRAN EARTHQUAKE
BARNHART, William D.1, HAYES, Gavin P.
2, SAMSONOV, Sergey V.
3, FIELDING, Eric J.
4 and SEIDMAN, Lily E.
2, (1)U.S. Geological Survey, National Earthquake Information Center, 1711 Illinois St, Golden, CO 80401, (2)National Earthquake Information Center, USGS, PO Box 25046, MS-966, Denver, CO 80225, (3)Natural Resources Canada, Ottawa, ON K1A0Y7, (4)Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, wbarnhart@usgs.gov
Large intermediate-depth, intraslab normal-faulting earthquakes are a common and hazardous phenomena in subduction zones, but are poorly understood owing to a paucity of near-source geophysical observations. Here, we use high-quality interferometric synthetic aperture radar (InSAR) and teleseismic waveform observations to explore the finite-rupture characteristics of the 2013 April 14 M
w7.7 Khash earthquake, which nucleated at a depth of 82km in the Makran subduction zone. We use three co-seismic interferograms acquired by the RADARSAT-2 C-band instrument from two independent tracks which fully span the surface displacement region. InSAR displacements are inverted using model-resolution based variable discretization for fault slip, while teleseismic waveforms are modeled using a 1D velocity model and the approach of Ji et al. (2002). Additionally, we derive a 3D slab geometry model for the Makran from various geophysical constraints.
Finite-fault slip models derived independently from each data source exhibit similar spatial rupture characteristics and require a rupture width of at least 50km, with the majority of slip occurring in the subducted oceanic mantle. These models, along with the hypocentral depth, suggest that at least half of the subducted lithosphere, including the mantle and entire crust, ruptured during the earthquake - confirming with high resolution that this earthquake nucleated from a neutral plane and ruptured the portion of the slab undergoing extension. Both data sets are best-fit by the north-dipping focal plane oriented at an oblique angle to the trench. Combining our earthquake slip distributions with the causative fault orientation and geometry of the local subduction zone, we assess the potential activity of several mechanisms for the generation of mantle and intermediate depth earthquakes. We hypothesize that the Khash earthquake likely occurred as the combined result of slab bending stresses acting on a preexisting fault that formed prior to subduction and that was weakened by dehydration of hydrous minerals.
