The Role of Multiple Foreshock Structures in Facilitating the Rupture of the Mw 5.0 Earthquake Swarm Mainshock near Urban Reno, Nevada in 2008

Abstract:

We resolve complex spatial and temporal evolution of a long-duration earthquake sequence on multiple fault planes by combining high-precision relative relocations with over 350 first-motion focal mechanisms and applying physics-based, open-source software supported by both the USGS and CIG (Computational Infrastructure in Geodynamics) to investigate static and quasi-static stress changes during an 8 week foreshock period. Low-magnitude earthquake activity began on February 28, 2008 in the Mogul neighborhood west of urban Reno, NV and continued to build in intensity leading to the M_W 5.0 mainshock on April 26, 2008. Initial locations are obtained by applying station and datum corrections to analyst-picked travel times using HYPOINVERSE then relocating the sequence via the HypoDD double-difference algorithm of Waldhauser and Ellsworth (2000) using both cross-correlated and catalog phase data on P and S arrivals. The final relocation set reveals a mechanism-supported temporal and spatial migration of seismicity on several distinct structures prior to the mainshock rupture. First-motion focal mechanisms are computed using the HASH code of Hardebeck and Shearer (2002). We apply the spatial and temporal stress inversion (SATSI) software (Hardebeck and Michael, 2006) to investigate variations in the stress field throughout the sequence. We integrate preliminary analysis of stress drops (M_L ≥ 3.0) using an EGF-derived spectral ratio technique. Finally, we determine significant fault planes identified in the foreshock sequence and build a local 3D fault model. We systematically test various slip scenarios on these structures using both CIG’s finite-element crustal deformation code PyLith and the USGS-supported Coulomb 3 program to explore how the foreshocks promote or inhibit failure on subsequent structures that may facilitate mainshock rupture.