Fault branching and rupture directivity

Abstract

[1] Could the directivity of a complex earthquake be inferred from the ruptured fault branches it created? Typically, branches develop in forward orientation, making acute angles relative to the propagation direction. Direct backward branching of the same style as the main rupture (e.g., both right lateral) is disallowed by the stress field at the rupture front. Here we propose another mechanism of backward branching. In that mechanism, rupture stops along one fault strand, radiates stress to a neighboring strand, nucleates there, and develops bilaterally, generating a backward branch. Such makes diagnosing directivity of a past earthquake difficult without detailed knowledge of the branching process. As a field example, in the Landers 1992 earthquake, rupture stopped at the northern end of the Kickapoo fault, jumped onto the Homestead Valley fault, and developed bilaterally there, NNW to continue the main rupture but also SSE for 4 km forming a backward branch. We develop theoretical principles underlying such rupture transitions, partly from elastostatic stress analysis, and then simulate the Landers example numerically using a two-dimensional elastodynamic boundary integral equation formulation incorporating slip-weakening rupture. This reproduces the proposed backward branching mechanism based on realistic if simplified fault geometries, prestress orientation corresponding to the region, standard lab friction values for peak strength, and fracture energies characteristic of the Landers event. We also show that the seismic S ratio controls the jumpable distance and that curving of a fault toward its compressional side, like locally along the southeastern Homestead Valley fault, induces near-tip increase of compressive normal stress that slows rupture propagation.

Finite Element Modeling of Branched Ruptures Including Off-Fault Plasticity

“…The surface rupture of the 1992 Landers earthquake activated multiple fault branches and illustrates the complexities of a rupture path (Sowers et al, 1994). Fliss et al (2005) studied backwards branching in this event and Bhat et al (2007) considered the interaction between the main fault and finite length branches. In addition to these strike-slip studies, Kame et al (2003) addressed the presence of branched faults in thrusting regime and Templeton et al (2010) investigated branch activation during normal faulting.…”

Earthquake Recurrence in Simulated Fault Systems

“…Previous modeling studies of earthquakes and slip in geometrically complex faults include investigation of slip of wavy faults (SAUCIER et al, 1992;CHESTER and CHESTER, 2000), slip through idealized fault bends (NIELSEN and KNOPOFF, 1998), rupture propagation into fault branches (OGLESBY et al, 2003;FLISS et al, 2005), and rupture jumps across gaps (HARRIS et al, 1991;DUAN and OGLESBY, 2006;SHAW and DIETERICH, 2007). Seismicity simulations that implement region-specific models of fault systems (WARD, 1996(WARD, , 2000RUNDLE et al, 2004;ROBINSON and BENITES, 1995) have demonstrated that plausible seismicity models can be implemented that replicate basic characteristics of regional seismicity.…”

Landers 1992 "reloaded": Integrative dynamic earthquake rupture modeling

“…Peyrat et al, 2001;Aochi and Fukuyama, 2002;Fliss et al, 2005;Heinecke et al, 2014;Wollherr et al, 2018]. Together with detailed analysis of the recorded strong ground motions [e.g., Campbell and Bozorgnia, 1994], rupture transfer mechanisms [e.g., Wesnousky, 2006;Madden and Pollard , 2012;Madden et al, 2013] and potential energy release [e.g., Dreger , 1994;Wald and Heaton, 1994] a comprehensive picture of the source kinematics and macroscopic earthquake properties has been developed.…”