RUI: Modeling the Earthquake Dynamics of Complex Thrust Fault Systems, with Application to the Western Transverse Ranges, California

The University Corporation, Northridge, Northridge CA

Investigators

Abstract

One of the central questions of earthquake hazard assessment is, "how big an earthquake can this fault system produce?" The answer often depends on how many of the system's fault segments can break together in a single earthquake. If fault segments are close together and oriented in a certain way, a rupture on one segment can hop to another and keep going, leading to a much larger quake than the individual fault segment could produce. There is a large body of previous work using computer simulations to understand multiple-segment rupture in strike-slip fault systems like the San Andreas, where motion along the fault is sideways. However, there is practically no research on "dip-slip" fault segments that experience some up and down displacement across the fault. In this two-phase project, Lozos and his team of four CSUN undergraduate researchers will take on this problem. First, the undergraduates will run a series of test models to determine some physics-based rules on when an earthquake rupture is likely to continue to a nearby segment. The results of these simulations will help Lozos interpret his more complex simulations of a network of dip-slip (thrust) fault segments in the western Transverse Ranges north of Los Angeles, California. These faults pose a serious hazard to the Los Angeles region, and identifying the likelihood of multi-fault ruptures and the maximum likely earthquake size will have an immediate impact on rupture forecasts and estimates of maximum expected ground shaking in the region. The size and path of an earthquake rupture through a fault system is affected by many factors, but fault geometry has consistently proven – through empirical, observational, and computational studies alike – to be one of the strongest first-order controls on rupture behavior and length. Understanding the physics of how fault geometry controls rupture behavior, and developing a set of physics-based rules for which geometrical complexities are more or less likely to stop a rupture, is therefore an important part of earthquake hazard assessment. Geometrical parameter studies – in which most initial on- and near-fault conditions are extremely simplified in order to isolate the effects of a single type of geometrical complexity – have been used to assess rupture behaviors and lengths on strike-slip faults for several decades. The asymmetry of dip-slip (thrust/reverse and normal) faults, however, means that the same geometry-based constraints on rupture behaviors developed through studying strike-slip faults may not apply to them. This means that dip-slip-specific parameter studies are necessary to better understand the hazard associated with these fault systems. This project centers around using 3D finite-element simulations of the dynamic rupture process to explore geometrical controls on reverse-faulting earthquakes. The first phase consists of a series of geometrical parameter studies isolating along-strike and down-dip bends, and discontinuities between pairs of faults; this phase will heavily involve undergraduate students, since the relative simplicity of these studies makes them a good introduction to the philosophy and techniques of computational earthquake dynamics research. The second phase involves using the results of these parameter studies to inform the setup and interpretation of a site-specific modeling study on a complex network of thrust faults in the Western Transverse Ranges, southern California: a fault system that poses significant hazard to population centers in Los Angeles and Ventura counties. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.