Theoretical earthquake nucleation, with applications to creep fronts, tremor, and slow slip

Princeton University, Princeton NJ

Investigators

Abstract

ABSTRACT This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Traditionally, faults were thought to accommodate slip in one of two ways: Stick-slip motion, in which long periods where the fault is essentially locked are punctuated by brief episodes of rapid slip (earthquakes), and steady creep at plate tectonic rates (centimeters per year) or less. The discoveries within the last decade of episodic slow slip and associated tremor within numerous subduction zones, and tremor along the San Andreas fault, show that fault behavior is much more varied. In episodic slow slip, faults slip at rates 1 to 2 orders of magnitude larger than plate tectonic rates at quasi-regular intervals on the order of 1 year. Because the slip episodes last for several weeks and extend over areas tens to hundreds of kilometers across, they release energy equivalent to earthquakes of magnitude 6 or more. Tremor is a quasi-continuous seismic signal thought to be made up of myriad small events similar to regular earthquakes of magnitude 1.5 or less except in being more sluggish. Because slow slip events increase the stressing rate on the shallower locked portion of faults that can slip in magnitude 9 earthquakes, understanding them may prove to be useful for earthquake hazards reduction. In addition, the increased understanding of slip localization that arises from this work can be used to interpret ongoing experiments monitoring earthquake nucleation at several kilometers depth in California and South Africa. Over the past several years the PI has worked with Jean-Paul Ampuero to understand the implications of the standard rate-and-state friction equations for earthquake nucleation on deformable faults. They derived analytic solutions that have been very useful for understanding complex numerical simulations. This proposal is to extend this work to the interpretation of slow slip and tremor in subduction zones and along the San Andreas fault, using both numerical and analytical techniques. A consensus is emerging that slow slip and tremor are just different manifestations of slip on a heterogeneous interface. Propagating creep fronts seem to be a ubiquitous feature of slow slip on heterogeneous faults, and are very effective at generating seismic slip speeds when they collide. Both properties make creep fronts attractive as tremor sources. The first goal of this study is to address how far such creep fronts might propagate in isolation, and the necessary conditions for two colliding fronts to generate seismic slip. This will be carried out both for standard rate-andstate friction and for other constitutive laws that have been proposed for slow slip. Once the creep fronts collide, they can generate seismic events with moment rate histories that, depending upon the underlying constitutive law, are quite different from those that seismologists have come to expect. This will be studied initially using the ?radiation damping? approximation to elastodynamics and ultimately full elastodynamics. Because much information is currently coming from observations of tidally- and teleseismically-triggered tremor, both forms of triggering will be modeled.