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

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ABSTRACTThis award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law111-5).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 ofrapid slip (earthquakes), and steady creep at plate tectonic rates (centimeters per year) or less. Thediscoveries within the last decade of episodic slow slip and associated tremor within numeroussubduction zones, and tremor along the San Andreas fault, show that fault behavior is much morevaried. In episodic slow slip, faults slip at rates 1 to 2 orders of magnitude larger than plate tectonicrates at quasi-regular intervals on the order of 1 year. Because the slip episodes last for severalweeks and extend over areas tens to hundreds of kilometers across, they release energy equivalentto earthquakes of magnitude 6 or more. Tremor is a quasi-continuous seismic signal thought tobe made up of myriad small events similar to regular earthquakes of magnitude 1.5 or less exceptin being more sluggish. Because slow slip events increase the stressing rate on the shallowerlocked portion of faults that can slip in magnitude 9 earthquakes, understanding them may proveto be useful for earthquake hazards reduction. In addition, the increased understanding of sliplocalization that arises from this work can be used to interpret ongoing experiments monitoringearthquake 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 implicationsof the standard rate-and-state friction equations for earthquake nucleation on deformablefaults. They derived analytic solutions that have been very useful for understanding complex numericalsimulations. This proposal is to extend this work to the interpretation of slow slip andtremor in subduction zones and along the San Andreas fault, using both numerical and analyticaltechniques.A consensus is emerging that slow slip and tremor are just different manifestations of slip ona heterogeneous interface. Propagating creep fronts seem to be a ubiquitous feature of slow slipon 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 toaddress how far such creep fronts might propagate in isolation, and the necessary conditions fortwo colliding fronts to generate seismic slip. This will be carried out both for standard rate-andstatefriction and for other constitutive laws that have been proposed for slow slip. Once the creepfronts collide, they can generate seismic events with moment rate histories that, depending upon theunderlying 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 andultimately full elastodynamics. Because much information is currently coming from observationsof tidally- and teleseismically-triggered tremor, both forms of triggering will be modeled.