Catalog-constrained models of tremor and slow slip

Princeton University, Princeton NJ

Investigators

Abstract

About 16 years ago the scientific community became aware of two previously unrecognized styles of fault slip. It had been thought that faults at depth either spent most of their time locked, slipping only in brief earthquakes, or else crept along steadily at the plate tectonic rate (centimeters per year). Starting around 2000 it was recognized that along the deep extension of faults in subduction zones, the fault did not simply progress from being locked at shallow depth to creeping at the plate rate at greater depth. Instead, in this intermediate regime the fault periodically accelerated to slip speeds perhaps 100 times the plate tectonic rate, but then slowed down again without producing earthquakes. These "episodic slow slip events" were comparable in energy release to magnitude 6.5 earthquakes, but lasted days to weeks rather than several seconds. In addition, these slow earthquakes (observed geodetically) were accompanied by a new signal seen on seismometers, termed "tectonic tremor". Tremor consists of myriad "low frequency earthquakes" that correspond in energy release to magnitude 1?2.5 earthquakes, but that last roughly 10 times as long (a few tenths of a second). Studies of episodic slow slip and tremor are relevant to earthquake hazards because they represent times of increased stressing rate on the shallow, locked portion of subduction zone faults capable of producing magnitude 9 earthquakes. It has also been proposed that the up-dip extent of tremor might be used to assess the down-dip extent of future great earthquakes, with important implications for ground shaking in Seattle. But we need to understand both slow slip and tremor better before we can confidently translate such ideas into statements about seismic risk. The proposed study seeks to improve our understanding of these styles of fault slip. When episodic slow slip was first discovered, the question was how faults could spontaneously and periodically accelerate, but then decelerate without producing an earthquake. Now, the goal is to use observations of slow slip to distinguish between competing hypotheses for this behavior. For the past several years the researchers have been improving tremor detection/location algorithms, with the aim of generating high-resolution catalogs that expand the observational constraints on slow slip (tremor is useful in this regard because it can be located more accurately than the underlying slow slip). The goal of this proposal is to develop numerical models of slow slip that use these observational constraints as guides. Key observations include: (1) Although the main tremor front propagates laterally along the fault at speeds of 5-10 km/day, secondary tremor fronts arise behind the main front that propagate tens to hundreds of times faster. (2) These secondary migrations take the form of rapid tremor reversals, all of which start at the main front and propagate in the opposite direction, or migrations that propagate along the main front independent of the main front orientation. Some of the largest rapid tremor reversals begin as migrations along the main front. None of the migrations start at the main front and propagate in the long-term propagation direction. (3) Closest to the main front, the tremor migrations are small and recur on timescales far too short to be tidally driven. As these intervals gradually increase from minutes to hours, eventually the largest rapid tremor reversals become modulated by the 12.4-hour tides. This work seeks to reproduce these observations using models of rate- and state-dependent friction, including both relatively homogeneous models and those that explicitly include asperities intended to mimic tremor sources. Emphasis will be placed on understanding analytically why the simulations do what they do, so that their results can be extrapolated beyond the narrow confines of their specific assumptions.