Investigating the Presence of Preseismic Velocity Changes

DESCRIPTION: Professional seismologists have been doing their best to contribute what they could to the challenge of earthquake prediction. There has been a long history of looking for precursory variations in the velocity of seismic waves or other waveform attributes preceding large earthquakes. The search for preseismic velocity changes has been called the holy grail of seismology. This project will continue taking small steps in that pursuit. Progress has been made. It is now scientifically well-documented that coseismic and postseismic velocity changes have been measured by a variety of means for several large earthquakes all demonstrating consistent behavior. The data in these studies, however, do not have the temporal sampling required to confirm the existence or non-existence of a preseismic velocity change. The findings are inconclusive simply because of a lack of data. We present in this proposal three different strategies designed to greatly improve the insufficient temporal sampling in the preseismic period. They involve new data and new technologies that are state-of-the-art and have recently come on the scene. The first approach is to do a comprehensive analysis of new repeating event sequences that have become available as a source for repeatable waves in the same location. The abundant new data sources for the repeating events substantially improves the temporal sampling for this now standard method for measuring velocity variations. The second approach takes advantage of the hot, new field of ambient seismic noise correlations which just recently has been applied for velocity change monitoring. The technique utilizes continuous data streams from permanent stations and so is able to finely sample velocity variations in the days leading up to large earthquakes. We propose enhancements to this method from our correlation detector research to better be able to extract the signal from the noise. The third strategy is to develop a prototype time-dependent double-difference tomography code to invert for where the changes are occurring spatially and to be able to use the majority of the microseismicity.

OUTCOMES: Didn't find evidence for preseimic velocity changes but was able to place upper bounds on possible changes as being too small, too short of duration, or outside ray coverage.