Investigating the Presence of Preseismic Velocity Changes

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

Earthquakes strike without warning and are some of the most destructive and devastating forces of nature in terms of loss of life. In May, 2008 the Wenchuan earthquake in China killed 80,000 people with 20,000 still missing. The 2004 Sumatra earthquake generated a giant tsunami that left 225,000 casualties in its wake, affecting eleven countries. In 1976 the Tangshan earthquake in China again, killed 240,000 people, the deadliest in recorded history. Statistics compiled at Columbia University indicate that of all natural disasters world-wide including floods, droughts, hurricanes, tornadoes, and fires?earthquakes rank the highest for total number of casualties. Many of the other natural disasters provide some type of warning so that at least lives can be saved even if property damage is extensive. Understandably people have diligently sought for some clues to foretell of impending great earthquakes in hopes of alleviating some of this loss and suffering. But earthquake prediction remains an extremely challenging problem. Most of the evidence of precursors is anecdotal and has not been scientifically verified in a rigorously controlled setting that produces reproducible results.

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.

Both laboratory experiments and theories have concluded that there should be measureable velocity changes in the crust preceding large earthquakes. They have yet to be consistently and reliably measured in the field. The reason for this may be due to lack of high quality data since we don?t know where and when the next earthquake will strike. However, an extremely well controlled experiment at Parkfield has recently observed velocity changes preceding two earthquakes presenting the most convincing evidence to date that these predicted changes do occur in the field. But conducting many more of these types of experiments is economically infeasible and so alternative strategies are needed to confirm the existence of preseismic velocity changes before other large earthquakes.

The question that seismologists are most often asked by the lay public is, 'Can we predict earthquakes yet?' The answer is not currently. While it is a far off goal, the general population clearly cares about this issue and desires that a portion of the resources they provide would be allocated towards that end. Engineers can build safer buildings to reduce the loss of life substantially, but in many of the poorer countries which also happen to be the ones that suffer the greatest number of casualties, this is economically prohibitive and so they would benefit the most from better earthquake forecasting.