Testing Predictions from the Cascade and Pre-Seismic Slip Models for Foreshock Occurrence with the High Precision Catalog for Northern California

Columbia University, New York NY

Investigators

Abstract

An unanswered question is how do large earthquakes initiate and can their nucleation process be distinguished from small earthquakes? The existence of foreshocks before some large earthquakes is one of the best documented observables preceding mainshocks and therefore may offer clues into these processes. A related question is whether foreshocks are different than any other earthquakes? Foreshocks are the most obvious and well recorded precursors to some large earthquakes. It is essential to understand the physical mechanisms for their occurrence. If a single triggering law can explain the observed behavior for foreshocks, mainshocks, and aftershocks, then foreshocks have no more predictive power than any other earthquake. If, however, they occur as a byproduct and indirect evidence of an aseismic nucleation phase that scales with eventual mainshock magnitude they may have some forecasting ability, in and among themselves, or provide confirmation and support of the pre-seismic slip model predicted by experimental and theoretical research to occur before large earthquakes. Other methods of detecting this nucleation phase may then perhaps be developed. The implications of this research will bear directly on the question of earthquake predictability, thus having broad impact and interest to the general public, but especially among hazard forecasting analysts, governmental agencies, and policy makers. This proposal will test two competing models for the occurrence of foreshocks. The first is the cascade model where foreshocks are a successive series of failures that push the mainshock closer to failure. The second, pre-seismic slip, is one where foreshocks are indirect evidence of a slow, aseismic nucleation phase that is predicted to scale with mainshock magnitude. Both predictions can be evaluated with the high resolution catalog we have developed for northern California (up to three orders of magnitude improvement). Static stress change calculations with improved source parameters and locations can determine if this is a viable mechanism for enhancing or destressing mainshock failure in the cascade model. Conversely measurements of the foreshock zone radius and rupture may show a correlation with mainshock magnitude as predicted in the pre-seismic slip model. Interpretation of these measurements has been difficult before in most prior studies because the location errors were greater than the expected size of the nucleation region. Our preliminary results on a larger data set of 162 foreshock sequences show that there is a negative correlation of foreshock occurrence with normal stress manifested in both the lithostatic load with depth and with regional tectonic stress indicated by the orientation of mainshock slip. Our proposed work will examine if a single, unifying triggering law on theoretical grounds can explain the observations in our data. Clear well-defined predictions of two competing models for foreshock occurrence can now be tested with better quality data and much more of it to determine meaningful statistics and reliable conclusions. These scientific hypotheses were essentially untestable before due to location errors, but now with a large, 300,000 event complete high- precision catalog of northern California, both the static stress triggering mechanism of the cascade model and the aseismic nucleation phase of the pre-seismic slip model can independently be examined if they are consistent with the observed data. It is still not known how large earthquakes initiate and therefore it is important to gain knowledge and insight into the physical processes for immediate foreshocks.