Criticality in Regional Seismicity: How Do We Test It and What Does It Mean?

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Abstract for proposal EAR0105405 (PH # 43x)Title: Criticality in Regional Seismicity: How Do We Test For It and What Does It Mean?PI's: C. Sammis and H. Saleur, University of Southern CaliforniaThere is mounting observational evidence that large earthquakes are followed by tens of years of regional quiescence of intermediate-size events, and preceded by a comparable time period of accelerating seismic release. This acceleration (due mostly to an increase in intermediate-size events) can be fit to a power law of the time-to-failure equation, which offers the possibility of predicting large earthquakes on an intermediate-term basis, of perhaps a few years. The radius R of the optimally accelerating region scales with magnitude as log R ~ 1/2 M, which is equivalent to R being proportional to fault length L. This phenomenology has been interpreted as signifying the approach of a region to, and retreat from, a critical state. Power law scaling and growth of the correlation length of the stress field leading to increasingly larger events are expected behavior when approaching a critical point. We plan to devise and implement quantitative observational tests of this analogy between regional seismicity and critical phenomena. This does not require having a detailed understanding of why earthquakes should be critical phenomena. Criticality, like self-organized criticality, leads to universal features like power laws and scaling relations, that are independent of the mechanisms involved. To develop the tests, we will explore the evolution of the spatial, temporal, and magnitude distributions of seismicity that are predicted by critical point models. We shall be especially interested in the behavior of the smaller events, which have not, as yet, been used to help monitor the approach to criticality.