Anomalous Seismicity and Accelerating Moment Release Preceding the 2001 and 2002 Earthquakes in Northern Baja California, Mexico CHARLES G. SAMMIS 1 ,DAVID D. BOWMAN 2,3 , and GEOFFREY KING 3 Abstract — An algorithm recently developed by RUNDLE et al. (2002) to find regions of anomalous seismic activity associated with large earthquakes identified the location of an M w = 5.6 earthquake near Calexico, Mexico. In this paper we analyze the regional seismicity before this event, and a nearby M w = 5.7 event, using time-to-failure algorithms developed by BOWMAN et al. (1998) and BOWMAN and KING (2001a,b). The former finds the radius of a circular region surrounding the epicenter that optimizes the time-to-failure acceleration of seismic release. The latter optimizes acceleration based on the expected stress accumulation pattern for a dislocation source. Both methods found a period of accelerating seismicity in an optimal region, the size of which agrees with previously proposed scaling relations. This positive result suggests that the Rundle algorithm may provide a useful technique to identify regions of accelerating seismicity, which can then be analyzed using signal optimization time-to-failure techniques. Key words: Seismicity, seismic hazard assessment, earthquake prediction, earthquake physics, earth- quake stress interactions. Introduction Many large earthquakes are preceded by a regional increase in seismic energy release. This phenomenon, called ‘‘accelerating moment release’’ (AMR), is due primarily to an increase in the number of intermediate-size events and occurs within a distance R of the main shock that scales with magnitude. AMR has been observed before large earthquakes in many locations (see summary by JAUME ´ and SYKES, 1999). All of these observations are ‘‘postdictions’’ in the sense that the location of the main event was known and used as the center of the search pattern. The challenge is to use AMR in a predictive mode. One approach has been to use a circle of radius R (corresponding to a prescribed magnitude M in the scaling relation described below) to search a grid for accelerating activity (see PAPAZACHOS and PAPAZACHOS, 2001; 1 Department of Earth Sciences, University of Southern California, Los Angeles, CA 90089-0740, U.S.A. E-mail: sammis@earth.usc.edu 2 Department of Geological Sciences, California State University at Fullerton, Fullerton, CA 92834- 6850 U.S.A. E-mail: dbowman@fullerton.edu 3 Laboratoire de Tectonique, IPGP, 4, Place Jussieu, 75252 Paris, Cedex 5, France. Pure appl. geophys. 161 (2004) 2369–2378 0033 – 4553/04/122369 – 10 DOI 10.1007/s00024-004-2569-3 Ó Birkha ¨ user Verlag, Basel, 2004 Pure and Applied Geophysics