Fall AGU 2004 Abstract

What do Seismicity Streaks and Holes Reveal About the Distribution of Seismic and Aseismic Slip?

Justin L. Rubinstein and Gregory C. Beroza

Monday 1:55 pm -- Session S13E

Many studies have shown that faults have "holes," i.e., regions of an otherwise active fault that are devoid of microseismicity, both in the aftershock sequences of large earthquakes (e.g., Mendoza and Hartzell, 1988) and during the interseismic interval (e.g., Oppenheimer et al., 1990). Seismicity holes also appear between more recently discovered "streaks" of seismicity on the Calaveras, Hayward, and San Andreas faults in California. Ellsworth et al. (2000) have made a convincing case that two streaks on the San Andreas fault near Parkfield delimit a stuck patch that has been partially ruptured by several magnitude 4+ events in the early 1990's. We examine these same features using precise earthquake relocations for the Calaveras fault. The Calaveras fault has a number of streaks and holes in its seismicity distribution and with the geometry of locked vs. slipping regions more difficult to discern than it is on the San Andreas fault at Parkfield. Our working hypothesis is that the streaks illuminate the transition from creeping to locked portions of the fault. We can test this by examining medium magnitude (M ­Y 3.5) earthquakes, which we expect to rupture inward from the streaks into areas devoid of microearthquakes, i.e. areas that were previously locked. Double difference relocations show medium sized earthquakes within these streaks, but clipping makes it difficult to determine earthquake locations as accurately for these events. To overcome this problem, we use a first-break master-event cross correlation method to improve hypocentral locations of these larger earthquakes that represent where these moderate magnitude events initiate. Analysis of accelerometer and short-period seismometer records provides finite faulting information, which will allow us to constrain the propagation direction relative to these hypocenters.