HR: 1340h
AN: S13B-1304    [Abstracts]
TI: Susceptibility to Long-Range Earthquake Triggering in California and Japan
AU: * van der Elst, N J
EM: nvanderelst@es.ucsc.edu
AF: Dept. of Earth and Planetary Science, University of California, Santa Cruz, Santa Cruz, CA 95060, United States
AU: Brodsky, E E
EM: brodsky@es.ucsc.edu
AF: Dept. of Earth and Planetary Science, University of California, Santa Cruz, Santa Cruz, CA 95060, United States
AB: Earthquake triggering associated with the passing seismic waves of large distant earthquakes has been observed regularly and robustly in California, particularly in volcanic and hydrothermal areas. Japan is known for its Holocene volcanic and hydrothermal activity, yet little long-range triggering has been observed. In order to develop a more detailed understanding of the conditions required for long-range triggering, we map long-range triggering susceptibility in California and Japan and compare it to maps of background activity, aftershock productivity, and surface heat flow. A spatial correlation is found between all four quantities in California, but no consistent pattern is found in Japan. Long-range triggering is tentatively identified in Kyushu, and possibly western Hokkaido. We find that Japan is less susceptible than California to short-range triggering, in that shallow crustal (<15km depth) earthquakes in Japan have on average fewer aftershocks than equivalent earthquakes in California. We infer that aftershock productivity and long-range triggerability are controlled by local fault properties in California and that tectonic style must influence triggering susceptibility. Triggerable regions in both California and Japan are predominantly trans-tensional and hydrothermal. Long-range triggering maps were produced by collecting all local events in the days before and after potential global triggering events, here defined as events that exceed an arbitrary magnitude-distance threshold and are located beyond some minimum distance. Raising the threshold restricts the analysis to stronger triggers but reduces sample size; we attempt to strike a qualitative balance between signal strength and robustness. We Regional triggering susceptibility is mapped by binning and differencing pre- and post-trigger seismicity and applying a gaussian smoothing filter. Short-range triggering susceptibility (traditional aftershock productivity) is mapped by counting the average number of early aftershocks generated by isolated mainshocks. We designate earthquakes as mainshocks if they are preceded by no larger events within 250km in the last 24 hours, and identify earthquakes as aftershocks if they fall within 15km and 1 hour of a mainshock. We bin and average the productivities of the mainshocks using the same smoothing scheme as used for the long-range map. The ANSS global catalog since 1984 was used for mapping in California and the JMA catalog since 1997 was used for Japan. Heat flow data is from the Southern Methodist University database and the Geological Survey of Japan. The maps show that long-range triggering thresholds are controlled by local fault conditions and that these conditions also influence traditional short-range aftershock productivity. Hydrothermal activity and trans-tensional tectonics are implicated as important conditions for triggering.
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting