HR: 17:15h
AN: S32F-06    [PDF]
TI: The Absence of Remotely Triggered Seismicity in Japan from 1997 to 2002
AU: * Wakefield, R H
EM: rebecca@moho.ess.ucla.edu
AF: UCLA Dept. of Earth and Space Sciences, 595 Charles Young Drive, Los Angeles, CA 90095-1567 United States
AU: Brodsky, E E
EM: brodsky@ess.ucla.edu
AF: UCLA Dept. of Earth and Space Sciences, 595 Charles Young Drive, Los Angeles, CA 90095-1567 United States
AB: Observations of increased seismicity following the Landers, Hector Mine, Izmit, and the Denali, earthquakes suggests remote seismic triggering occurs in geothermal locations as far as 3150 km. This study attempts to determine if the same effects occur in Japan, a geothermal region of high seismicity. For the period of 1997 to 2002, we searched for significant increases in the seismicity levels following earthquakes with M$_{w}$ $\ge$ 6.5 at distances larger than conventionally associated with aftershocks. Additionally, we examined available waveform data in order to detect uncataloged events hidden by the coda of the mainshock. Five events had associated waveform data: March 24, 2001 Geiyo, M$_{w}$ = 6.8; March 28, 2000 Volcano Islands, M$_{w}$ = 7.6; July 30, 2000 Honshu, M$_{w}$ = 6.5; October 6, 2000 Tottori, M$_{w}$ = 6.7; and the January 28, 1999 Kuril Islands, M$_{w}$ = 6.8 earthquake. Located 260 km from the Geiyo epicenter, station TKO recorded one possible triggered event within 65 km during the hour following the mainshock. However, the TKO data contains many anomalous spikes, and we are not confident the record is clear enough to differentiate small local events from noise. An ambiguous, two-day, regional seismicity increase followed the Volcano Islands event. We interpret the swarm associated with the signal as coincidental because no similar swarms occurred at the same location following Tottori or Geiyo, both of which had an order of magnitude larger shaking. Both waveforms and cataloged events indicate no triggering occurred following the Honshu, Tottori and Kuril Islands mainshocks. We do not interpret the one indefinite local event recorded by TKO as evidence for mid range dynamic triggering, implying that the 2.5 cm/s shaking at TKO did not exceed the local triggering threshold. Additionally, the lack of triggering following Honshu, Tottori, and Kuril Islands suggests that the 1, 2.5 and 2.6 cm/s shaking at distances of 182, 238, and 267 km, respectively, creates lower bounds for the dynamic triggering thresholds at the respective locations. This assumes the bound is frequency independent. In none of the cases were thresholds exceeded over a large enough region or by large enough amplitude to produce a statistically significant increase in the cataloged rate of seismicity during the period from 1997 to 2002. All previously documented examples of triggering have occurred following shallow earthquakes with M$_{w}$ $>$ 7. With the exception of Volcano Islands, all of the events of this study have M$_{w}$ $<$ 7, and have no triggering associated with them. This suggests two possibilities: either events with M$_{w}$ $>$ 7 are required to produce sufficient shaking to trigger seismicity, or Japan is less susceptible to triggering than the western US or Greece. We assume that the depth of the Volcano Islands earthquake prohibits any substantial surface shaking. We conclude that more data is required associated with shallow, crustal events with M$_{w}$ $>$ 7 in order to determine whether or not Japan is susceptible to regional triggering.
DE: 7223 Seismic hazard assessment and prediction
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting