The Absence of Remotely Triggered Seismicity in Japan

Abstract

Remote dynamic triggering occurred in the western United States and Greece following the following the Denali, Hector Mine, Landers, and Izmit main- shocks in seismically active, largely geothermal areas that experienced shaking from the mainshock typically exceeding 0.2 cm/sec. Because triggered earthquakes are small (often below Mw, 2), investigations of whether remote triggering is a generally occurring phenomena must be concentrated in well-instrumented locations. As a seismically active, geothermal, well-instrumented location, Japan constitutes an ideal location for such study. We systematically examine whether remote dynamic trig- gering occurs following mainshocks in Japan producing amplitudes in the study region above 0.2 cm/sec at distances greater than two estimated fault lengths, from 1998 to 2004. We use both catalog data and filtered waveforms to search for seis- micity increases. One significant regional increase follows a Mw 7.6 mainshock in March 2000, largely due to a volcanic swarm that began the day before and increased in intensity in the following days. Two localized increases in cataloged seismicity occur in Kyushu, which is the only area in the study region with onshore tectonic extension. Two factors possibly inhibit regional triggering in Japan: (1) compres- sional tectonics, and (2) the frequent occurrence of large mainshocks. The ambient seismicity in Japan combined with the lack of observable widespread triggering ren- ders rate-state friction and stress corrosion to be improbable triggering mechanisms. Fracture unclogging as a triggering mechanism is consistent with the observations.