HR: 16:45h
AN: S52I-04    [PDF]
TI: Seismically and Geodetically Constrained Source Mechanisms of Volcanic Induced Seismicity - Miyakejima, Japan, 2000
AU: * Minson, S E
EM: minson@seismo.berkeley.edu
AF: California Institute of Technology, Seismological Laboratory MS 252-21, Pasadena, CA 91125 United States
AU: Dreger, D S
EM: dreger@seismo.berkeley.edu
AF: University of California, Berkeley, 281 McCone Hall, Berkeley, CA 94720 United States
AU: Burgmann, R
EM: burgmann@seismo.berkeley.edu
AF: University of California, Berkeley, 389 McCone Hall, Berkeley, CA 94720 United States
AB: During a period of volcanic unrest on Miyakejima, Japan in 2000, the largest recorded earthquake swarm in Japan occurred (Japan Meteorological Agency (JMA), 2000). More than 100,000 earthquakes were recorded in the next two months (Ito and Yoshioka, 2002). But the most notable feature of this seismicity is that it migrated northwest with an offshore dike intrusion. In this study, seismic and GPS data are used to independently determine source mechanisms for the two largest earthquakes in this sequence. Broadband seismic data from stations operated by the F-net Broadband Seismograph Network were used in linear moment tensor inversions (Dreger et al, 2000; Dreger and Woods, 2002) to determine deviatoric and full moment tensor mechanisms. Nonlinear inversions of GPS data (B\"urgmann et al., 1997) recorded at stations operated by the Geographical Survey Institute of Japan (GSI) were used to find the geometry and slip distribution for each event. Seismic data alone was used to invert for the mechanisms of an additional sixteen earthquakes. Of the eighteen events studied, three have particularly significant mechanisms: EVT3, EVT7, and EVT15. EVT7 occurred on July 8, the same day as the first summit eruption and the formation of a caldera on Miyakejima. The source mechanism for EVT7 has a large volumetric component and appears to be directly related to these processes. For EVT15, the source mechanism from the moment tensor inversions is that of a north-south orientated strike-slip earthquake, making it one of the few double-couple earthquakes in this sequence. The GPS data for EVT15 independently predict a mechanism that is very similar to the mechanism determined by the moment tensor inversions. Furthermore, synthetic seismograms generated using the mechanism determined by the GPS inversions fit the observed seismograms extremely well. This is particularly noteworthy because it shows that the GPS data can predict a completely independent data set. When EVT3 (a large earthquake near the northwest edge of the inferred dike) occurred, the GPS data indicate that there was a significant amount of deflation on Miyakejima. This deflation is clearly not directly related to EVT3, and makes it difficult to identify the earthquake's contribution to the observed deformation. Because of this, the geodetic mechanism for EVT3 is less well constrained than the mechanism for EVT15. However, both the seismic and GPS data prefer a non-double-couple mechanism for EVT3. The mechanism of this earthquake is clearly anomalous, and might be related to the interaction between an inflational source and a faulting source.
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 7280 Volcano seismology (8419)
SC: Seismology [S]
MN: 2003 Fall Meeting