HR: 0830h
AN: G21B-0264    [PDF]
TI: Repeating Slow Slip Events Which Correlate Deep Low-Frequency Tremor Activity in Southwest Japan
AU: * Hirose, H
EM: hirose@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tenno-dai, Tsukuba, 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1 Tenno-dai, Tsukuba, 305-0006 Japan
AB: Non-volcanic deep low-frequency tremors are observed along the Nankai trough subduction zone, Southwest Japan (Obara,~2002). This observation provides us a new point of view in seismotectonics at subduction zones. Recently, Obara and Hirose~(2003) found the following characteristics in the tremor activity. First, the tremor is not distributed homogeneously in the narrow belt-like zone along the strike of the subducting Philippine Sea Plate but some clusters exist in the zone. Second, the major activity of most clusters repeats periodically. Here we report the observed crustal tilt changes which correlate the tremor activity. These crustal deformations can be explained by repeating slow slip events at the bottom end of the transition zone on the plate interface. The tremor activity is observed by high-sensitivity seismograph network, NIED Hi-net. Every Hi-net observatory is equipped with a high-sensitivity accelerometer, whose long period horizontal component can be analyzed as ground tilt. One of the most active clusters is located on the western Shikoku area, where the major tremor activity repeats with about a half-year interval. For two years from the beginning of 2001, the major tremor emerged four times, i.e., January and August 2001, and February and August 2002. During these active periods, we found slow tilt changes which last 2-3 days around the source area of the activity. The tilt change is about 0.1~$\mu$rad in maximum. The spatial pattern of the tilt changes suggests that a slow slip event, which accompanies the tremor activity, occurs near the plate boundary at depth. We estimate the fault parameters for each period, which can explain the observed tilt changes. The estimated slow slip fault is located around the tremor cluster. The moment magnitudes of the events are estimated to be 5.9. Expected surface displacement from this fault model is 1-2~mm in maximum, which is in accordance with the fact that little change is observed by GSI's nationwide GPS network. We conclude that the tremor activities and the slow slip events occur simultaneously and quasi-periodically at the deeper part of the subduction zone, as seen at Cascadia subduction zone (Rogers and Dragert,~2003).
DE: 1206 Crustal movements--interplate (8155)
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
SC: Geodesy [G]
MN: 2003 Fall Meeting