HR: 0800h
AN: T21A-0360    [Abstracts]
TI: Deep low-frequency tremor and very-low-frequency earthquake as indicator for slow slip event at the transition zone on the plate interface in southwest Japan
AU: * Obara, K
EM: obara@bosai.go.jp
AF: AU: Sekine, S
EM: ssekine@bosai.go.jp
AF: AU: Hirose, H
EM: hirose@bosai.go.jp
AF: AU: Maeda, T
EM: maeda@bosai.go.jp
AF: AU: Ito, Y
EM: yito@aob.geophys.tohoku.ac.jp
AF: AU: Ito, Y
EM: yito@aob.geophys.tohoku.ac.jp
AF: Graduate school of Science, Tohoku University, Aoba, Sendai, Miyagi, 980-8578, Japan
AB: Three types of deep slow earthquakes; low-frequency tremor [Obara, 2002], very-low-frequency (VLF) earthquake [Ito et al., 2007], and short-term slow slip event [Obara et al., 2004] have been detected at the transition zone on the plate interface in the Nankai subduction zone. These slow earthquakes characterized by different frequency properties occur simultaneously and reflect the stress relaxation process at the deeper side of the seismogenic zone. Considering magnitude of each slow earthquake, the short-term slow slip event may be the primary phenomenon as a stick slip on the plate interface and other slow events are triggered by the transient slip. We examine the detail time history of these slow earthquakes to reveal the interaction between each slow event. The tremor source area is divided into some segments along the narrow belt-like zone considering with the space-time distribution of activities. Western Shikoku, northern Kii and Tokai segments are characterized by the recurrence interval of six months. In these areas, the short-term slow slip event can be detected by some tiltmeters and the fault models are estimated. On the other hand, tilt changes are rarely recognized during active tremor stage with a recurrence interval of three months at eastern and central parts of Shikoku and central Kii segments. There are many minor tremor activities without tilt change, however major tremor episodes are always accompanied by the slow slip event and migrate unilaterally or bilaterally. Comparison between the tilting record and tremor activity indicates that the beginning of the detectable slow slip event coincides with activation of tremor. This supports an idea that the slow slip event occurs first and induces the tremor. The front of the migrating tremor activity is very sharp and behind of the front the tremor activity keeps for a while. This observational fact suggests that the migration of tremor is induced by the rupture propagation of the slip event and the tremor behind the migration front keeps highly activity by the slipping. The VLF earthquake with predominant period of 20 seconds has been detected during the episode of tremor and slip. Based on the centroid moment tensor analysis, the VLF earthquake is located at the same position of the tremor source area and the focal mechanism is the reverse fault type coincident with the slow slip event and the plate interface geometry. In many cases, VLF earthquakes occur at the most active stage of tremors and coincident with tremor migration. Sometimes, VLF earthquakes seem to correspond to the acceleration stage of ground tilt. Therefore, tremor and VLF earthquake might reflect the change in slip velocity on the plate interface. The crustal movement caused by the short-term slow slip event is too small to resolve the precise space-time distribution of slip; however the monitoring of tremor and VLF earthquake is considered as the sensitive indicator for plate motion.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting