HR: 17:00h
AN: S44A-05    [Abstracts]
TI: Modeling Short-Interval Silent Slip Events in Deeper Subduction Interfaces Caused by Pore-Fluid Pressure Changes due to Frictional Dilatancy and Compaction
AU: * Shibazaki, B
EM: bshiba@kenken.go.jp
AF: International Institute of Seismology and Earthquake Engineering, BRI, 1-Tatehara, Tsukuba, 305-0802 Japan
AB: Recent high-resolution observations of crustal movements reveal the occurrence of two types of silent slip events in deeper subduction interfaces; long- and short-interval silent slip events. Short-interval silent slip events occur in the deeper Cascadia subduction zone (Dragert et al., 2001) and in the southwest Japan subduction zone (Obara and Hirose, 2004). These silent slip events occur simultaneously with the activity of distinct, low-frequency, non-earthquake tremors. Recurrence interval of these silent events is from 3 to 14 months. Short-interval silent slip events are thought to be caused by high pore-fluid pressure associated with dehydration in deeper subduction interfaces since these events are accompanied by low-frequency tremors. We developed a 2D model of short-interval silent slip events considering frictional dilatancy due to slip and compaction in the deeper frictional stable region. Pore-fluid pressure increases at very low slip velocity due to compaction. When pore-fluid pressure increases and shear stress reaches a certain level, slip starts to accelerate. With an increase of slip velocity, porosity increases due to frictional dilatancy; as a result, pore-fluid pressure decreases. This decrease in pore-fluid pressure results in strengthening the fault and deceleration of slip. We found that short-interval silent slip events can occur only at the condition in which pore fluid pressure is very close to lithostatic pressure. The recurrence interval of these silent slip events is from several months to a few years. The maximum slip velocity of these silent slip events reaches 10$^{-8}$ m/s. Important parameters are the critical displacement for the porosity equation and the dilatancy coefficient which determines the amount of frictional dilatancy. To reproduce short interval silent slip events, we need to take the value of the critical displacement for the porosity equation to be around 1 mm. Approaching the main event, there are some changes in intervals and the maximum slip velocity of silent slip events. By the slow precursory slip of the main event, occurrence of silent slip events becomes irregular. Our numerical results suggest that monitoring silent slip events will be very important for forecasting the main earthquake. We also investigated rupture velocity of these silent slip events by a 3D modeling. We can reproduce silent slip events which propagate horizontally with a velocity of a few km per day below the bottom of the seismogenic region. Observations of silent slip events have shown that the velocity of the migration ranges from 5 to 15 km per day (Dragert et al., 2004). Our numerical results are consistent with the observations.
DE: 7260 Theory and modeling
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting