2009 Slow Slip Event under the Northeast Taiwan at the Backarc Extension of the Ryukyu Subduction Zone from Continuous GPS Data

Abstract:

A slow-slip event is detected from June 2009 to October 2009 based on the analysis of coordinate time series from 22 continuous GPS stations in northeastern Taiwan, belonging to the backarc extension region of the Ryukyu subduction zone. The GPS data from 2006 to 2011 were processed using GAMIT software. The seasonal signals and common-mode errors were removed from the coordinate time series. Accumulative displacement pattern shows horizontal displacement direction towards SSE with the maximum value of ~9 mm. However, no significant vertical displacements are detected. The accumulative slip distribution and fault geometry parameters are inverted by surface accumulative displacements using the uniform stress drop inversion. The optimized model indicates a normal fault with a shallow dip angle of ~5° and a depth of ~8 km. A maximum of ~40 mm of slip is inferred near the eastern part of the study area. Comparison with rainfall data indicates that this slow slip event is not the seasonal response. Seismicity distribution and focal mechanism solutions imply that several nearly east-west-striking seismogenic zones develop and mainly act as left-lateral strike-slip faulting and normal faulting accompanying the opening of the backarc basin. The bottom depth of these seismogenic zones is ~10 km, corresponding to the depth of our inferred fault plane. A low Qp/Qs layer is also discovered at the same region, which suggests a high fluid within this layer. Therefore we suggest a shallow south-dipping décollement under the backarc basin in northeastern Taiwan. The relationship between this slow slip event and other regular large earthquakes in this region will be studied soon.