Three-dimensional structure of V-p, V-s and V-p/V-s beneath the active collision boundary of eastern Taiwan

Abstract

Seismic hazard in the active collision zone of southeastern Taiwan has been poorly known. Although the area has experienced only a few magnitude 6 earthquakes during the modern seismic observation period, there are good geological and instrumental evidences that the area has repeatedly experienced magnitude 7 earthquakes, including a sequence in 1951. We estimated 3-D P-and S-wave velocity models by applying an earthquake tomography method to a large number of high-quality arrival-time data collected by a regional seismic network and a local seismic array. Earthquakes in the region that occurred between 1991 and 2011 were then relocated using the 3-D velocity models. The 3-D velocity models and seismicity depict the current deformation structures in the region. Numerous earthquakes have occurred along a narrow east-dipping seismic zone beneath the southern Coastal Range (CR) near Chengkung. The seismogenic structure extends from the surface eastward to depths greater than 20 km. It is characterized by low-V-p, low-V-s and low-V-p/V-s. Soft materials within the seismogenic structure may include fluids as well as cracks with high aspect ratios resulting from extended periods of tectonic loads. Soft sediments with high fluid contents between rigid materials are regarded as significant weak zones where stress concentration and nucleation of numerous earthquakes can easily occur. Beneath the western boundary of the Longitudinal Valley (LV), a vertical or steeply west-dipping strike-slip fault is identified. Apparently, significant slip partitioning is taking place to accommodate the oblique tectonic motions. In the eastern offshore, seismic velocities indicate that the materials are derived from deeper depth and that significant amounts of water have been introduced to form serpentines at shallow depth. The central LV and CR have been largely aseismic since a series of magnitude 7 earthquakes in 1951. A large amount of tectonic stress is stored in the region. Thus, the area has high potential to release seismic energy through future large earthquakes.