Study of the Triggering Level of Dynamic Stress Induces Non-Volcanic Tremor in Longitudinal Valley in Eastern Taiwan

Abstract:

Taiwan is located at the margin of the Eurasian Plate and the Philippine Sea Plate, which is a subduction zone between these two plates and the fault structures are rather complicated and dense seismicity, especially the Longitudinal Valley (LV) in eastern Taiwan. Non-volcanic tremor (NVT) is a seismic signal with low amplitude and long duration. NVT is often occurred below the seismogenic zone, which is between the lower crust and upper mantle, and the arrival time data of the body wave is difficult to be collected. Therefore, this study aims to investigate the physical mechanisms of NVT in several steps. First, in the investigation of the teleseismic earthquake data from the U.S. Geological Survey in 2005 to 2014, thirty-five potential teleseismic earthquakes are selected. Second, the seismograms are collected from the Broadband Array in Taiwan for Seismology (BATS) and Central Weather Bureau Seismic Network (CWBSN) for these thirty-five potential teleseismic earthquakes. Third, the Seismic Analysis Code is used to select the seismograms from seven possible events which satisfied the conditions of triggering tremor during the passage of the surface wave. Forth, a band-pass filter is applied to retain the frequency with the range of 2-8 H_Z of the surface waveform. Finally, visually determination for the tremor signals. The experimental results show that five certainly NVT events and two potential triggered events were found in the LV zone of eastern Taiwan. The locations of the hypocenters were then estimated using HYPO71 for these five certain events. According to the estimated hypocenters, the sources of NVT are possibly beneath the southern region of LV, close to the Chih-Shang fault. Moreover, these estimated hypocenters are within the high Vp/Vs ratio region and in depth of 30-40 km. The further analysis found that the amplitude of the surface wave is one of the key factors that when the peak ground velocity > 0.02cm/s, which equivalents to 2-3kPa dynamic stress, might trigger tremors.