[S-SS04] New trends in data acquisition, analysis and interpretation of seismicity

[SSS04-P06] Earthquake monitoring in a field laboratory in northern Hualien, Taiwan

*Ru-Hung Tsai¹, Yen-Yu Lin^1,2,3, Yin-Tung Yen⁴ (1.Department of Earth Sciences, National Central University, Taiwan, 2.Earthquake-Disaster & Risk Evaluation and Management Center (E-DREaM), National Central University, Taiwan, 3.The Graduate Institute of Applied Geology, National Central University, Taiwan, 4.Sinotech Engineering Consultants, INC, Taiwan)

Keywords:Borehole seismometer, Earthquake location

Seismicity in northern Hualien is relatively high compared to it in other regions in Taiwan. It is due to that area locates on a collision boundary between the Eurasia and Philippine Sea plates, and the Ryukyu subduction zone in nearby. The tectonic environment in this area is not clear and has been debated. Magnitude of earthquakes occurred in this area distributes in a wide range from M1 to 8. The largest earthquake (M8.0) in 1920 occurred in this area offshore. The distribution of earthquakes is also wide from near surface to 150 km at depth. Due to diversity of earthquakes in size and depth, the area in northern Hualien mat be the best location for seismicity activity study. To monitor seismicity in this area, we established a field laboratory near the Heping village since 2020. It has operated a broadband seismometer placed in a borehole at a depth of 100 meters, an acceleration seismometer and a velocity 3-component geophone on the surface. Combining borehole seismometers deployed by the Central Weather Administrator (CWA), Taiwan in our study area and our instrument to be a borehole network, it is able to increase capability of earthquake detection, especially for mircoearthquakes. In the first stage of this study, we calibrate the horizontal azimuth of the borehole seismometer in the field laboratory by using long-period (0.01~ 0.5 Hz) waveforms from the 2021, M7.4, Qinghai earthquake in China. The corrected angle is 339.11° counterclockwise. In the second stage, we pick up 2026 earthquakes manually from the continuous records of the borehole seismometer in the laboratory during two periods: October to November 2020 as well as, October to December 2022. Among them, we identify 217 events that are not listed in the CWA earthquake catalog. We successfully located 125 events by using the 3-D velocity model in Taiwan. These events are clustered to the north of the laboratory, which might be related to interactions of colliding and the subducting behaviors. In the future, we will focus on determining the focal mechanisms and other source parameters (e.g., magnitude and stress drop) to understand stress condition of this complex tectonic region.