A composite declustering method for detection of earthquake swarms in Taiwan

Abstract:

The observation of earthquake swarms act as an indicator of fluid activity and/or aseismic transient at depth. Frequent earthquakes in Taiwan serves as a natural laboratory for studying the physical mechanisms controlling the characteristics of swarm activity in this complex tectonic environment. Here we propose a composite swarm detection method that relies on the correlation of mainshock density rates derived from three different declustering algorithms Re85, Z&B13, and ETAS. Using the 1990-2019 M3, shallower than 50 km earthquakes in Taiwan, we identified 85 swarm sequences characterized by event number of greater than 10. The swarm sequences are found to be active at the place showing high seismicity rate and high a-value (> 4) but not anomalous b-value, suggesting the environment hosting high occurrence rate of earthquake coincides with the mechanism for swarm generation. Tectonically this is consistent with the frequent swarms at the termination of Ryukyu subduction zone near eastern Taiwan. In addition, the swarms are found to overlap with (1) repeating earthquakes in southern Longitudinal Valley where the creeping fault is located (2) tectonic tremors in southern Central Range where overpressure fluid may play a role in weakening the deeper fault zone. Consequently, we propose that both active collision boundary experiencing high creep rate and the areas experiencing episodic aseismic slip and high fluid pressure are capable of producing swarm activity in Taiwan.