Interaction between aseismic slip phenomena in a collision orogen

Using continuous seismological data of Central Weather Administration (CWA) Seismographic Network and Broadband Array in Taiwan for Seismology (BATS), we applied the envelope correlation method of Mizuno and Ide (2019) to identify tectonic tremors in Taiwan 2012 to 2022. With a large number of seismic stations used in this study and removal of short-lasted events (< 10 s), we successfully detected ~7000 events. Except for the tremor zone previously observed at southern Central Range, we reported the new tremor “hotspots” across the mountain range of the island, over a distance of 200 km. Different from the fluid-rich environment previously established for tremors in subduction zones, the newly discovered tremor zones in Taiwan coincide with the spots with high geothermal heat flux, indicating that the temperature effect may be the common mechanism for tremor generation in a mountain belt of Taiwan.

Other than tectonic tremors, several seismic phenomena are believed to be driven by aseismic slip process such as repeating earthquakes and earthquake swarms. The three catalogs may provide new insight into the controls of quasi-periodic aseismic slip and the role of deep fluid in their generation mechanism. We found that only < 5% of repeating earthquakes and swarms are located in 5 km of the tremor clusters, indicating that the deep-seated tremors might be engineered differently, comparing with the shallower repeaters and swarms. The spatial association is only observed underneath the southern Central Range, where the shallow swarms (< 15 km) and deep tremors (20-50 km depth) are likely interactive. We found 69-80% tremors and 86-96% swarm events occurred at the lower ground water level, respectively. This is contradictory with opposite clamping effect of hydrological/tidal stresses on thrust faulting (tremor) and normal faulting (swarm) slip. We hypothesized that in the lower crust where the thrust-faulting tremors are generated, the vertical fluid mobility could be easily elevated during the decreasing ground water level under the condition of near-lithostatic pore-fluid pressure. The upward migration of fluids may play an important role in the occurrence of swarm activities above the tremors. The continuous magnetotelluric monitoring at the location of active swarms will help us to confirm and further establish the temporal variation of fluid flow.