OBTAIN：流體存在與循環的關鍵證據：造山帶的快、慢地震之整合分析

This three-year project investigates crustal fluid circulation and slow deformation processes in the rapidly uplifting arc–continent collision orogenic belt of Taiwan by integrating seismological, geodetic, and magnetotelluric observations. In the first year, catalogs of tectonic tremors, earthquake swarms, and repeating earthquakes were updated and integrated, showing that hydrological loading and tidal effects can modulate subsurface stress on seasonal to interannual timescales. In the second year, the tremor catalog was extended to 2024 and published in Geophysical Research Letters, revealing five persistent tremor clusters distributed over ~200 km beneath the Central Range. The results indicate that structural pathways enabling vertical fluid migration are the primary control on tremor occurrence. In the third year, moment tensor inversion and tidal sensitivity analyses show that tremors mainly occur at depths of ~30–50 km within the lower crust and are characterized by low-angle thrust-type mechanisms. All tremor clusters exhibit significant tidal modulation, with sensitivities generally higher than those of most mature subduction zones, suggesting that the Taiwanese lower crust is in a mechanically weakened, near-critical state. In addition, integrated analyses of earthquake swarms, repeating earthquakes, and GNSS data identified precursor-type slow earthquake activity prior to the 2024 Hualien earthquake. These results were published in Nature Communications (2025) and Tectonophysics (2026). Overall, this project demonstrates that slow earthquakes are not exclusive to subduction zones but are widespread in fluid-rich, mechanically weak lower crust within complex orogenic belts, providing new insights into deep crustal deformation and fluid processes.