Depth-dependent interaction of tectonic tremors, repeating earthquakes, and earthquake swarms in a collision zone of Taiwan

Seismological signatures of aseismic slip include tectonic tremors, repeating earthquakes, and earthquake swarms. They are most commonly documented in subduction margins, where their depth distribution and temporal behavior reflect slab geometry, fluid release, and plate interface rheology. Their occurrence in continental collision belts, however, remains unclear. Taiwan provides a unique natural laboratory to address this gap. Here we investigate how earthquake swarms interact with repeating earthquakes (REs) and tectonic tremors, and how their coupling varies with depth along the west-dipping Central Range Fault.
Using a 24-year seismic catalog, we show that tremors (mostly >25 km), REs (15–25 km), and swarms (<20 km) align spatially along the same fault system but exhibit distinct recurrence behaviors and interaction thresholds. We find that temporal interaction among these phenomena is strongly depth-dependent and controlled by spatial overlap. Swarms and REs display the strongest coupling within shared fault segments, consistent with asperities being repeatedly loaded by surrounding aseismic creep and transient slip-rate accelerations, supported by the rapid diffusivity of catalogued swarms (several m²/s). In contrast, tremors occur in the lower crust under near-critical frictional conditions and generally interact with seismogenic activity only during periods of elevated aseismic slip, most notably following M6-class earthquakes. Tremors alone exhibit strong tidal modulation (~90% during increasing tidal levels), indicating markedly higher stress sensitivity at depth. Elevated Vp, Vs, and Vp/Vs ratios further delineate a high-stiffness, high–pore-fluid-pressure lower-crustal environment that enables tremor generation beneath the mountain root. Overall, these observations indicate that swarms, REs, and tremors represent depth-stratified manifestations of aseismic slip, with interaction style and stress sensitivity systematically varying with depth.