Aseismic slip and seismic swarms leading up to the 2024 M7.3 Hualien earthquake

Understanding the role of aseismic slip in earthquake cycles is essential for assessing seismic hazards and short-term forecasting. Eastern Taiwan's double-vergence suture zone, where the Philippine Sea Plate subducts beneath the Eurasian Plate, experiences frequent M ≥ 6 earthquakes and widespread aseismic slip, making it an ideal natural setting to study earthquake triggering processes. Here we demonstrate how aseismic deformation contributed to the April 3, 2024 Mw7.3 Hualien earthquake by analyzing a 24-year catalog of repeating earthquake sequences (RESs) and earthquake swarms. We find that six out of nine swarms in the epicentral area, northern Longitudinal Valley, were accompanied by increasing aseismic slip rates, as revealed by RESs on the west-dipping Central Range Fault (CRF). A notable aseismic slip episode in 2021 indicated by GNSS signals, the accelerated RESs-derived slip rate, and a four-month-long swarm sequence with high diffusivity (~5.2 m²/s), suggests joint contributions from over-pressured fluids and deep fault creep. Following this episode, a sequence of M6+ events occurred in 2022, and both seismicity and aseismic slip gradually increased again starting in 2023. Coulomb stress modeling indicates that cumulative aseismic and seismic slips since 2021 generated up to ~30 kPa positive stress on the eventual 2024 rupture, promoting fault weakening and shallower seismicity. This study provides compelling evidence for aseismic-slip-induced stress triggering of a major earthquake and highlights the importance of integrating aseismic processes into earthquake hazard models for collisional fault systems.