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

Recognizing the locations and timing of aseismic fault slip during the earthquake cycle may be critical for short-term earthquake forecasting and hazard assessment. Abundant M ≥ 6 earthquakes and widespread aseismic slip in a double-vergence suture in eastern Taiwan provide a rare opportunity to better understand the role of aseismic slip in earthquake cycle deformation and triggering relationships. By tracking aseismic slip captured by repeating earthquake sequences and earthquake swarms in eastern Taiwan from 2000 to mid 2024, we observed an acceleration of aseismic slip rate that occurred approximately three years preceding the April 3, 2024 Mw7.3 Hualien earthquake. The accelerated aseismic slip revealed by the repeating earthquake data was accompanied by a four-month-long active seismic swarm starting on April 18, 2021, in the 2024 Mw7.3 epicentral area. The swarm occurred on the west-dipping Central Range fault exhibited a northward and upward migration pattern with a diffusion rate of ~ 6 m2/s. After 2021 swarm sequence, the regional aseismic slip rate initially decreased until an Mw 6.1 event in June 2022, followed by a gradual increase in the aseismic slip rate leading up to the mainshock over approximately 1.5 years. Concurrent with the accelerated aseismic slip, the Central Range Fault was characterized by a heightened seismicity rate until the 2024 Mw7.3 mainshock. The resulting static Coulomb stress change reveals that accumulated aseismic and seismic slips from swarms, RESs, and M6+ events on the CRF from mid-2021 to mid-2022 generally produce positive stress changes (up to 20 kPa) on the same fault. The sequential seismic-aseismic interplay of faulting in the epicentral area of the 2024 Hualien event provides a rare example of aseismic-slip-induced stress triggering leading up to the 2024 Mw7.3 earthquake.