Coulomb stress effects of the April 3, 2024 Hualien, Taiwan, China MS 7.3 earthquake on the surrounding faults and subsequent April 23, 2024 MS 6.2 and MS 6.3 earthquakes

Objective To determine the rupture mode of the MS 7.3 earthquake in Hualien, Taiwan, China on April 3, 2024 and its triggering effect on subsequent seismic events in the surrounding area, the co-seismic displacement field and the induced areal strain response were analyzed by inverting the geometric structure and sliding characteristics of the seismogenic fault. The static Coulomb Failure Stress (CFS) triggering effect of the main earthquake event on the two MS 6.2 and MS 6.3 earthquakes that occurred on April 23, 2024 was evaluated to clarify the promoting effect of this earthquake on the seismic activity and its influence on the stress field of the adjacent area. Methods The two possible seismogenic nodes of the main earthquake and its subsequent earthquakes are determined by using the method of "focal mechanism solution". Based on the homogeneous elastic half-space theoretical model, the co-seismic displacement field and areal strain field under seismic action are established. The co-seismic displacement field results of the vertical fault direction of the main shock are analyzed and its sliding characteristics are determined. The CFS variation of the main shock in subsequent seismic events is calculated and its promoting effect on subsequent earthquakes is evaluated. The method of facet clustering is adopted to determine the seismogenic fault plane of the earthquake event. The stress field of the study area is projected onto the seismogenic fault plane and the causes of its occurrence are analyzed. Results A co-seismic displacement analysis of the April 3, 2024 MS 7.3 Hualien earthquake in eastern Taiwan reveals distinct patterns in both horizontal and vertical displacement fields, consistent with a reverse fault mechanism. Based on the focal mechanism solution, this seismic event is identified as a typical reverse fault earthquake, aligning with the tectonic compression between the Eurasian Plate and the Pacific Plate. The horizontal displacement field demonstrates a complex material flow pattern: substantial crustal materials converged towards the seismogenic fault along its southeastern and northwestern flanks, followed by outward migration in northeastern and southwestern directions. This kinematic pattern reflects the intense plate convergence where the Pacific Plate subducts westward beneath the Eurasian Plate along the eastern margin of Taiwan Island. Vertical displacement measurements show significant differential movement across the fault. The southeastern block (upper plate) experienced remarkable uplift reaching 48.4 cm, while the northwestern block (lower plate) underwent subsidence of up to 11.4 cm. This vertical displacement configuration, characteristic of thrust faulting, is further confirmed by cross-sectional observations perpendicular to the fault strike. The interface between the upper and lower plates exhibits sharp kinematic contrasts, with the upper plate displaying predominant upward motion components and the lower plate showing downward movements. Along-strike displacement reached approximately 22 cm, significantly exceeding the maximum perpendicular displacement of ~5 cm; this indicates thrust-dominated rupture with minor strike-slip components. The strain field distribution corresponds to a compressive belt parallel to the fault trace near the epicenter, flanked by extensional zones to the immediate east and west. Stress field analysis reveals significant shear stress concentrations (relative shear stress >0.7) and negative normal stresses on the fault planes of three major earthquakes in the sequence, consistent with the compressional regime generated by plate convergence. The westward subduction of the Pacific Plate beneath Taiwan Island creates optimal conditions for thrust faulting along the Longitudinal Valley Fault system, where accumulated shear stress ultimately exceeds the fault strength threshold. Notably, the Coulomb Failure Stress (CFS) calculations demonstrate that the April 3 mainshock significantly promoted subsequent seismic activity. The April 23 MS 6.3 and MS 6.2 events occurred in regions where the calculated CFS changes reached 0.020 MPa and 0.3 MPa, respectively, both exceeding the 0.01 MPa threshold for earthquake triggering. This earthquake sequence represents a normal release process of accumulated tectonic stress in the plate convergence zone. The spatial-temporal evolution of co-seismic deformation, strain redistribution, and stress interactions fully aligns with the regional tectonic framework dominated by the ongoing collision between the Eurasian and Pacific Plates. Conclusion Research shows that the MS 7.3 earthquake in Hualien, Taiwan, was caused by thrust faults. This earthquake event, along with the two subsequent MS 6.2 and MS 6.3 earthquake events that occurred on April 23, 2024, were all normal releases of local stress accumulation. Moreover, the CFS generated by the MS 7.3 earthquake in Hualien, Taiwan, has a significant impact on the surrounding seismic activities and has an obvious promoting effect on the occurrence of the subsequent two earthquakes. [ Significance] This study not only evaluates the impact of the MS 7.3 earthquake in Hualien, Taiwan, on subsequent earthquakes, but also provides a fundamental dataset for geodynamic studies in the region. Strengthening the capacity of earthquake monitoring and forecasting and disaster mitigation promotes the formulation of relevant policies and safeguards people's lives and properties.