Resolving the tsunamigenic source of the 2024 Mw 7.4 Hualien earthquake: Integrated tsunami-waveform inversion and InSAR constraints on complex multi-fault rupture

On 3 April 2024, an Mw 7.4 earthquake struck offshore Hualien, eastern Taiwan Island, triggering a localized tsunami with a peak amplitude of 0.8 m at the Su’ao tide gauge. Despite the significant onshore geodetic coverage, the event’s precise seismogenic fault and rupture extent remain elusive due to the complex tectonic setting and the invisible nature of offshore deformation to terrestrial networks. To bridge this gap, we employ a tsunami waveform inversion approach that requires no pre-assumed rupture fault, utilizing Green’s functions computed from a grid of 75-unit Gaussian sources via the JAGURS model. Our inversion reveals a primary sea-surface uplift of ∼2.9 m near the hypocenter, complemented by moderate deformation to the south and northeast. This seafloor displacement model is further validated by InSAR (Interferometric Synthetic Aperture Radar) analysis, which shows onshore vertical displacements exceeding 50 cm near Hualien, consistent with both field surveys and our inverted offshore results. Compared to existing fault models, our inversion-based approach demonstrates better agreement with observed tsunami arrival times and waveforms. With such characteristics of the seafloor deformation, we infer that the primary tsunamigenic source is the west-dipping Takangkou-High Thrust Fault, potentially involving co-seismic rupture along the northern Longitudinal Valley Fault. This study underscores the synergy between tsunami inversion and onland geodetic data in characterizing complex offshore earthquake sources, offering critical insights for seismic and tsunami hazard assessment about nearshore earthquakes.