[S-CG50] Earthquakes, Tsunamis, Seismotectonics, and Hazard Potential of the Ryukyu Trench and Okinawa Trough

[SCG50-02] A Comparative Analysis of the November 14, 1986, and April 2, 2024, Hualien, Taiwan earthquakes: Seismic Source Characteristics and Regional Impacts

★Invited Papers

*Kuo-Fong Ma^1,2, Shiann-Jong Lee¹, JoLan Liao² (1.Institute of Earth Sciences, Academia Sinica, Taiwan, 2.E-DREaM, National Central University, Taiwan)

Keywords:November 14, 1986, and April 2, 2024, Hualien, Taiwan earthquakes, Source Characteristics , Earthquake Rupture Process, Waveform and spectra similarities

The November 14, 1986 (M_L 6.8; Mw 7.3) and April 2, 2024 (M_L 7.1; Mw 7.4) Hualien earthquakes underscore the seismic hazards posed by offshore fault structures associated with the Ryukyu subduction zone near northeastern Taiwan. Despite occurring nearly four decades apart, both events caused significant damage in Taipei, over 120 km from their epicenters. The 1986 earthquake, recorded with limited instrumental coverage, was attributed to an east-dipping fault in earlier study. However, due to the sparse seismic network at the time, uncertainties remained regarding its precise rupture characteristics. In contrast, the 2024 event (Mw 7.4)—Taiwan’s largest earthquake since the 1999 Chi-Chi earthquake—was captured by a modern, high-density seismic network. This allowed for precise relocation and detailed rupture analysis. Advanced joint source inversion and strong-motion comparisons revealed that the 2024 earthquake involved a complex rupture on both east- and west-dipping fault planes, significantly influencing ground motion distribution and damage patterns. We leverage advanced analytical techniques from the 2024 earthquake to re-examine the 1986 event, aiming to better understand their source characteristics. While differences in location introduce uncertainties, primarily due to the sparse station coverage in 1986, these two earthquakes share similar focal mechanisms. Comparative analysis of GDSN (Global Digital Seismographic Network ) in 1986 and IRIS (Incorporated Research Institutions for Seismology) in 2024 stations recordings for both earthquakes reveals high waveform similarity in long-period signals (>1 sec). Additionally, when comparing velocity records from local strong-motion stations, the 1986 earthquake’s data from SMART1-C00 (16-bit, 0.1–50 Hz bandwidth) aligns closely with the modern 24-bit digital recordings (TSMIP-E026) from 2024. The similarity in teleseismic long-period signals, and velocity records from local strong motion stations suggest that both events likely originated from similar seismogenic mechanisms. Further spectral analysis of the two earthquakes in Hualien and the Taipei Basin shows higher ground motion in the southwestern Taipei Basin, reached comparable spectral levels in both events, correlating with significant structural damage. The enhanced building codes implemented after 1999 helped mitigate the impact of the 2024 event, yet the persistence of strong shaking highlights ongoing vulnerabilities. The remarkable similarity in waveforms and spectral characteristics between the 1986 and 2024 Hualien earthquakes suggests that these events share not only comparable focal mechanisms and magnitudes but also a common seismogenic process. However, discrepancies in the hypocenter determination of the 1986 earthquake, compounded by limited instrumental responses during the 1986 event, underscore the challenges in accurately associating the rupture processes of the 1986 and 2024 Hualien earthquakes. Our study emphasize the necessity for in-depth insights into the seismogenic structures of Taiwan’s offshore faults and their potential impacts on metropolitan regions. These findings highlight the critical need for enhanced offshore fault characterization and improved seismic hazard assessments to mitigate risks to urban centers like Taipei.