Effect of Coseismic Stress Changes from the 1999 Chi-Chi and 2024 Hualien Earthquakes on the Ambient Stress Field

Ruoyan Wang, & Heidi Houston

Submitted August 30, 2026, SCEC Contribution #15442, 2026 SCEC Annual Meeting Poster #TBD

Large earthquakes can reorganize crustal stress fields, producing detectable changes in both stress orientation and, in some cases, absolute stress state. Here we extend analyses of coseismic stress rotations in Taiwan by combining stress tensor inversions of suites of focal mechanisms before and after a mainshock with finite-fault-based static stress calculations to place constraints on the ambient stress field.

We focus on the 1999 Mw 7.7 Chi-Chi earthquake in central-western Taiwan and the 2024 Mw 7.4 Hualien earthquake in eastern Taiwan, which ruptured distinct tectonic settings within the active Eurasian-Philippine Sea plate collision zone. Using focal mechanism catalogs from Wu et al. (2008, 2010) and AutoBATS (Jian et al., 2018), we invert for pre- and post-mainshock stress tensors in multiple subregions surrounding each rupture. We also calculate the mainshock coseismic stress changes using Okada elastic dislocation solutions based on published finite-fault slip models.

Both earthquakes produced measurable stress-field changes, including rotations of principal stress axes and changes in the stress shape ratio. Around the Chi-Chi rupture, stress axes northeast of the fault rotated by up to ~20 degrees after the mainshock. By comparing these observed rotations in three dimensions with modeled coseismic stress perturbations, we estimate the ambient deviatoric stress amplitudes implied by the observed focal-mechanism-derived stress rotations. Large rotations imply low levels of absolute deviatoric stress even prior to the mainshock.

This combined approach moves beyond documenting stress reorientations to constraining the absolute levels of deviatoric stresses before and after large mainshocks. Comparing the Chi-Chi and Hualien earthquakes also highlights how fault geometry, slip distribution, and tectonic setting control the degree to which large earthquakes modify the regional stress field.

Key Words
Coseismic stress change, Chi-Chi earthquake, Hualien earthquake

Citation
Wang, R., & Houston, H. (2026, 08). Effect of Coseismic Stress Changes from the 1999 Chi-Chi and 2024 Hualien Earthquakes on the Ambient Stress Field. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)