[S-GD01] Geodetic Advances in Crustal Dynamics and Environmental Change for Taiwan and Neighboring Regions

[SGD01-P01] Detection of slow slip events underneath Taiwan orogenic belt

*Wei Peng¹, Kate Huihsuan Chen¹, Zaccaria El Yousfi², Mathilde Radiguet², Erwan Pathier² (1.Department of Earth Sciences, National Taiwan Normal University, 2.Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, IRD, Univ. Gustave Eiffel, ISTerre)

Keywords:Tectonic tremor, GNSS, Orogenic belt

In Taiwan, tectonic tremors have been discovered along the mountain belt and at its most active beneath the highest mountain at 3952 m. They occurred in the lower crust at 20-50 km depth, where the subducted continental crust facilitates the release of metamorphic fluids. The nature of these deep-seated, short-lasted tremors (up to 30 mins) made the geodetic detectability very difficult. In this study, we present a SSE detection technique that takes advantages of the recently published 10-yr tremor catalog by Ide and Chen (2024) and separate the inter- and intra-tremors GNSS signals. Out of five tremor zones along the mountain, only the area hosting highest daily occurrence rate revealed the geodetic feature. By inverting the aseismic slip at the tremor zone, we found that the SSE here is equivalent to moment magnitude of 5.4, which is much higher than that predicted from tremor duration versus SSE magnitude scaling. Furthermore, using a simple forward model, we found that the detected SSE signal better fits a normal faulting system, contradicting the thrust faulting inferred from moment tensor inversion of tremors (Ide et al., 2015; Ide and Chen, 2024). We argue that the mountain building process in this young orogen complicates the relationship between tremor and SSE. This tremorgenic area has been experiencing layered deformation: the lower crust is dominated by convergence-parallel shear deformation, whereas upper crust is dominated by collision-related compressional deformation. The thrust-faulting, aseismic slip taking place at greater depth, will encounter with the extensional environment in the upper crust, to be recorded at the surface. The SSE signals therefore, reveal the combination of deep thrust-faulting and shallow normal-faulting motions. To conclude, we successfully detected the SSEs synchronized with active tremors, providing supportive evidence that the slow slip is happening on a localized shear zone underneath the highest mountain in Taiwan. The calculation of how much energy is stored and released during such slow slip, however, requires careful treatment. This is because this particular area, the ongoing formation of the mountain building leads to a layered deformation, which makes SSE detection and interpretation challenging.