Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake

Chi-Hsien Tang, Ya-Ju Hsu, Sylvain D. Barbot, James Moore, & Wu-Lung Chang

Published August 15, 2018, SCEC Contribution #8618, 2018 SCEC Annual Meeting Poster #129

The strength of the lithosphere controls tectonic processes and seismic cycles. Laboratory rock experiments provide fundamental insights into rock rheology under various conditions, but an extrapolation to the strain-rate and length scale relevant to the lithosphere is still required. The response of the lithosphere to the stress perturbation generated by large earthquakes can be treated as a large-scale creep experiment, but so far this approach has been limited to testing simplified assumptions. Here, we exploit the stress change following the 1999 Mw 7.6 Chi-Chi earthquake and 14 years of postseismic GPS observations to directly image the spatio-temporal viscoelastic transient beneath the Taiwan orogenic belt and to constrain its physical properties. We show that the evolution of stress and strain-rate beneath the middle section of the Central Range is best explained by a power-law Burgers rheology, suggesting that rocks exhibit nonlinear transient creep shortly after the mainshock before reaching steady-state dislocation creep. This evolution is compatible with the flow laws for quartz and feldspar and thermal gradients of 35 °C/km and 20 °C/km beneath the Central Range and Coastal Plain, respectively. The effective viscosity rapidly increases from ~10^17 to ~10^19 Pa s in the first year, and to 2×10^19 Pa s after a decade. Our study demonstrates the potential for geodesy to constrain in situ rheological properties of lithosphere rocks, bridging the gap between laboratory and fault-system scales of creep behaviors.

Key Words
Seismic cycle, Postseismic deformation, Rheology, GPS

Citation
Tang, C., Hsu, Y., Barbot, S. D., Moore, J., & Chang, W. (2018, 08). Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Tectonic Geodesy