Under the long-term support by Academia Sinica and the Ministry of Science Technology, Dr. Ya-Ju Hsu and her research group investigated the lower-crustal rheology beneath the Taiwan orogenic belt using the Global Positioning System (GPS) observations more than a decade following the 1999 Chi-Chi earthquake. Their result manifested the non-linear relationship between stress and strain rate in the lower crust, against the linear relationship often considered previously. The short-term modulation of lower-crustal strength during the seismic cycle throughout the geological time may largely facilitate the mountain building. This work was published in Science Advances, the well-known multidisciplinary open-access scientific journal under the American Association for the Advancement of Science (AAAS), on 27 February 2019.
The strength of the lithosphere and its temporal evolution play a critical role in seismic hazard and risk assessment. However, the uncertainties are controlled by the situ physical conditions, such as stress, rock composition, grain size, water content, confining pressure, and temperature. Therefore, to understand how rocks deform under stress in their natural settings is always challenging. To overcome this issue, Dr. Hsu and her group cooperated with National Taiwan University (NTU), National Central University (NCU), Earth Observatory of Singapore (EOS) and University of Southern California (USC) to develop a new algorithm that probes the deformation in the deep crust directly using geodetic data. The new approach allowed them to explore the rheology of the lower crust with the strong constraint provided from numerous and dense GPS data.
Dr. Hsu and her group found that a uniform linear rheology cannot explain the GPS data following the Chi-Chi earthquake. In contrast, they suggested that transient creep followed by a non-linear steady-state creep has dominated the behavior of accelerated viscoelastic flow in the lower crust after the Chi-Chi earthquake. This indicates that the ductile deformation involves the movement of dislocations through the crystal lattice of the material, which is in accordance with the presence of seismic anisotropy.
Incorporating the laboratory data and associated uncertainties, inferred thermal gradients suggest an eastward increase from about 20°C/km in the Coastal Plain to 30°C/km in the Central Range of Taiwan. Although the inferred thermal gradients are subject to uncertainties, they are consistent with a number of independent studies. The new model reconciles geodetic observations, seismic anisotropy, and heterogeneous thermal gradients in the Taiwan orogenic belt. The rheological parameters may be integrated in geodynamic models of the Taiwan orogeny to incorporate the short-term effect of the seismic cycle.
Tang, C.-H., Y.-J. Hsu, S. Barbot, J. D. P. Moore, W.-L. Chang, (2019), Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake, Sci. Adv., 5, eaav3287.
Figure: The GPS station at Chi-Tsai Hu (the rainbow lake), Nantou. Credit: Mr. Hsuan-Han Su.