Exploring the Spatio-temporal Evolution of the Lithospheric Strength After the Chi-Chi Earthquake: Revealing the Rheological Behavior of the Lower Crust in the Taiwan Orogenic Belt

The Chi-Chi earthquake (集集大地震), one of the worst natural disasters Taiwan had ever experienced, occurred on September 21, 1999, leaving thousands of people dead overnight and hundreds of billions NT dollars in economic losses. Due to its immense impact, the Chi-Chi earthquake has since been the focus of numerous researches and investigators in geosciences, aiming to better understand the seismogenic process of earthquakes and to further improve the ability of disaster prevention and mitigation.

Lithospheric strength and its temporal evolution are an important index for the earthquake hazard assessment. However, lithospheric strength is controlled by various physical conditions, such as stress, rock composition, grain size, water content, confining pressure and temperature. Because of this, understanding the behavior of rock deformation under their natural settings over geological time scales is a challenging task.

In order to explore the spatio-temporal evolution of lithospheric strength in the Taiwan orogenic belt, an international research team was established by Academia Sinica (AS, Taiwan), National Taiwan University (NTU, Taiwan), National Central University (NCU, Taiwan), Nanyang Technological University (NTU, Singapore), and University of Southern California (USC, USA). Researchers from the team developed a new algorithm to analyze the rock deformation in the lower crust, using the time series of surface displacement measured by GPS over more than a decade after the Chi-Chi earthquake. The research shows that when the rocks in the lower crust experience deformation, the relationship between stress and strain-rate is non-linear, instead of a linear relationship that most expected in the past. The result implies that the strength of rocks can span several orders of magnitude throughout the geological time scales. A large amount of short-term rapid deformation following a large earthquake may facilitate the mountain building in Taiwan.

After a major earthquake, the crust will experience a transient deformation that may last for years or even decades. This phenomenon is called “postseismic deformation”, which is usually attributed to the continuous slip on faults and the ductile deformation deep in the crust excited by the coseismic stress perturbation. Through the high-precision GNSS (Global Navigation Satellite System) observation network in Taiwan, the long-lasting postseismic surface displacements following the Chi-Chi earthquake are well-recorded, enabling us to explore the rheological properties of the lower crust in the Taiwan orogenic belt.

Previous studies on postseismic deformation often assume that the relationship of stress to strain rate in the lithosphere is linear. However, as the increasing amount of observational data collected from California, Sumatra, Japan, and Chile, there is more evidence showing that the ratio between stress and strain rate does not remain constant over time. The research team discovered that the observational data collected for over ten years following the Chi-Chi earthquake cannot be explained through linear rheology alone. Instead, it is possible that the postseismic deformation in the lower crust is dominated by transient creep behavior along with the following non-linear rheology. 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.

Furthermore, by incorporating the creep index of crustal rocks gathered from the laboratory experiments, the research team has estimated the thermal gradients from about 20°C/km in the Coastal Plain to 30°C/km in the Central Range. Although the inferred thermal gradients are subject to uncertainties, they are consistent with a number of independent studies. This demonstrates that even the large difference of scale between laboratory and nature, the rheological behavior of rocks is in a broad agreement, suggesting that the geodetic data may bridge the disparity in scales between the laboratory environment and the lithosphere. The new approach can estimate the rheological parameters in the lower crust, serving as a basis for geodynamic models so as to shed new light on the seismic cycle and the Taiwan orogeny.

This research is the joint effort of the following researchers: Chi-Hsien Tang (唐啟賢), Post-graduate student at Department of Geosciences, NTU, Taiwan, and substitute military serviceman at Institute of Earth Sciences, AS; Ya-Ju Hsu (許雅儒), Research Fellow at Institute of Earth Sciences, AS, and Joint-appointment Professor at Department of Geosciences, NTU, Taiwan; Sylvain Barbot, Assistant Professor at Department of Geosciences, USC; James D. P. Moore, Postdoctoral Fellow at Earth Observatory of Singapore, NTU, Singapore; Wu-Lung Chang (張午龍), Associate Professor at Department of Earth Sciences, NCU. The study has been published in Science Advances, the well-known academic journal under the American Association for the Advancement of Science (AAAS), on February 27, 2019.