Probing the Deep Rheology of Tibet

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

The researchers in this project are using the satellite technique of InSAR (Interferometric Synthetic Aperture Radar) to study ground deformation following two large earthquakes that occurred recently in northern Tibet, in order to constrain the mechanical properties of the rocks at depth beneath the Tibetan plateau. This region is one of the most tectonically active in the world. It lies within the broad India-Eurasia collision zone, in which the rapid motion of the Indian plate towards the Eurasian plate causes deformation from the Himalayan front northwards for thousands of miles. Numerous active faults dissect the plateau, including several very long structures such as the Karakorum, Altyn Tagh and Kunlun faults. There is currently vigorous debate as to whether Tibet behaves more as a weak viscous fluid or a series of rigid blocks between the major faults. This rheological debate can potentially be informed by studying the deformation associated with large earthquakes. In this sense, large earthquakes signal the beginning of a lithospheric-scale rock mechanics experiment. Transient aseismic deformation which occurs after the main shock is a response to the stress changes at depth brought about by the earthquake. The magnitude and spatial distribution of the observed postseismic signal, and the timescale over which the initially rapid motion levels off, all depend on the rheological structure and properties of the rocks in the lithosphere. In turn, the specifics of the rheology control the rate at which stress is redistributed in the lower crust and upper mantle following an earthquake. A particular focus of the data analysis is the elucidation of the degree to which deformation is localized beneath the seismogenic part of the Kunlun fault in northeastern Tibet, and to diagnose significant variations in viscous strength in the lower crust and upper mantle. The InSAR datasets are accompanied by GPS data from a network of 44 sites across the Kunlun fault. Computer models are being run to test various candidate rheologies against this rich dataset. To help answer their questions, the investigators also plan to use complementary geophysical data from various lithospheric studies in Tibet, as well as geological information from studies across the Kunlun fault and Altyn Tagh fault further west. Among the broader impacts of this work are that it will contribute to further understanding of the earthquake cycle, helping inform earthquake hazard researchers. The proposal will also contribute to the support and training of a postdoc.