Postseismic motion after the 2001 M_W 7.8 Kokoxili earthquake in Tibet observed by InSAR time series

Abstract

On November 14th 2001, a Mw 7.8 earthquake occurred in the Kokoxili region of northern Tibet. The earthquake ruptured more than 400 km along the western part of the Kunlun fault with a maximum of 8 m left‐lateral slip. In this paper, we use a multitemporal Interferometric SAR (InSAR) time series technique to map the postseismic motion following the large Kokoxili event. SAR data from Envisat descending orbits along five adjacent tracks covering almost the entire ruptured fault length are used to calculate the displacement time series for a period between 2 and 6 years after the earthquake. A peak‐to‐trough signal of 8 cm in the radar line of sight is observed during the period between 2003 and 2008. Two different mechanisms are employed to explain the observed surface displacements, namely afterslip and viscoelastic relaxation. The observations inverted for afterslip on and below the coseismic rupture plane shows that the maximum slip in the afterslip model is 0.6 m. The position of the maximum postseismic slip is located in the middle of two relatively high coseismic slip patches, which suggests that afterslip is a plausible mechanism. Models of viscoelastic stress relaxation in a Maxwell half‐space give a best fitting viscosity for the mid‐to‐lower crust of 2–5 × 1019 Pa s, and the principal postseismic relaxation process is due to viscous flow in the lower crust to upper mantle. However, the InSAR observations are incapable of distinguishing between localized (afterslip) and distributed (viscoelastic relaxation) deformation. And the lowest misfits are produced by mixed models of viscoelastic relaxation in the mantle below 70 km and afterslip in the crust. Modeling of viscoelastic relaxation in a Maxwell half‐space, and also a mixed mechanism model, enables us to place an effective viscosity of 2 × 1019 Pa s on the lower crust to mantle of northern Tibet.

Indo‐Asian Collision: Transition from Compression to Lateral Escape Tectonics

“…Geophysical observations of the present physical state of Tibetan crust are consistent with relatively hot, fluid-rich middle crust (e.g., Nelson et al, 1996;Wei et al, 2001;Unsworth et al, 2005; and see reviews in , conditions favorable for low viscosity (Beaumont et al, 2004). However, the distribution of regions of low seismic velocity (e.g., Yao et al, 2010;Hetényi et al, 2011;Yang et al, 2012) and high electrical conductivity (e.g., Bai et al, 2010) appears to be quite heterogeneous, and bulk rheologic properties determined at post-seismic timescales do not require low viscosity (e.g., Hilley et al, 2005;Ryder et al, 2010;2011;Wen et al, 2012). Yet, the geologic history of the growth of portions of the eastern Tibetan Plateau appears to require some version of lower crustal flow on a regional scale (e.g., Burchfiel and Royden, 2008).…”

Interseismic Deformation of the Altyn Tagh Fault Determined by Interferometric Synthetic Aperture Radar (InSAR) Measurements

“…Most interferograms were unwrapped using the branch-cut method [36], but some of them needed manual unwrapping (bridge) across the fault due to the loss of coherence. The unwrapping errors were detected and removed by a phase-closure technique [37,38]. After phase closure check, we correct the atmospheric errors and orbital ramps (Figure 3) respectively.…”

Coseismic slip in the 2010 Yushu earthquake (China), constrained by wide-swath and strip-map InSAR

“…To determine to what width (depth) slip could be allowed to occur, a series of inversions were carried out for a width range between 10 km and 32 km (e.g. Wen et al, 2012). Figure 3a shows the plot of RMS against inversion width.…”