Post-seismic motion following the 1997 Manyi (Tibet) earthquake: InSAR observations and modelling

Abstract

S U M M A R YOn November 8 1997 a M w 7.6 earthquake occurred in the Manyi region of northern Tibet, near the western end of the Kunlun Fault. Over 7 m of left-lateral slip occurred on a 200-kmlong fault. Here we use InSAR observations of post-seismic surface deformation following the Manyi earthquake to investigate possible causal mechanisms. Time-series of deformation are constructed from 26 interferograms, covering the entire length of the fault for nearly 4 yr after the earthquake. Three different modelling approaches are used to try and understand the observed variations in surface displacement. First order poroelastic models predict displacement fields which do not match those observed. Modelling of viscoelastic stress relaxation in a half-space with standard linear solid rheology beneath an elastic lid provides a reasonable fit to the observations, and demonstrates that two relaxation times are needed to characterize the post-seismic transient. The best-fitting viscosity is 4 × 10 18 Pa s, and the fully-relaxed shear modulus is 32 per cent of the purely elastic value. A model with a Maxwell viscoelastic halfspace underlying the lid cannot explain the observations: at later times, larger viscosities are required than at earlier times. This increase in effective viscosity with time may be consistent with stress relaxation occurring in a power-law rheology. The time-series are also inverted for distributed afterslip on an extension of the coseismic rupture. Along-strike correlation of coseismic and post-seismic slip maxima suggests that afterslip is a plausible mechanism. The maximum slip in the afterslip model is 0.72 m after 3 yr, and the equivalent moment release is approximately 20 per cent of the coseismic moment. This study shows that it is possible to rule out certain mechanisms as the dominant post-seismic process, but with the current InSAR data set it is difficult to distinguish between other plausible options.

Inference of Multiple Earthquake-Cycle Relaxation Timescales from Irregular Geodetic Sampling of Interseismic Deformation

“…Similar inferences have also recently been made on the basis of decadal observations of postseismic data alone (e.g., Ryder et al, 2007;Hearn et al, 2009). These results suggest that, in contrast to the classical single relaxation time model, variations in surface velocities throughout the earthquake cycle may be subtle and challenging to identify in the presence of multiple faults and observational noise.…”

“…However, steady-state CEH models are inconsistent with observations of spatially broad (> 100 km), time-variable, postseismic deformation following large earthquakes (e.g., Nur and Mavko, 1974;Hetland and Hager, 2003;Pollitz, 2003;Freed and Bürgmann, 2004;Freed et al, 2006;Ryder et al, 2007). Models for time-dependent earthquake-cycle deformation have generally focused on either the diffuse deformation of a linear viscoelastic subseismogenic layer (e.g., Thatcher, 1983;Savage, 1990;Dixon et al, 2002Dixon et al, , 2003Ergintav et al, 2002;Segall, 2002;Hilley et al, 2005, Motagh et al, 2007 or localized shear on a down-dip extension of the coseismic fault zone (e.g., Marone et al, 1991;Bürgmann et al, 2002), whereas recently more postseismic models have incorporated both (e.g., Freed et al, 2006;Johnson et al, 2009).…”

“…For example, in the context of earthquake-cycle models assuming a single relaxation timescale, analysis of postseismic deformation suggested upper mantle effective viscosities of, or less than, 10 18 Pa s (e.g., Pollitz et al, 2001) whereas late in the earthquake-cycle deformation has been interpreted as consistent with viscosities of a subseismogenic layer exceeding 10 19 Pa s (e.g., Hilley et al, 2005;Meade and Hager, 2005). More recently, the analysis of postseismic deformation signals has led to the suggestion that single relaxation timescale models may not be sufficient to describe behavior within the postseismic regime alone (e.g., Pollitz et al, 2001;Pollitz, 2003Pollitz, , 2005Freed and Bürgmann, 2004;Ryder et al, 2007;Hearn et al, 2009).…”

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

“…Wright et al [21] werethe first to use InSAR for studying the interseismic deformation. InSAR has been proven to be a important tool in researching the seismic cycle deformation after many successful applications in coseismic [20], postseismic [29] and interseismic [16,21,22,27,28] deformation. Walters et al [22] improved Wright and coworkers' results [21] by using SAR data from two different look directions, in their paper, they stated that the uncertainties in slip rate and locking depth are reduced up to 60% which is a huge improvement.…”

Patterns and mechanisms of coseismic and postseismic slips of the 2011 M W 7.1 Van (Turkey) earthquake revealed by multi-platform synthetic aperture radar interferometry

“…Several physical mechanisms can contribute to postseismic deformation (Fialko, 2004;Ryder et al, 2007), including afterslip, poroelastic rebound and viscoelastic relaxation. Following Peltzer et al's approach (1998) different Poisson's ratios were used for undrained and drained upper crust conditions to simulate potential surface deformation caused by poroelastic relaxation .…”