JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, B08405, doi:10.1029/2011JB009043, 2012 Postseismic motion after the 2001 MW 7.8 Kokoxili earthquake in Tibet observed by InSAR time series Yangmao Wen,1,2,3 Zhenhong Li,3 Caijun Xu,1,2 Isabelle Ryder,4 and Roland Bürgmann5 Received 21 November 2011; revised 6 June 2012; accepted 20 June 2012; published 9 August 2012. [1] 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 Â 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 Â 1019 Pa s on the lower crust to mantle of northern Tibet. Citation: Wen, Y., Z. Li, C. Xu, I. Ryder, and R. Bürgmann (2012), Postseismic motion after the 2001 MW 7.8 Kokoxili earthquake in Tibet observed by InSAR time series, J. Geophys. Res., 117, B08405, doi:10.1029/2011JB009043. 1. Introduction generally proposed to be a result of (1) afterslip on unruptured fault patches near the surface or downdip of the coseismic [2] Postseismic motion following large earthquakes rupture [e.g., Marone et al.,1991;Bürgmann et al., 2002]; represents the response of host rocks to the redistribution of (2) viscoelastic deformation of the lower crust and/or upper coseismic stress changes. Geodetic and seismic observations mantle [e.g., Pollitz et al., 2000], where coseismic stress show that postseismic displacements are typically an order changes imparted to the hot lower crust and upper mantle can- of magnitude smaller than coseismic displacements and not be sustained and drive viscoelastic flow; and/or (3) por- decrease with time over a period of years following the oelastic rebound [e.g., Peltzer et al., 1998; Jónsson et al., 2003], event. Geodetic measurements of postseismic motion are where coseismic pressure changes drive fluid flow within the upper crust. Each mechanism is capable of producing 1School of Geodesy and Geomatics, Wuhan University, Wuhan, China. observable surface displacements and can operate separately 2Key Laboratory of Geospace Environment and Geodesy, Ministry of or in combination [e.g., Fialko, 2004; Freed et al., 2006a; Education, China, Wuhan, China. Biggs et al., 2009]. Although different mechanisms can pro- 3COMET+, School of Geographical and Earth Sciences, University of Glasgow, Glasgow, UK. duce spatially similar displacement fields [Savage, 1990; 4School of Environmental Sciences, University of Liverpool, Liverpool, Hearn, 2003], surface displacements with a good spatial and UK. temporal resolution over time periods ranging from days to 5Berkeley Seismological Laboratory, University of California, Berkeley, years following the earthquake are more helpful in identifying California, USA. what is happening at depth, which might provide insight into Corresponding author: C. Xu, School of Geodesy and Geomatics, future seismic hazards. Wuhan University, Luoyu Rd. 129, Wuhan, Hubei 430079, China. [3] On November 14th 2001, a Mw 7.8 earthquake struck ([email protected]) the Kokoxili region of northern Tibet; no casualties were ©2012. American Geophysical Union. All Rights Reserved. reported, due to the extremely low population density and 0148-0227/12/2011JB009043 B08405 1of15 B08405 WEN ET AL.: 2001 KOKOXILI POSTSEISMIC MOTION B08405 the lack of high buildings in the remote region. Surface located in the Kunlun mountain range in northern Tibet, ruptures of the 2001 Kokoxili earthquake extended over 426 extends for nearly 1500 km along strike, delineating a km in total, with an azimuth averaging N100 Æ 10E[van transition from a low-relief, high-elevation plateau in the der Woerd et al., 2002b; Lasserre et al., 2005; Xu et al., south to a northern domain characterized by active high 2006; Klinger et al., 2006]. The rupture initiated west of mountain ranges and intramontane basins [Kirby et al., Taiyang Lake, on a secondary strike-slip fault within the 2007]. The Kunlun fault is also coincident with the north- complex horsetail system that forms the western end of the ern boundary of the Songpan-Ganzi-Hoh Xil Terrane along Kunlun fault. It then propagated eastward, breaking through the Kunlun suture [Yin and Harrison, 2000]. Studies of a 45 km long and 10 km wide pull-apart, to merge with the satellite images, cosmogenic surface dating, radiocarbon main trace of the Kunlun fault. The rupture continued on the dating and trench surveys along the Kunlun fault zone sug- Kunlun Pass fault along the southern front of the Burchan gest an average slip rate of 10–20 mm/yr during the Qua- Budai Shan, ending just east of 95E[van der Woerd et al., ternary [van der Woerd et al., 2002a; Lin et al., 2006]. Using 2002b; Antolik et al., 2004]. The observed displacements 14C dating of offset fluvial fans over late Pleistocene to essentially correspond to pure left-lateral strike-slip motion Holocene time, Kirby et al. [2007] reported that the slip rates on subvertical faults. In a field study, Xu et al. [2006] decrease systematically along the eastern 150 km of the reported a maximum coseismic horizontal left-lateral dis- fault from >10 mm/yr to <2 mm/yr. GPS observations since placement of 7.6 m (at 35.767N, 93.323E). Analysis from the 1990s indicate a left-lateral strike slip rate of 10–20 mm/ 1 m resolution IKONOS and 0.61 m resolution Quickbird yr on the Kunlun fault [Wang et al., 2001; Zhang et al., images suggests that the coseismic strike-slip offsets range 2004; Hilley et al., 2009]. Thus, both geological and GPS from 2 m up to 16.7 m, but are generally between 3 and 8 m data suggest that the slip rate along the Kunlun fault is at the [Lin and Nishikawa, 2007]. Using 4 adjacent tracks of ERS level of 10 mm/yr, which indicates that the Kunlun fault InSAR images, Lasserre et al. [2005] estimated a maximum plays a key role in models for extrusion of central Tibet [Li left-lateral slip of 8 m occurring at depths between 0 and 5 et al., 2005]. km using a strike-slip model with distributed slip. Both the [6] In the past 100 years, a series of large earthquakes (M ≥ rupture length and maximum coseismic displacement are the 7) have occurred along the Kunlun fault zone, including the largest ever reported for an intraplate earthquake [Yeats et most recent November 14th, 2001 Kokoxili event on the al., 1997]. Kusai Hu segment (Figure 1) [Division of Earthquake [4] Shen et al. [2003] modeled 6 months of postseismic Monitoring and Prediction, 1995, 1999; Lasserre et al., Global Positioning System (GPS) data for the Kokoxili 2005; Wen et al., 2007]. The 1963 Ms 7.0 Alan Lake event earthquake and suggested that both afterslip and viscoelastic and 1937 M 7.5 Dongxi Co event ruptured along the central relaxation occurred in a lower crustal weak layer. Using one segment of the Kunlun fault [Fitch, 1970; Jia et al., 1988]. year of GPS data collected at the same stations as Shen et al. The 1973 Ms 7.3 Yiji Taicuo event occurred along the [2003], Ren and Wang [2005] reported that nearly 50 per- western part of the Manyi fault with an unknown length of cent of the total surface deformation took place during the surface rupture [Velasco et al., 2000]. In 1997, the Mw 7.6 first two weeks, implying that the deformation rate rapidly Manyi earthquake ruptured the westernmost 180 km of the decreased with time. Combining GPS observations pre- Manyi fault with a maximum left-lateral displacement of 7 m sented in Ren and Wang [2005] with 5 descending tracks [Peltzer et al., 1999; Funning et al., 2007], which is located and one ascending track of Envisat ASAR images collected to the southwest of the western end of the Kunlun fault and from early 2003 to early 2007, Ryder et al. [2011] inferred considered as a branch fault of the Kunlun system. Trenching that the most likely mechanism of post-seismic stress survey studies and offset geomorphological features indicate relaxation is time-dependent distributed creep of viscoelastic that the average recurrence interval of great earthquakes on material in the lower crust.