Caijun Xu

A deforming block model for the present-day tectonics of Tibet

[1]xa0We use GPS data from 45 sites across the Tibetan Plateau surveyed between 1991 and 2001 to study the distribution of strain in that part of the India-Eurasia collision zone. The plateau is cut by a few major, rapidly slipping strike-slip fault zones, with broadly distributed strain between those zones. The GPS velocities can be fit well by a simple deforming block model that combines uniform strain with the motion of blocks separated by the major known fault zones within the plateau. The boundaries of the four blocks in this model correspond to the major, rapidly slipping faults. A rigid block model with a smaller number of blocks fits the data poorly and can be rejected. We estimate that 5.9 ± 0.7 mm/yr of extension in the N69°W direction occurs on the Yadong-Gulu rift south of the plateau; localized extension may extend as far north as the Nyainqentanglha Range. We find 7.4 ± 0.7 mm/yr of right-lateral slip on the Karakorum-Jiali fault zone, significantly slower than that previously estimated from offset geologic features. Our deforming block model and a two-dimensional single-fault screw dislocation model give lower and upper bounds of 4.4 ± 1.1 and 10.3 ± 0.4 mm/yr on the slip rate on the Kunlun fault, respectively, comparable, in its highest, to the long-term slip rate observed geologically. The distributed deformation is surprisingly uniform over the Tibetan Plateau, Qaidam Basin, and Qilian Shan and approximates a combination of pure shear and uniaxial contraction with the same axes of maximum contraction, ∼N32°E. This strain field is a combination of shortening and extension that produces little net dilatation (1% area loss per million years) at present. The broadly distributed strain most likely represents slip on many faults, each with a relatively low slip rate. Distributed conjugate strike-slip faulting is the most plausible mechanism to produce the observed strains, as supported by the record of medium to large earthquakes within the plateau. The distributed deformation is just as important in the accommodation of the total India-Eurasia convergence as is slip on the major faults. The eastward motion of central Tibet is as large as 50% of the convergence rate between India and Eurasia. However, this eastward motion is not motion of a relatively undeforming block bounded by rapidly moving strike-slip faults, as suggested by past extrusion models. Instead, much of this eastward extrusion is driven by the internal extension of the plateau. Our inference is in agreement with the model of F. Shen et al. [2001] that suggested east-west extension, north-south contraction, and eastward plateau growth dominate the present tectonics of the Tibetan Plateau.

Coseismic Slip Distribution of the 2008 Mw 7.9 Wenchuan Earthquake from Joint Inversion of GPS and InSAR Data

On 12 May 2008, the M wxa07.9 Wenchuan earthquake occurred at the eastern margin of the Tibetan plateau along the Longmen Shan fault. Based on previous geological and geophysical studies and offset maps from Advanced Land Observing Satellite/Phased Array type L-band Synthetic Aperture Radar ALOS/PALSAR data, a layered crustal structure and a five-segment rupture model are established to study the coseismic deformation of this event. Using the constrained least-squares method, a preferred coseismic slip model is derived from Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data. In particular, the Helmert variance component estimation (HVCE) method is used to determine the relative weight ratio between the GPS and InSAR data. In contrast to the initial weight ratio (0.752/0.246/0.002) among three-type observations (GPS horizontal, GPS vertical, and InSAR LOS displacement), the final optimal weight ratio is found to be 0.473/0.279/0.248, and the overall root mean square (rms) misfit for all three datasets decreases from 9.15xa0cm without HVCE to 5.46xa0cm with HVCE. As revealed by the preferred slip, the slip changes obviously along the strike direction from southwest to northeast and exhibits four asperities close to Hongkou town, Yuejiashan town, Beichuan County, and Nanba town, respectively. Two major asperities are located on the Hongkou and Beichuan segments with the maximum slip close to 10xa0m. The slips of Hongkou, Yuejiashan, and Beichuan segments are dominated by the thrust movement with a significant right-lateral strike-slip component. The Qingchuan segment experiences a dominant right-lateral strike slip with an average magnitude of 1–2xa0m. Most of the slip asperities are shallower than 10-km depth, except for the southwest part near the hypocenter where the slip may exceed 20xa0km. The seismic moment of the Wenchuan earthquake is 8.19×1020 N m ( M wxa07.91) based on the layered crust model, which is slightly larger than that using a corresponding homogeneous crust model ( M wxa07.87).

Spatially variable extension in southern Tibet based on GPS measurements

[1]xa0We use Global Positioning System (GPS) data and kinematic block models to study the present-day deformation of southern Tibet. GPS data from 33 sites in southern Tibet and Nepal surveyed between 1991 and 2000 reveal 13 ± 2 mm/yr of N110°E extension between Lhasa and Shiquanhe (80°E–91°E), of which 9.7 ± 3.0 mm/yr represents permanent extension of the Tibetan crust. The remaining ∼3 mm/yr results from elastic deformation from the locked, curving Main Himalayan Thrust fault. This extension is spatially nonuniform. One half to two thirds of the permanent extension is concentrated across the Yadong-Gulu rift, with an opening rate of 6.5 ± 1.5 mm/yr; most of the remainder occurs on or near the Thakkola graben, with little extension across the rifts between them. Differential velocities of sites north and south of the Yarlung-Zangbo suture in western Tibet imply that the suture or an adjacent structure may be active as a strike-slip fault. A numerical model suggests that right-lateral strike slip on the Yarlung-Zangbo suture may extend from the Karakorum Fault Zone in the west at least to the Yadong-Gulu rift in the east with ∼3 mm/yr slip rate, accommodating part of the eastward extrusion of Tibet. The convergence directions inferred from GPS are consistent with slip vectors of earthquakes; however, the rate of slip of India beneath Tibet along the Himalaya is lower than those previously estimated. We estimate a slip rate of 12.4 ± 0.4 mm/yr between longitudes 83°E and 88°E and 17 ± 1 and 19 ± 1 mm/yr in the western and eastern Himalaya, respectively. The variable slip rate correlates with the variable extension rate in southern Tibet, and we suggest that it results from variation in the deformation rate of the overriding Tibetan crust. We infer that the slower convergence rate in the central Himalaya is significant.

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

[1]xa0On 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.

Sensitivity of Coulomb stress change to the parameters of the Coulomb failure model: A case study using the 2008 Mw 7.9 Wenchuan earthquake

The Coulomb stress change has been widely employed to interpret mainshock-mainshock and mainshock-aftershock triggering as well as interactions amongst earthquake faults and volcanoes. This quantitative index is computed based on the Coulomb failure criterion and is a function of fault parameters including the source and receiver fault geometries, the friction coefficient on the receiver fault, and Skemptons coefficient of the host rock. Thus, for the robust determination of the Coulomb stress change, the sensitivity of the Coulomb stress change to these model parameters should be thoroughly assessed. However, notwithstanding numerous case studies, almost no systematic investigation of the sensitivity of the Coulomb stress change has been performed. Here we present an error estimator for the Coulomb stress change and then quantitatively investigate the sensitivity of the Coulomb stress change to the fault model parameters for the 2008 Mw 7.9 Wenchuan earthquake. Our results indicate that for this case the Coulomb stress change is the most sensitive to the uncertainty in the dip angle of the receiver fault, while the influences of the uncertainties in the slip model of the source fault, the strike, and rake angles of the receiver fault, and the friction and Skemptons coefficients cannot be neglected. Accordingly, it is crucial to perform a realistic estimate of the uncertainty in the Coulomb stress change. By performing such calculation, future Coulomb stress analyses such as the stress triggering of earthquake sequence and the likelihoods of potential earthquakes could be based on more robust Coulomb stress change maps.

3-D coseismic displacement field of the 2005 Kashmir earthquake inferred from satellite radar imagery

We use radar amplitude images acquired by the ENVISAT/ASAR sensor to measure the coseismic deformation of the 8 October 2005 Kashmir earthquake. We use the offset images to constrain the fault trace, which is in good agreement with field investigations and aftershock distribution. We infer a complete 3-D surface displacement field of the Kashmir earthquake using the offset measurements derived from both descending and ascending pairs of SAR images. The peak-to-peak offsets are up to (3.9, 3.6, 4.1) m in the east, north, and up directions respectively, i.e., 2.9 and 4.1 m along and across the fault assuming striking 325?. We model the coseismic displacements using a four-segment dislocation model in a homogeneous elastic half-space. We first estimate the source parameters using a uniform slip model. Then we fix the optimal geometric parameters and solve for the slip distribution using a bounded variable least-squares (BVLS) method. The resultant maximum slip is about 9.0 m at depth of 4–8 km beneath Muzaffarabad. We find a scalar moment of 2.34 × 1020 N m (Mw7.55), of which almost 82% is released in the uppermost 10 km.

Deformation and Source Parameters of the 2015 Mw 6.5 Earthquake in Pishan, Western China, from Sentinel-1A and ALOS-2 Data

In this study, Interferometric Synthetic Aperture Radar (InSAR) was used to determine the seismogenic fault and slip distribution of the 3 July 2015 Pishan earthquake in the Tarim Basin, western China. We obtained a coseismic deformation map from the ascending and descending Sentinel-1A satellite Terrain Observation with Progressive Scans (TOPS) mode and the ascending Advanced Land Observation Satellite-2 (ALOS-2) satellite Fine mode InSAR data. The maximum ground uplift and subsidence were approximately 13.6 cm and 3.2 cm, respectively. Our InSAR observations associated with focal mechanics indicate that the source fault dips to southwest (SW). Further nonlinear inversions show that the dip angle of the seimogenic fault is approximate 24°, with a strike of 114°, which is similar with the strike of the southeastern Pishan fault. However, this fault segment responsible for the Pishan event has not been mapped before. Our finite fault model reveals that the peak slip of 0.89 m occurred at a depth of 11.6 km, with substantial slip at a depth of 9–14 km and a near-uniform slip of 0.2 m at a depth of 0–7 km. The estimated moment magnitude was approximately Mw 6.5, consistent with seismological results.

Joint analysis of the 2014 Kangding, southwest China, earthquake sequence with seismicity relocation and InSAR inversion

Over 1000 earthquakes struck the northwest of Kangding on the Xianshuihe fault in southwest China between 22 and 29 November 2014, including two largest events of Mw 5.9 and Mw 5.6. The hypocenters of 799 relocated earthquakes suggest that two independent main shock-aftershock subsequences occurred on the Selaha and Zheduotang branches of the Xianshuihe fault, respectively. Fault slip inversion results from one interferometric synthetic aperture radar (InSAR) interferogram (26 September 2014 to 5 December 2014) show that the Mw 5.9 main shock produced a maximum slip of ~0.47u2009m at the depth of ~9u2009km. However, there is no distinct slip associated with the Mw 5.6 main shock. The InSAR determined moment is 2.36u2009×u20091018u2009Nm with a rigidity of 30u2009GPa, equivalent to Mw 6.2, which is about twofold the total seismic moment of all the recorded earthquakes during the InSAR time span. This large discrepancy between geodetic and seismic moment estimates indicates that there was probably rapid aseismic afterslip in the 2 weeks following the Mw 5.9 main shock. The released seismic energy of this earthquake sequence is far less than the accumulated strain energy since the 1955 M 712 earthquake on the same fault branch, which implies that the seismic risk on the Selaha-Kangding segment of the Xianshuihe fault remains high.

Source model of the 2015 Mw 6.4 Pishan earthquake constrained by interferometric synthetic aperture radar and GPS: Insight into blind rupture in the western Kunlun Shan

The Pishan, Xinjiang, earthquake on 3 July 2015 is the one of largest events (Mw 6–7) that has occurred along the western Kunlun Shan, northwestern edge of the Tibetan Plateau in recent time. It involved blind thrusting at a shallow depth beneath the range front, providing a rare chance to gain insights into the interaction between the Tarim Basin and the Tibetan Plateau. Here we present coseismic ground displacements acquired by high-resolution ALOS-2 SAR imagery and derived from GPS resurveys on several near-field geodetic markers after the event. We observed a maximum displacement exceeding 10u2009cm in the epicentral region. Analysis of the data based on a finite fault model indicates that coseismic slip occurred on a subsurface plane of 22u2009kmu2009×u20098u2009km in size with a dip of about 27° to the north and a strike of 114°, representing partial break of one ramp fault buried in Paleozoic strata at 8–16u2009km depths beneath the foothill of the western Kunlun Shan. This blind rupture is characterized largely by a compact thrusting patch with a peak slip of 0.63u2009m, resulting in a stress drop of 2.3u2009MPa. The source model yields a geodetic moment of 5.05u2009×u20091018u2009Nu2009·u2009m, corresponding to Mw 6.4. The Pishan earthquake suggests a northward migration of deformation front of the Tibetan Plateau onto the Tarim Basin. Our finding highlights slip along ramp-decollement faults to build up the western Kunlun Shan as the Tarim slab is subducting beneath western Tibet.

Geodetic imaging of potential seismogenic asperities on the Xianshuihe-Anninghe-Zemuhe fault system, southwest China, with a new 3-D viscoelastic interseismic coupling model

We use GPS and interferometric synthetic aperture radar (InSAR) measurements to image the spatial variation of interseismic coupling on the Xianshuihe-Anninghe-Zemuhe (XAZ) fault system. A new 3-D viscoelastic interseismic deformation model is developed to infer the rotation and strain rates of blocks, postseismic viscoelastic relaxation, and interseismic slip deficit on the fault surface discretized with triangular dislocation patches. The inversions of synthetic data show that the optimal weight ratio and smoothing factor are both 1. The successive joint inversions of geodetic data with different viscosities reveal six potential fully coupled asperities on the XAZ fault system. Among them, the potential asperity between Shimian and Mianning, which does not exist in the case of 1019u2009Pau2009s, is confirmed by the published microearthquake depth profile. Besides, there is another potential partially coupled asperity between Daofu and Kangding with a length scale up to 140u2009km. All these asperity sizes are larger than the minimum resolvable wavelength. The minimum and maximum slip deficit rates near the Moxi town are 7.0 and 12.7u2009mm/yr, respectively. Different viscosities have little influence on the roughness of the slip deficit rate distribution and the fitting residuals, which probably suggests that our observations cannot provide a good constraint on the viscosity of the middle lower crust. The calculation of seismic moment accumulation on each segment indicates that the Songlinkou-Selaha (S4), Shimian-Mianning (S7), and Mianning-Xichang (S8) segments are very close to the rupture of characteristic earthquakes. However, the confidence level is confined by sparse near-fault observations.