Search

We investigate the slip distribution of the 2008 May 12 Wenchuan M 7.9 earthquake using GPS data and InSAR measurements under a linear inversion scheme, with emphasis on the effect of three factors, including constraint on rake, different discretizations, and layered elastic model. Within our inversion parameterization context, we find the most influential factor would be constraint on rake. Without constraint on rake, the slip model seems physically wrong under the depth of 15 km, due to the limited depth resolution of the geodetic data used, especially the one orbit of InSAR measurements. Thus it is necessary to add a priori to the slip rake to obtain a reasonable fault source model. Different discretizations of the subfault patches have a notable impact on the slip distribution. Also, the layered elastic model predicts more slip at depth than does the half-space model, by about 15-20 per cent. The characteristics of slip distribution established through our inversions include some points as follows: (1) the most reliable results would be expected at depth range of 0-15 km; (2) three peak-slip asperities are inverted, at Yingxiu county (near epicentre), Yuejiashan county, and Beichuan county, respectively; (3) the inverted rake along the Yingxiu-Beichuan fault changes from dominantly thrusting motion at the southwest segments to dominant right-lateral or even pure right-lateral strike slip at the northeast segments, and (4) only thrust slip is occurred on the Guanxian-Jiangyou fault. [ABSTRACT FROM AUTHOR]

Ongoing controversies on the timing and kinematics of the Indo–Asia collision can be solved by palaeomagnetically determined palaeolatitudes of terranes bounding the Indo–Asia suture zone. We show here, based on new palaeomagnetic data from the Linzizong volcanic rocks (54–47 Ma) near the city of Lhasa, that the latitude of the southern margin of Asia was 22.8 ± 4.2°N when these rocks were deposited. This result, combined with revised palaeomagnetic results from the northernmost sedimentary units of Greater India and with apparent polar wander paths of India and Eurasia, palaeomagnetically constrain the collision to have occurred at 46 ± 8 Ma (95 per cent confidence interval). These palaeomagnetic results are consistent with tomographic anomalies at 15–25°N that are interpreted to locate the Tethyan oceanic slab that detached following collision, and with independent 56–46 Ma collision age estimates inferred from the timing of slowing down of India, high pressure metamorphism, the end of marine sedimentation and the first occurrence of suture zone and arc detritus on the Greater Indian margin. When compared with apparent polar wander paths of India and Eurasia, the ∼46 Ma onset of collision at 22.8 ± 4.2°N implies 2900 ± 600 km subsequent latitudinal convergence between India and Asia divided into 1100 ± 500 km within Asia and 1800 ± 700 km within India. [ABSTRACT FROM AUTHOR]

Shear wave splitting parameters (fast polarization directions and splitting times) obtained at the permanent broad-band station Wushi (WUS), which is situated in central Tien Shan, China, demonstrate systematic azimuthal variations. The fast polarization directions range from 26° to 85° and the delay times range from 0.4 to 1.6 s, suggesting a complex anisotropic structure that cannot be adequately explained by a single anisotropic layer with horizontal axis of symmetry. The azimuthal variations have an apparent 90° periodicity and thus can be best explained by a two-layer anisotropic model with horizontal symmetry axes. The optimal fast direction for the upper layer is in the 60°–80° range that is subparallel to the strike of the Tien Shan orogenic belt, and can consequently be attributed to the ongoing N–S directed shortening. The fast direction for the lower layer ranges from 85° to 105° and can be explained by lattice-preferred orientation of olivine crystals in the asthenosphere associated with present-day absolute plate motion of the Eurasian plate. [ABSTRACT FROM AUTHOR]

On the southeastern margin of the Tibetan Plateau lies a large region which seismicity and GPS data show to be actively deforming. This paper describes the active faulting in the region, and how it relates to the velocity field observed with GPS. In places the velocity field is accommodated by rotations about vertical axes, and most or all of the strain at the surface in the region appears to be released seismically. GPS velocities are then compared to velocities calculated using a model for deformation driven by gravitational driving forces. Using rheologies estimated from experimentally derived mineral flow laws, the model provides velocities that are in good agreement with observed GPS velocities. It is not possible to uniquely determine the rheology or flow velocity at depth, and there are two forms of model solution which match the observed horizontal surface velocities. In one of these, vertical planes deform by pure shear, and in the other vertical gradients of horizontal velocity are present within the crust. Two distinct regions of normal-faulting earthquakes are present in the region, and have mechanisms which are most easily explained by gravity-driven deformation. [ABSTRACT FROM AUTHOR]