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[1] We derive a detailed horizontal velocity field for the southeast borderland of the Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2004. Our results reveal a complex deformation field that indicates that the crust is fragmented into tectonic blocks of various sizes, separated by strike-slip and transtensional faults. Most notably, the regional deformation includes 10-11 mm/yr left slip across the Xianshuihe fault, $7 mm/yr left slip across the Anninghe-Zemuhe-Xiaojiang fault zone, $2 mm/yr right slip across a shear zone trending northwest near the southern segment of the Lancang River fault, and $3 mm/yr left slip across the Lijiang fault. Deformation along the southern segment of the Red River fault appears not significant at present time. The region south and west of the XianshuiheXiaojiang fault system, whose eastward motion is resisted by the stable south China block to the east, turns from eastward to southward motion with respect to south China, resulting in clockwise rotation of its internal subblocks. Active deformation is detected across two previously unknown deformation zones: one is located $150 km northwest of and in parallel with the Longmenshan fault with 4-6 mm/yr right-slip and another is continued south-southwestward from the Xiaojiang fault abutting the Red River fault with $7 mm/yr left slip. While both of these zones are seismically active, the exact locations of faults responsible for such deformation are yet to be mapped by field geology. Comparing our GPS results with predictions of various models proposed for Tibetan Plateau deformation, we find that the relatively small sizes of the inferred microblocks and their rotation pattern lend support to a model with a mechanically weak lower crust experiencing distributed deformation underlying a stronger, highly fragmented upper crust.

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Abstract. We combine 6 years of Global Positioning System (GPS) data with 20 years of trilateration data and a century of triangulation, taped distance, and astronomic azimuth measurements to derive 66 interseismic station velocities in the greater Los Angeles region. We interpolate the velocities to construct a regional strain rate map beyond the Los Angeles basin. Our results generally agree with the model proposed by the Working Group on California Earthquake Probabilities in 1995. Important regional findings of this study are as follows: (1)

The 2008 Wenchuan earthquake occurred on imbricate, oblique, steeply dipping, slowly slipping, listric-reverse faults. Measurements of coseismic slip, the distribution of aftershocks, and fault-plane solution of the mainshock all confirm this style of deformation and indicate cascading earthquake rupture of multiple segments, each with coseismic slip occurring in the shallow crust above a depth range of 10 to 12 km. Interactions among three geological units-eastern Tibet, the Longmen Shan, and the Sichuan basin-caused slow strain accumulation in the Longmen Shan so that measurable preearthquake slip was minor. Coseismic deformation, however, took place mostly within the interseismically locked Longmen Shan fault zone. The earthquake may have initiated from slip on a fault plane dipping 30-40 • northwest in a depth range from 15 to 20 km and triggered oblique slip on the high-angle faults at depths shallower than 15 km to form the great Wenchuan earthquake.

[1] Interferometric synthetic aperture radar data are used to map the interseismic velocity field along the Haiyuan fault system (HFS), at the north-eastern boundary of the Tibetan plateau. Two M $ 8 earthquakes ruptured the HFS in 1920 and 1927, but its 260 km-long central section, known as the Tianzhu seismic gap, remains unbroken since $1000 years. The Envisat SAR data, spanning the 2003-2009 period, cover about 200 Â 300 km 2 along three descending and two ascending tracks. Interferograms are processed using an adapted version of ROI_PAC. The signal due to stratified atmospheric phase delay is empirically corrected together with orbital residuals. Mean line-of-sight velocity maps are computed using a constrained time series analysis after selection of interferograms with low atmospheric noise. These maps show a dominant left-lateral motion across the HFS, and reveal a narrow, 35 km-long zone of high velocity gradient across the fault in between the Tianzhu gap and the 1920 rupture. We model the observed velocity field using a discretized fault creeping at shallow depth and a least squares inversion. The inferred shallow slip rate distribution reveals aseismic slip in between two fully locked segments. The average creep rate is $5 mm yr À1 , comparable in magnitude with the estimated loading rate at depth, suggesting no strain accumulation on this segment. The modeled creep rate locally exceeds the long term rate, reaching 8 mm yr À1, suggesting transient creep episodes. The present study emphasizes the need for continuous monitoring of the surface velocity in the vicinity of major seismic gaps in terms of seismic hazard assessment.

Abstract. We collected GPS data from the southern Tarim basin, the Qaidam basin, and the western Kunlun Shan region between 1993 and 1998 to determine crustal deformation along the Altyn Tagh fault system at the northern margin of the Tibetan plateau. We conclude from these data that the Altyn Tagh is a left-lateral strike slip fault with a current slip rate of •9 mm/yr, in sharp contrast with geological estimates of 20-30 mm/yr. This contrast poses an enigma: because the GPS data cover a wider region than the geologic data, they might be expected to reveal somewhat more slip. We also find that the Tarim and Qaidam basins behave as rigid blocks within the uncertainty of our measurements, rotating clockwise at a rate of •11 and •4.5 nrad/yr, respectively, with respect to the Eurasia plate. The rotation of the Tarim basin causes convergence across the Tian Shan, increasing progressively westward from •6 mm/yr at 87øE to •18 mm/yr at 77øE. Our data and other GPS data suggest that the Indo-Asia collision is mainly accommodated by crustal shortening along the main Himalayan thrust system (•53%) and the Tian Shan contractional belt (•19%). Eastward extrusion of the Tibetan plateau along the Altyn Tagh and Kunlun faults accommodates only •23% of the Indo-Asia convergence.

International audienceWe use high-resolution Synthetic Aperture Radar- and GPS-derived observations of surfacedisplacements to derive the first probabilistic estimates of fault coupling along the creeping section of theSan Andreas Fault, in between the terminations of the 1857 and 1906 magnitude 7.9 earthquakes. Usinga fully Bayesian approach enables unequaled resolution and allows us to infer a high probability ofsignificant fault locking along the creeping section. The inferred discreet locked asperities are consistentwith evidence for magnitude 6+ earthquakes over the past century in this area and may be associated withthe initiation phase of the 1857 earthquake. As creeping segments may be related to the initiation andtermination of seismic ruptures, such distribution of locked and creeping asperities highlights the centralrole of the creeping section on the occurrence of major earthquakes along the San Andreas Fault