GPS-derived crustal deformation in southwestern China

The Sichuan - Yunnan region is divided into nine active secondary crustal blocks, based on several GPS repeat surveys at more than 200 GPS sites during the period 1999 - 2005. Velocities of the nine secondary blocks are calculated and analysed. The strain field within the area related to the 2004 Sumatra - Andaman earthquake event is also analysed. Results indicate that the crustal movement in the northern and western areas of the Sichuan - Yunnan region is stronger than that in the south and east. The horizontal velocities change from 19 - 20 mm/y in the northern and the central rhombic block to 11.7 mm/y in the southern rhombic block. The orientations of block motion vary from 99 degrees in the north to 126 - 150 degrees in the central area and 156 - 188 degrees in the south, implying that the motion of the Sichuan Yunnan rhombic block is dominated by a clockwise rotation. The velocity differences between blocks inside and outside the rhombic block are about 6.5 - 7.7 mm/y in the northern and central Sichuan Yunnan region. The southeastward extrusion rate of the Tibetan Plateau shows a remarkable downtrend of up to 47% along the Xianshuihe Fault, suggesting an increase in strain accumulation and hence an area prone to strong earthquakes. The horizontal coseismic deformation caused by the Mw9.0 Sumatra - Andaman earthquake is <10 mm with a south- southeast orientation towards the earthquake epicentre. The dilatational strain rates from coseismic displacements reveal a possible interaction between the extrusion from the Tibet plateau interior and the underthrust effects from the Sumatra - Andaman earthquake.