Interseismic deformation across the Longmenshan fault zone before the 2008 M8. 0 Wenchuan earthquake

By using regional GPS and leveling data and through an analysis considering seismotectonic background, this paper studies interseismic crustal deformation in the region across the Longmenshan fault zone before the 2008 M8. 0 Wenchuan earthquake, discusses the active tectonic and geodynamic model that caused the interseismic deformation, and, from these, analyzes the mechanism of brewing and producing the Wenchuan earthquake. The result mainly shows that, in the period from 1997 to 2007 horizontal shortening in the direction perpendicular to the fault zone and horizontal right-lateral shearing parallel to the fault zone occurred in the area from the middle segment of the fault zone to about 230 km away northwest. The shortening rate is estimated to be 1. 3 X 10(-8)/a (that is 0. 013 mm/km/a) and the distortion rate to be 2. 6 X 10(-8) /a. Vertical uplifting also happened in the same area during the period from 1975 to 1997, with uplifting rates changing from 0. 6 mm/a in between the frontal-range fault and the central fault of the fault zone to 2 to 3 mm/a near and northwest of the back-range fault of the fault zone. All these reflect that at least in the last 10 to over 30 years before the Wenchuan earthquake the frontal-range and central faults of the middle segment of the Longmenshan fault zone had been locked with strain building up. The main reason to cause such deformation is believed to be that, taking a low-velocity zone in the crust as an "uncoupling layer", the horizontal southeast-ward movement of the upper crust of the Bayan Hat block was counterworked from the South China block at the western edge of the Sichuan basin and transformed into the thrust movement along the middle segment of the fault zone. The interseismic locking of this fault segment made both the horizontal shortening perpendicular to the fault and the right-lateral shearing parallel to the fault occur in the upper crust of the Bayan Har block. However, during the period from 1997 to 2007, horizontal shortening across the northern segment of the Longmenshan fault zone was very weak, but a right-lateral distortion deformation of 0. 9 mm/a happened there. Such weak activity of the northern Longmenshan fault segment should apparently be attributed to that those fault zones of Minjiang, Huya and Longriba on the west of this fault segment have absorbed most of the horizontal east-ward movement of the Bayan Har block. In addition, the difference of the crustal deformation features between the middle and northern segments of the Longmenshan fault zone before the Wenchuan earthquake is consistent with the difference of along-fault distribution of the coseismic energy release during the earthquake.