Abstract:

The study area, SongpanGanzi region, lies in the northeast corner of the QinghaiTibet plateau (96°107°E，30°35°N). Totally 993 earthquake faults with various properties are distinguished by statistic analysis based on 4781 observed faults longer than 2 km in 1 to 200000 scale and 5220 seismic records since 1900, and field investigation data, that allows to establish a centennial seismotectonic framework in the study area. Changes of the earthquake faults in decade interval since 1970 shows that after a strong earthquake swarm occurred in BaimaHuya area in the mid1980s, seismic activities increased obviously along the central GongmaDaqu earthquake faults and the southeast segment of the Xianshuihe earthquake faults when they migrated eastwards along each major strikeslip fault zone. The relationship between the seismicity and the fault movement was dominantly controlled by a transpressional shearing and showing a typical wedge effect. Three relatively weak deformation velocity domains, a major velocity gradient along Xianshuihe fault zone and several local velocity perturbations are indicated by 165 GPS velocity vectors observed before 2008. Rates of displacement across the Xianshuihe fault zone in the south of the study area are about 6.58.6 mm/a as a result of the converging of the GongmaDaqu earthquake faults, whilst across the East Kunlun fault zone in the north are only about 1.82.2 mm/a. The condition of the Wenchuan 8.0 earthquake nucleation is accumulated by an increase of velocity vector components vertical to the fault plane, since the trend of the Xianshuihe fault is sharply southward bending at its mid and southeast segment, which also caused the earthquake fractures running northeastwards from the epicenter.

Key words:  earthquake fault, transpressional shear, GPS, digital, allocation of followup earthquake