松潘—甘孜地区百年地震构造和现今动力学

Seismotectonics since last 110 years and recent geodynamics in Songpan-Ganzi region

研究区位于青藏高原的东北隅(96°～107°E,30°～35°N)。基于该地区长度大于2km的4 781条1∶20万数字化实测断裂、1900年以来的5 220条数字地震记录,以及野外地质观测数据,识别出993条不同属性的地震断层,构建了该地区百年地震构造格局。1970年以来十年期地震断层跃迁图像表明,自20世纪80年代中期白马—虎牙强烈震群爆发之后,地震活动在沿各主要走滑断层带自西(北西)向东(南东)迁移的同时,逐渐向中部贡玛—达曲断裂带和南部鲜水河断裂带的东南段集中。地震活动的断裂构造联系主要表现为挤压剪切转换机制和典型的楔顶效应。研究区165个GPS速度矢量展现了与3个地块和以鲜水河断裂带为主的速度域、速度梯度带和速度扰动区。跨研究区南缘鲜水河断裂带的位移速率因贡玛—达曲断裂带汇聚而达到了6.5～8.6mm/a,而跨北缘东昆仑断裂带的位移速度只有1.8～2.2mm/a。因鲜水河断裂走向在其中南段发生向南的急剧偏转,垂直断层面的位移矢量分量不断增强,形成了汶川8.0级地震成核及NE向单边破裂的动力学条件。

The study area,Songpan-Ganzi region,lies in the northeast corner of the Qinghai-Tibet 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 Baima-Huya area in the mid-1980s,seismic activities increased obviously along the central Gongma-Daqu 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 6.5-8.6 mm/a as a result of the converging of the Gongma-Daqu earthquake faults,whilst across the East Kunlun fault zone in the north are only about 1.8-2.2 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.