松潘—甘孜地区百年地震构造和现今动力学
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摘要
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.
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.
引文
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[13]Sherman S I.Tectonophysical analysis of the seismic process in zones of active lithospheric faults and medium-term earth-quake prediction[J].Geofizicheskii Zhurnal,2005,27(1):20-38.
[14]张家声,黄雄南,牛向龙,等.川主寺—黄龙左行走滑剪切和松潘—平武剪切转换构造[J].地学前缘,2010,17(4):15-32.
[15]王敏.GPS数据处理方面的最新进展及其对定位结果的影响[J].国际地震动态,2008(7):3-8.
[2]Bakun W H,Aagaard B,Dost B,et al.Implications for pre-diction and hazard assessment from the 2004Parkfield earth-quake[J].Nature,2005,437:969-974.
[3]Kagan Y Y,Jackson D D.Worldwide doublets of large shal-low earthquakes[J].Bulletin of the Seismological Society ofAmerica,1999,89:1147-1155.
[4]Felzer K R,Kilb D.A case study of two M～5mainshocks inAnza,California:Is the footprint of an aftershock sequencelarger than we think[J]?Bulletin of the Seismological Societyof America,2009,99(5):2721-2735.
[5]许志琴,索书田,韩郁菁,等.中国松潘—甘孜造山带的造山过程[M].北京:地质出版社,1992:1-190.
[6]马宗晋,张家声,汪一鹏.青藏高原三维变形运动学的时段划分和新构造分区[J].地质学报,1998,72(3):211-227.
[7]张家声,李燕,韩竹均.青藏高原向东挤出的变形响应及南北地震带构造组成[J].地学前缘,2003,10(特刊):168-175.
[8]Wells D L,Copppersmith K J.New empirical relationships among magnitude,rupture area and surface displacement[J].Bulletin of the Seismological Society of America,1994,84(4):974-1002.
[9]Sibson R H.Transient discontinuities in ductile shear zone[J].Journal of Structural Geology,1980,2(1/2):165-171.
[10]汪集扬,黄少鹏.中国大陆地区大地热流数据汇编(第二版)[J].地震地质,1990,12:351-366.
[11]汪洋.中国大陆大地热流分析[D].北京:中国科学院地质研究所,1999:118.
[12]Sherman S I,Lobatskaya R M.Correlation between lengths and depths of faults in the Baikal rift zone[J].Dokl AN SSSR,1972,205(3):578-581.
[13]Sherman S I.Tectonophysical analysis of the seismic process in zones of active lithospheric faults and medium-term earth-quake prediction[J].Geofizicheskii Zhurnal,2005,27(1):20-38.
[14]张家声,黄雄南,牛向龙,等.川主寺—黄龙左行走滑剪切和松潘—平武剪切转换构造[J].地学前缘,2010,17(4):15-32.
[15]王敏.GPS数据处理方面的最新进展及其对定位结果的影响[J].国际地震动态,2008(7):3-8.
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