藏南安岗地堑的史前大地震遗迹、年龄及其地质意义

The Relics,Ages and Significance of Prehistoric Large Earthquakes in the Angang Graben in South Tibet

地表调查发现,沿近南北向亚东-谷露裂谷中段的安岗地堑存在地震大滑坡、多世代断层崖和断层崩积楔等多种类型的史前大地震遗迹.进一步的观测和年代分析表明：该区的古地震滑坡体至少存在新、老两期,其中规模最大的“尼续大滑坡体”应该是最新一次大地震所形成.该区T1到T6各阶地的形成时代从新到老分别为7.7～2.1ka、11.0～10.5ka、17.6～12.1ka、25.7～22.9ka、58.4～70.6ka和130～150ka,它们沿主边界正断层的平均垂直断距依次为2.8m、6.1～7.9m、10.3～12.5m、16.6～19.0m、28.0m和76.0m.其中T1和T2阶地上的断崖剖面揭示,最近两次大地震发生在距今约5.8±1.0ka和2.4±0.2ka.综合分析认为：安岗地堑的大地震活动具有较明显的丛集性特征,并且在距今约23～26ka以来一直处于大地震活跃期,期间的断层垂直活动速率为0.8～1.3mm/a,大地震的原地复发间隔大致为3.3～3.6ka,特征地震的矩震级为7.0～7.2,推算整个尼木地堑群的大震复发间隔最短可能只有约1.0～1.2ka.研究结果指示,藏南裂谷的大地震活动性明显比藏北的近南北向正断层更显著。

The Angang basin is a typical half-graben of the Nyemo graben group of the central Yadong-Gulu rift in south Tibet. Multiple-generation fault scarps, earthquake-induced large landslides, and colluvial wedges of fault scarps along the west mar- gin boundary normal fault of the graben contain rich information of prehistoric earthquake events and are effective marks to identify prehistoric large earthquakes （M≥7.0）. Based on the measured vertical displacements of multiple-generation fault scarps and OSL and 14C dating results of 36 samples from river terraces and fault scarp profiles, it can be estimated that the vertical displacement rate of western main boundary fault zone of Angang graben is 0.8-1.3 mm/a and at least 6 large earthquakes have occurred in the Angang graben since about 23 26 ka. According to the length and the maximum vertical displace ment of earthquake fault, the magnitude of such large earthquakes should be in a range of Mw7.0-7.2. The newly study result on paleoearthquake show that the ages of the last two large earthquakes should be about 5.8 ± 1.0 ka and 2.4± 0.2 ka before presents respectively, and the average recurrence interval between two large earthquakes would roughly be 3.3-3.6 ka in Anga ng, but the recurrence interval of large earthquakes in the whole Nyemo graben group should be short and inferred to he 1.0-1.2 ka. Prominent normal faulting and frequent large earthquakes indicate that the large earthquake activity is consistent with clusterd-slip model and are in the seismic active period since about 23-26 ka in the Angang graben and Nyemo graben group.