2008年汶川8．0级地震发震断裂的滑动速率、复发周期和构造成因

Slip rates and recurrence intervals of the Longmen Shan active fault zone,and tectonic implications for the mechanism of the May 12 Wenchuan earthquake,2008,Sichuan,China

2008年5.12汶川大地震发生在中国大陆南北地震带中段.由于龙门山断裂带历史上只发生过3次6～6(?)级强震.而且其晚第四纪构造活动速率很低,以至于对其潜在地震危险性认识不足.为什么在龙门山地区突发大地震,该地震具有哪些特征?其成因机制是什么?本文在地震地质科学考察的基础上,利用震前的GPS观测结果,试图对上述问题进行一些初步的思考和探讨.结果表明,5.12汶川大地震是龙门山断裂带的映秀—北川断裂突发错动的结果,地表上形成200多公里长的地表破裂带;灌县—江油断裂在地震中也发生了破裂,形成的地表破裂带长达60多公里.震前的GPS观测表明,横跨整个龙门山断裂带的滑动速率不超过～2 mm/yr,单条断裂的活动速率不超过～1 mm/yr,与地震地质研究结果和历史地震记录相一致.利用地震地质考察和地震波反演得到的最大同震位移可以获得相当于5.12汶川大地震的强震复发周期为2000～6000年.龙门山断裂带发育在破裂强度很大的变质杂岩体中,断裂带本身在剖面上呈"犁形"或"铲形"结构,有利于能量积累,形成破坏性巨大的地震.所以,5.12汶川大地震是一次低滑动速率、长复发周期和高破坏强度的巨大地震,是一种值得高度重视和深入研究的新的地震类型.

The great Wenchuan earthquake of May 12, 2008 occurs on the Longmen Shan fault zone which forms a prominent section of the seismically very active seismic belt called the North- South Trending Seismic Belt by Chinese seismologists. There have been 3 earthquakes with magmtudes from 6 to 61/2 along the Longmen Shan fault zone during more then 2000 year documented history of the Sichuan province. Previous active faulting studies indicate slow （less than 3 mm/yr ） slip rate across the Longmen Shan fault zone. Why such a strong earthquake occurred in the Longmen Shan fault zone？ What are the characteristics of the May 12 earthquake？What is the tectonic mechanism of the earthquake？ On the basis of geological studies of the earthquake surface rupture zone and pre-earthquake GPS measurements in the region, we try to understand the questions mentioned above. The May 12 earthquake, 2008 is caused by displacement along the Yingxiu-Beichuan fault along which a more than 200 km long surface ruptures was formed. Another strand of the Longmen Shan fault zone, Guanxian-Jiangyou fault also ruptured as indicated by more then 60 km long surface ruptures. GPS measurements before the earthquake suggest that slip rate across the entire Longmen Shan fault zone does not excess about 2 mm/yr, and does not excess 1 mm/yr across individual fault strand. These data agree with seismogeological studies and historical seismicity of the Sichuan province. Using maximum coseismic displacement obtained from seismogeological studies and inversions from seismic waves, the recurrence intervals of great earthquakes, such as the 2008 Wenchuan earthquake, can be estimated to be 2000 46000 years. The Longmen Shan fault zone has a high dipping angle （more than 50°-60°） near surface and low-angle at depth （15-20 km）. This kind of listric shape favors significant strain or energy accumulation to form great earthquake. The May 12 Wenchuan earthquake, 2008 is characterized by slow strain accumulation, long recurrence interval, and significant damage power. It is a new type of earthquake event that deserves further studies.