汶川M_S8.0地震前后龙门山及其邻区构造应力场时空分布特征

Spatio-temporal distribution of tectonic stress field in Longmenshan and its adjacent regions before and after Wenchuan M_S8.0 earthquake

基于2000年7月—2009年6月龙门山及其邻区的震源机制解资料,采用Gephart&Forsyth方法,反演得到了汶川地震前后该地区构造应力主方向的空间分布.结果显示:沿鲜水河断裂带及其北部地区,构造应力场变化显著,区域构造应力的最大主应力方位由NNW变为NW,断层错动类型由正断型兼走滑型变为走滑型;沿龙门山断裂带的构造应力最大主应力方向仍然为近EW和NE向,但其EW向范围在向NE方向扩张,其南部汶川地震震中附近异常带范围在收缩.另一方面,对上述时间段龙门山及其邻区不同时段构造应力场的反演结果表明,其构造应力场的特征参数(包括R值、应力洛德参数μ以及3个主应力的方位角和仰角等)均从第13时窗开始出现显著变化,这表明第13时窗(2006年12月—2007年1月)是一个构造应力显著变化的特征窗口.在该时窗内,地震能量积累达到一个临界状态,是汶川MS8.0地震发生的时间节点.

Based on the focal mechanism solutions for Longmenshan and its adja-cent regions from July 2000 to June 2009,the spatial distribution of the main direction of tectonic stress fields are retrieved in the regions before and after the Wenchuan MS 8.0 earthquake by using the Gephart ＆amp; Forsyth method.The results show that the tectonic stress fields change significantly along Xianshuihe fault zone and the region to the north,and that the maximum principal stress direction of the regional tectonic stress shifts from NNW to NW,while the faul-ting changes from combined normal fault and strike-slip to strike-slip only. Significant changes can also be observed in the tectonic stress field along the Longmenshan fault zone.Although the maximum principal stress direction still approximates to the east-west and north-east,its scope is expanding towards the east-north,and the abnormal belt near the epicenter of Wenchuan earth-quake shrinks.On the other hand,the retrieval results of the tectonic stress fields in Longmenshan and its adjacent regions during different periods from July 2000 to June 2009 show that the characteristic parameters of tectonic stress fields,including the R value,Lode parameter μ,azimuths and elevation angles of three principal stresses,begin to change significantly since the thirteenth window,suggesting that the thirteenth window （December 2006 to January 2007）is a characteristic window with significant changes in the tectonic stress. In the thirteenth window,the seismic energy is accumulated to a critical state, signaling the time node for the occurrence of Wenchuan MS 8.0 earthquake.