利用成都、昆明遥测数字地震波形记录资料,研究大量小震震源机制解和视应力值,分析了区域应力场动态和视应力值的时-空分布。给出川青地块平均应力场方位的时间变化。川青地块平均主应力场的方位在汶川8.0级地震前出现明显扰动,同时,地块力轴张量的倾角变化也很明显。得出2006～2007年川青块体逆冲型地震比例偏高,汶川8.0级地震前区域中小地震震源错动类型发生变化的结果,与区域地形变的分析结果是一致的。根据地震波形全波段计算辐射能量,求得的视应力σapp值揭示区域视应力值的微动态起伏过程与区域主压应力场方位的转折类似,可用于地块蕴震物理过程的探索。从汶川8.0级地震前的中短期视应力σapp值的空间分布看,震中所在的龙门山断裂带是低应力分布区,而相对高视应力则分布在外围地区。这些图像可能揭示了汶川8.0级地震前中短期发震构造附近呈现的闭锁现象。

Abstract This article uses the telemetry digital seismic waveform data from Chengdu and Kunming to study the focal mechanism solutions and the apparent stress values of a large number of small earthquakes, and analyzes the regional stress field dynamics and the spatio-temporal distribution of apparent stress values. The temporal variation of the average stress field azimuth in Sichuan-Qinghai block is given. The average principal stress field azimuth of Sichuan-Qinghai block had obvious disturbances before the Wenchuan Ms8. 0 earthquake. This may be related to status change of block stress field between the Sumatra Ms9. 0 earthquake on December 26, 2004 in Indonesia and the Wenchuan Ms8. 0 earthquake in 2008. The ratio of thrust-type earthquake in the Sichuan-Qinghai block was high from 2006 t0 2007. The variations of the earthquake dislocation type of regional moderate and small earthquakes before the Wenchuan Ms8.0 earthquake are consistent with the state change of block displacement field revealed by GPS data, and also identical with analysis results of crustal shortening velocity change which is vertical to Longmenshan fault zone. The apparent stress value is determined on the basis of the whole band of the seismic waveform to calculate radiation energy. The fluctuation shape of the fitting trend of the apparent stress line is related to strong and weak of regional seismicity. It reveals that the micro-dynamic fluctuation process of regional stress value is similar to the azimuth transition of regional principal compressive stress field, which can be used to grope for pregnant physical process. The higher value areas of apparent stress are the possible areas of potential seismic risk. It can be seen from the spatial distribution of the medium-and short-term apparent stress before the Wenchuan Ms8. 0 earthquake that the Longmenshan fault zone is low stress distribution area, and the relatively high apparent stress is in peripheral area. These images may show the lock phenomenon near the medium and short Wenchuan Ms8.0 earthquake, for example, the focal parameter consistency of the sub-blocks in Sichuan and Yunnan Provinces changed with time, the thrust-type earthquakes in Sichuan-Qinghai block continually increased, the spatial distribution areas of the high apparent stress were discovered, and so on. The authors' continuous work in this aspect after the Wenchuan Ms8.0 earthquake may further reveal the relationship between these phenomena and future great earthquake.

国家自然科学基金(41090290)项目资助