大地震前后实测地应力状态变化及其意义--以龙门山断裂带为例

EVALUATING MEASURED IN-SITU STRESS STATE CHANGES ASSOCIATED WITH EARTHQUAKES AND ITS IMPLICATIONS: A CASE STUDY IN THE LONGMENSHAN FAULT ZONE

实测地应力状态在连续地震事件前后的变化特征,对于应用地应力实测数据探索开展地震预报等研究有重要意义,但一直以来缺少典型实例研究。以龙门山断裂带西南段的跷碛和映秀地区为研究区,利用该地区汶川地震前至芦山地震后获得的地应力实测数据,分析了表征地应力状态的特征参数在汶川和芦山地震事件前后变化特征,探讨了其对地震预报研究的意义。研究表明,跷碛地区地应力状态特征参数K_(HV)、K_(Hh)和μ_m变化表现为芦山地震后值(QQ-14)大于汶川地震前(QQ-99),QQ-99结果大于汶川地震后值(QQ-09),而主应力梯度系数变化为QQ-09>QQ-14>QQ-99;分析认为K_(HV)、K_(Hh)和μ_m变化规律能准确反映汶川和芦山地震事件前后跷碛地区构造应力场演化特征,而仅用主应力随深度变化梯度系数变化特征,不能完全准确地反映构造应力场调整变化情况;映秀地区,除K_(Hh)外,主应力随深度变化梯度系数、K_(HV)和μ_m均表现为汶川地震后结果大于震前,其变化反映的应力场调整变化特征需要补充数据检验;利用地应力状态参数变化规律开展地震预报探索研究时,长期的、可对比的高质量地应力测量数据是研究有所突破的关键。研究成果对于龙门山地区构造应力场和减灾防灾研究有重要意义,对于应用地应力数据探索开展地震预报研究等有参考价值。

Evaluating measured in-situ stress state changes associated with large earthquake events plays a crucial role in earthquake prediction using measured in-situ stress data, whereas typical examples were not stated systematically yet. In this study, the Qiaoqi and Yingxiu regions which contain measured in-situ stress data crossing two large earthquakes （the Wenchuan earthquake and the Lushan earthquake） were selected as examples to study this issue. The changes of the stress state before and after the Wenchuan and Lushan earthquakes in the Qiaoqi and Yingxiu regions were analyzed by comparing the magnitudes of gradient coefficient, characteristic indexes K HV （ratio of the maximum horizontal stress to vertical stress）, K Hh （ratio of the maximum horizontal stress to minimum horizontal stress）, and μ m （shear stress normalized by average stress） obtained before and after these two earthquakes. The results indicate that the average magnitudes of K HV , K Hh , and μ m obtained after the Lushan earthquake （QQ-14） in Qiaoqi region are larger than those obtained before the Wenchuan earthquake （QQ-99）; and the mean magnitudes of these parameters obtained from QQ-99 are larger than those obtained after the Wenchuan earthquake （QQ-09）. However, the evolution feature of gradient coefficient before and after these two earthquakes can be characterized by QQ-09〉QQ-14〉QQ-99. Based on above estimation, it was stated that the changes of K HV , K Hh , μ m can reflect the evolution trend of the regional tectonic stress filed in Qiaoqi region, while the gradient coefficient can not reflect the evolution feature accurately enough. The magnitudes of gradient coefficient, K HV , and μ m obtained from measured in-situ stress data after the Wenchuan earthquake exceed those after this large earthquake in Yingxiu region, while the K Hh shows contradictory trend. The tectonic stress evolution in Yingxiu region should be verified by supplementing additional stress data. Long-term measured in-situ stress data which can be compared is the key element in significant breakthrough of earthquake prediction using the change laws of stress state characteristics parameters. Conclusions drawn in this study is of great significance for tectonic stress field estimation and disaster prevention and reduction in Longmenshan region, and can provide reference for earthquake prediction research.