汶川MW7.9地震周边地区布格重力异常与岩石圈垂向构造应力场

前人研究给出, 龙门山断裂带中南段地壳均衡异常显著, 具有发生7级以上大地震的深部动力背景。 2016年6月, 我们围绕该均衡异常显著区域开展重力/GNSS加密观测, 提高了该地区布格重力异常和地壳均衡异常场的空间分辨率。 依据上述观测结果与前期同类观测数据, 反演了汶川M_W7.9地震周边地区地壳密度构造。 结果显示, 龙门山断裂带是地壳密度变化的高梯度带, 其东侧地壳较薄, 但其西部明显变厚, 上、 中、 下地壳变化趋势均呈现上述特征; 研究区东侧的莫霍面深度为35~40 km, 西侧为60~65 km。 此外, 利用重力/GNSS联合观测数据计算了汶川M_W7.9地震震中区周边地区岩石圈承载的垂向构造应力场, 结果表明, 汶川M_W7.9地震震中区北部、 宁强、 峨眉山周边地区蓄积了-30 MPa至-40 MPa的负向构造应力, 龙门山断裂带中南段蓄积了约40 MPa的正向构造应力, 区域最大垂向构造应力分布在龙门山断裂带中南段, 临近芦山M_W6.6地震。 统计结果表明, 地震多发生在垂向构造应力高梯度带附近, 或垂向构造应力的高值区域。

Bouguer Gravity Anomaly and Isostasy around the 2008 Wenchuan MW7.9 Earthquake

Previous isostatic study shows that the crust around the middle and south segments of the Longmenshan fault zone is in great isostatic imbalance, which indicates an M≥7.0 earthquake in the future. We conducted a gravity and hybrid GNSS observation campaign in June of 2016 around the above great imbalance area, and improved spatial resolution of the gravity anomalies. We inversed the density structures around the Wenchuan M_W7.9 earthquake using the above and previous observation data. Our results show that the Longmenshan fault zone is the high gradient belt of crustal density change. The crust east of which is relatively thinner than that in the west. As a whole the upper, middle and lower crusts show the same characteristics. Moho depths in the eastern parts of the study area are 35~40 km. but those in the western parts are 60~65 km. Besides, we calculated the vertical tectonic stress of the lithosphere of the study area using gravity and GNSS data. We found that the crusts store up vertical tectonic stress of -30~-40 MPa around Lingqiang, Emei Mountain, and the area north to Wenhcuan M_W7.9 earthquake. However, the crust stores up vertical tectonic stress of 40 MPa around the middle and south segments of the Longmenshan fault zone. The maximum of vertical stress locates near the epicenter of the 2013 Lushan M_W6.6 earthquake. Statistical results show that the earthquakes often occur near the high gradient zones of vertical tectonic stress.