安宁河-则木河断裂带的现今地震危险性真有这么高吗？

The Daliangshan sub-block is enclosed by the Daliangshan, Anninghe and Zemuhe fault zones. Since the end of last century, the Anninghe-Zemuhe fault zone has been considered to be under a very high seismic risk and any great earthquakes occurring here might not be surprising. Nonetheless, the Daliangshan fault zone, which belongs to the same tectonic system as the Anninghe-Zemuhe fault zone and is located on the east side of this fault system, is always neglected. Latest GPS and geological data show that, the Daliangshan, Anninghe and Zemuhe fault zones all have the capability to grow an earthquake with magnitude greater than 7, and they jointly absorb the shear strain generated by the relative motion between the Tibetan plateau and Sichuan basin with the comparable magnitude. Therefore, how to interpret such phenomena? Does the Daliangshan fault zone play an important role in stress partitioning of the region? How much percent if so? Does the Anninghe-Zemuhe fault zone still have a high seismic potential after considering such stress partitioned by the Daliangshan fault zone? Answers to these questions are apparently critical to the earthquake prediction and forecasting of this region. Taking such consideration into account, we propose to jointly invert the GPS and historical earthquake data for the strain accumulation, partitioning and slip deficit of the Daliangshan, Anninghe and Zemuhe fault zones, to test the confidence level of the inversion results together with geological data, and to comprehensively evaluate the present-day seismic hazard of the Anninghe-Zemuhe fault zone.

大凉山次级块体由大凉山、安宁河、则木河断裂带围合而成。自上世纪末以来，安宁河-则木河断裂带一直被认为已进入复发周期，随时有发生强震的可能，人们对该断裂带的关注也因此远高于展布在其东侧且属于同一构造变形系统的大凉山断裂带。最新的GPS、地质等观测结果表明，大凉山、安宁河、则木河断裂带都具备发生7级及以上地震的地质条件，同时青藏高原相对于四川盆地的剪切变形在该区间内被三条断裂带联合吸收，且量级相当。因此，如何认识这一现象？大凉山断裂带在区域构造应力分配中是否承担着不可忽视的角色？顾及应力分配后的安宁河-则木河断裂带是否依然具有较高的强震孕育背景？这些问题的回答对做好该区域的地震预测预报工作无疑具有重要意义。本研究将基于上述思考，联合三维GPS、历史地震资料反演大凉山、安宁河、则木河断裂带的应力累积及分配状态，结合地质等资料测试反演结果的置信水平，并以此评判安宁河-则木河断裂带的现今地震危险性。

搜集了大凉山次级块体及周边区域1997年以来建设的共计144个GPS连续站与流动站数据，处理并获取了这些站点的三维速度场信息. 独立使用GPS水平速度产品计算的面应变率场结果揭示出的丽江-小金河断裂以西、小江断裂以东的区域拉张及安宁河-则木河断裂带两侧的区域挤压等信息，与GPS垂向速度产品独立揭示的区域上升、下降结果基本一致. 使用GPS水平速度产品和4种应变率场计算方法获取了区域的应变率第二不变项、面应变率及累积矩震级等信息，表明大凉山次级块体西缘区域地震风险较其他区域明显偏高，最高值出现在西昌附近. 粘弹性模型结果表明，该高值与西昌1850年7.5级地震的震后效应并无直接关联. 基于GPS速度产品及欧拉矢量自动聚类分析（机器学习）算法，首次对研究区域的刚性块体及块体边界进行了自动识别和有效性验证，结果表明，安宁河-则木河-大凉山断裂带围合而成的区域、大凉山-荥经-马边断裂带围合而成的区域具有独立且明显的刚性运动特征. 计算了各刚性块体的欧拉矢量及块体边界（断层）的长期滑动速率，结果表明，安宁河-则木河断裂带、大凉山断裂带、荥经-马边断裂带共同调制着青藏高原与四川盆地之间的相对运动，三者联合滑动速率约为9-11mm/yr. 计算了各刚性块体边界（断层）的震间耦合状态，但有效性检验结果表明，仅依靠现有GPS站点难以对断层震间耦合状态进行可靠约束. 我们注意到了一个不同寻常的现象，即去除刚性运动之后的GPS残余速度结果表明，安宁河断裂带上可能存在较为显著的无震滑移（蠕滑）信号，现场考察发现沿该断裂带有几处滑石粉采矿区让我们进一步加深了这种怀疑. 如果属实，该现象对重新认识该区域的现今地震风险（如果无震滑移存在，则意味着该区域的地震风险可能有所降低）将有着重要参考价值，同时也表明，现有发表成果给出的断层耦合结果可能均需要重新评估.