Source mechanisms of dike-induced earthquakes in the Dabbahu-Manda Hararo rift segment in Afar, Ethiopia: implications for faulting above dikes
Source mechanisms of dike-induced earthquakes in the Dabbahu-Manda Hararo rift segment in Afar, Ethiopia: implications for faulting above dikes
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DOI：
10.1093/gji/ggs076
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发表时间：
2013-03
影响因子：
2.8
通讯作者：
M. Belachew;C. Ebinger;D. Cote
中科院分区：
文献类型：
--
作者：
M. Belachew;C. Ebinger;D. Cote
苏建民，马建民，王荣勇。岩脉侵位过程改变了侵入区的应力场，引起了迁移地震群。研究了埃塞俄比亚阿法尔（Afar）海底早期扩张段两个大体积堤防的放置诱发的最大地震（ML≥3.5）的震源机制，确定了它们的时空关系。考虑到堤坝就位过程中可能存在复杂的源机制，我们求解了四种不同的源模型：双偶（DC）、DC +各向同性（DC + ISO）、偏态（DVMT）和全矩张量（FMT）。采用不同模型得到的解表明，地震具有非双偶分量。最好的双偶机制以正断层为主，走滑分量小，其滑动面与测地确定的板块开口方向几乎垂直。这些地震大多为峰值频率≤2 Hz的低频地震，发生在堤防的传播阶段。震源机制的时空分布与堤防就位时地震活动性迁移模式、浅层震源深度估算；区域最大压应力- 90°旋转p轴机制的缺失和机制的非双偶性质表明，最大震级地震主要是由岩脉上方正断层产生的，可能在岩脉流体的影响下存在张开口成分。这些观测结果表明，岩脉上正断层是岩脉侵入过程中地震能量释放的主要过程。假设岩脉上的断层遵循构造地震的长度位移关系，估计2007年11月和2008年10月岩脉的地震总滑动量分别为0.8 m和1.3 m。从最大地震和正断层滑动的弹性位错模型估计的总滑移的相似性，以及更大的地震与大地测量的总力矩逆差表明，大部分由岩脉侵入引起的板块开口在抗震上得到了调节。研究结果表明，在岩脉侵入过程中，岩脉上的地表断裂是控制裂谷带地貌地貌发育的主要过程。
S U M M A R Y The process of dike emplacement changes the stress field in the intruded region, causing swarms of migrating earthquakes. We determine source mechanisms of the largest earthquakes (ML ≥ 3.5) induced by the emplacement of two large volume dikes along an incipient seafloor spreading segment in Afar, Ethiopia to determine their space-time relations. Given the possibility of complex source mechanisms during dike emplacement, we solved for four different source models: double couple (DC), DC + isotropic (DC + ISO), deviatoric (DVMT), and full moment tensor (FMT). The solutions obtained using the different models indicate that the earthquakes have non-double couple components. The best double-couple mechanisms, which are mainly normal faulting with small strike-slip components, have slip planes nearly perpendicular to the geodetically determined plate opening direction. Most of these earthquakes are low-frequency earthquakes with peak frequencies ≤2 Hz, and they occurred during the propagation phase of the dikes. The space-time distribution of the source mechanisms with respect to the migrating patterns of seismicity during dike emplacement, the shallow source depths estimated, the lack of mechanisms with ∼90o rotated P-axes from the regional maximum compressive stress and the non-double couple nature of the mechanisms indicate that the largest magnitude earthquakes are generated mainly by normal faulting above the dikes probably with some component of tensile opening under the influence of dike-related fluids. These observations suggest that normal faulting above dikes is the main process of seismic energy release during dike intrusions. Assuming that faults above dikes follow lengthdisplacement relations found for tectonic earthquakes, total seismic slips of 0.8 m and 1.3 m are estimated for the November 2007 and October 2008 dikes, respectively. Similarity of the total slip estimates from the largest earthquakes and from elastic dislocation model estimates of normal fault slips and the larger total seismic versus geodetic moment deficits indicates that most of the plate opening by dike intrusion is accommodated aseismically. Our results show that during dike intrusions surface faulting above dikes is the main process that controls the development of rift zone geomorphology and topography.