Stress Features Inferred From Induced Earthquakes in the Weiyuan Shale Gas Block in Southwestern China
Stress Features Inferred From Induced Earthquakes in the Weiyuan Shale Gas Block in Southwestern China
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DOI：
10.1029/2022jb025344
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发表时间：
2023-02
期刊：
影响因子：
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通讯作者：
R. Chu;Minhan Sheng
中科院分区：
文献类型：
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作者：
R. Chu;Minhan Sheng
应力特征，特别是局部应力场和地震应力降，对认识诱发地震的机制具有重要意义。自2015年页岩气开采以来，威远页岩气区块频繁发生地震。本文通过拟合三分量波形确定了257个ML > 1.5的地震的震源机制解，反演了两个密集地震群的最大水平应力方向，然后用谱比法计算了矩震级在2.2 ~ 2.75之间的17个地震的应力降。所有地震的震源机制都是逆断层。局部最大水平压应力方向为东东东方向，与GPS测量所指示的地壳运动方向一致。震源机制和微震活动的位置都表明存在倾角为70°的陡倾断层，这是相对罕见的，在通常条件下很难滑动，不像美国中西部和加拿大西部的走滑或低角度逆冲断层的主要诱发地震。然而，大倾角的反向事件可以诱导由水力压裂产生的大孔隙压力。应力下降范围从2.5到54.7 MPa，与美国中西部潜在的诱发地震相当。研究结果表明，威远地区的地震受局部构造应力控制，并受水力压裂产生的大孔隙压力诱发，从而进一步加深了对陡倾逆断层复活的认识。
Stress features, particularly local stress field and earthquake stress drops, are important to understand mechanism of induced earthquakes. Since shale gas exploitations in 2015, the Weiyuan shale gas block has experienced frequent earthquakes. In this paper, we determine focal mechanisms of 257 events with ML > 1.5 by fitting three‐component waveforms, invert for direction of maximum horizontal stress for two dense earthquake clusters, and then calculate stress drops for 17 earthquakes with moment magnitudes between 2.2 and 2.75 through the spectral ratio method. The focal mechanisms of all earthquakes are reverse faulting. The orientation of local maximum horizontal compressive stress is in the ESE direction, consistent with crustal movement indicated by GPS measurements. Both the focal mechanisms and microseismicity locations suggest the existence of steeply dipping faults with dip angles of ∼70°, which are relatively rare and difficult to slip under usual conditions, unlike major induced earthquakes with strike‐slip or low‐angle thrusting faults in the midwestern United States and western Canada. However, the steeply dipping reverse events can be induced by large pore pressure from hydraulic fracturing. The stress drops range from 2.5 to 54.7 MPa, comparable to those of potentially induced earthquakes in the midwestern United States. Our results imply earthquakes in the Weiyuan area are controlled by local tectonic stress and induced by large pore pressure from hydraulic fracturing, which advances our knowledge of reactivation of steeply dipping reverse faults.