Trigger mechanism and dynamic causes of the Taiwan earthquake sequence on September 17, 2022
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摘要:
2022年9月17—18日, 中国台湾先后发生台东县MS6.5和花莲县MS6.9两次强震及多次余震。两次强震均为逆走滑型地震, 且震中都位于台湾纵谷断裂带, 该地区构造复杂, 为晚中生代古太平洋板块朝东亚陆缘的消减带, 具有逆冲型构造应力背景。对纵谷断裂带周围地区历史上发生过的地震进行统计发现, 大部分地震为逆断型。为探究该地区此次发生逆走滑型地震的原因及其与构造应力场的关系, 首先通过搜集研究区相关的地震震源机制, 反演该地区的构造应力场, 明确其是以走向为北西西向的压应力为主的应力场; 然后将应力场投影到走向、倾角不同的断层面上, 发现一些节面上表现出较大的相对剪应力和较小的相对正应力, 说明这些节面上具有较强的剪切作用和较小的摩擦力, 容易发生错动而产生逆断型、逆走滑型和走滑型的地震。同时, 为明确短时间内两次强震间的触发关系, 通过计算MS6.5地震在MS6.9地震破裂面和滑动方向上产生的库伦破裂应力变化发现, MS6.9地震约在0.02MPa的库伦破裂应力触发下发生。相关结论对研究台湾纵谷带地震的发震机理和地球动力学具有一定的指导意义。
Abstract:On September 17th and 18th, 2022, two earthquakes struck Taiwan Province of China in Taitung County and Hualien County, respectively, measuring MS6.5 and MS6.9 in magnitude, followed by multiple aftershocks. Both seismic events were situated along the Longitudinal Valley Fault and exhibited a reverse strike-slip mechanism. The geological setting in this area is intricate, as the Longitudinal Valley Fault zone represents a subduction zone where the late Mesozoic Paleo-Pacific plate converges with the East Asian continental margin, resulting in a predominant thrust-type tectonic stress background. However, historical earthquake data in this region have indicated a prevalence of reverse-faulting earthquakes. To address the causes of these reverse strike-slip fault earthquakes and their relationship with the tectonic stress field in the area, we first reconstructed the tectonic stress field by analyzing the focal mechanisms of past earthquakes within the study area. The resulting stress field was characterized by compressive stress oriented with an azimuth of NWW. Subsequently, this stress field was projected onto fault planes with various strike and dip angles. This analysis revealed that certain fault joints experienced more significant relative shear stress and lower relative normal stress, suggesting that these joints were more prone to dislocation, leading to earthquakes of the reverse fault type, reverse strike-slip type, and strike-slip type. Furthermore, the proximity and timing of the two earthquakes within a two-day span prompted an examination of their potential triggering relationship. Researchers calculated the Coulomb rupture stress changes caused by the MS6.5 earthquake on the rupture plane of the MS6.9 earthquake and its sliding direction. Their analysis indicated an increase of approximately 0.02 MPa in Coulomb rupture stress, suggesting that the Taitung MS6.5 earthquake may have triggered the Hualien MS6.9 earthquake. This study holds significant importance for understanding the seismogenic mechanism of the Longitudinal Valley Fault and gaining insights into the geodynamics of the study region.
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Key words:
- Longitudinal Valley Fault /
- stress field /
- stress system /
- Coulomb failure stress
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表 1 中国台湾台东县MS6.5地震震源机制中心解结果和标准差
Table 1. Results and standard deviation of the central focal mechanism solution for the MS6.5 earthquake in Taitung County, Taiwan, China
序号 各震源机制解(走向，倾角，滑动角)/(°) 机构 作为初始解计算出的中心震源机制解(走向，倾角，滑动角)/(°) 标准差(S)/ (°) S最小的中心解与各震源机制解的最小空间旋转角/(°) 1 202，63，13 USGS 205.4，58.2，20.8 16.499554 8.38 2 208，44，9 CPPT 205.4，58.2，20.8 16.499550 19.63 3 199，76，15 GFZ 205.4，58.2，20.8 16.499544 19.07 4 212，62，53 OCA 205.4，58.2，20.8 16.499547 29.65 5 214，50，26 IPGP 205.4，58.2，20.8 16.499576 10.71 6 206，39，11 中国地震台网中心 205.4，58.2，20.8 16.499550 21.71 7 207，57，39 RMT 205.4，58.2，20.8 16.499550 17.39 8 197，74，10 台湾地震科学资料中心(P波结果) 205.4，58.2，20.8 16.499543 18.62 9 206，62，21 台湾地震科学资料中心(W震相结果) 205.4，58.16，20.9 16.499525 3.73 10 204，62，19 GCMT 205.4，58.2，20.8 16.499572 4.19 11 205.0，55.0，16.3 GRMT 205.4，58.2，20.8 16.499573 5.43 注：台湾地震科学资料中心的计算结果包括两种，一种根据地震波的P波初动计算，另一种由地震波W震相计算得到 
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表 2 中国台湾花莲县MS6.9地震震源机制中心解结果和标准差
Table 2. Results and standard deviation of the central focal mechanism solution for the MS6.9 earthquake in Hualien County, Taiwan, China
序号 各震源机制解(走向，倾角，滑动角)/(°) 机构 作为初始解算得的中心震源机制解(走向，倾角，滑动角)/(°) 标准差(S)/ (°) S最小的中心解与各震源机制解的最小空间旋转角/(°) 1 201，61，31 GCMT 205.4，59.1，34.3 13.815634 4.40 2 205，60，31 CPPT 205.4，59.1，34.3 13.815623 3.28 3 204，54，31 GFZ 205.4，59.1，34.3 13.815624 5.79 4 215，60，48 OCA 205.4，59.1，34.3 13.815633 12.12 5 200，52，30 IPGP 205.4，59.1，34.3 13.815634 8.43 6 210，77，19 USGS 205.4，59.1，34.3 13.815631 25.07 7 207，40，26 中国地震台网中心 205.4，59.1，34.3 13.815622 21.26 8 206.7，53.2，36.5 GRMT 205.4，59.1，34.3 13.815642 6.14 9 205.0，61.3，46.6 RMT 205.4，59.1，34.3 13.815644 12.67 10 199.4，72.7，43.4 台湾地震科学资料中心(W震相结果) 205.4，59.1，34.3 13.815638 18.65
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表 3 5次MS＜6的地震震源机制中心解汇总表
Table 3. Table of central focal mechanism solution of the five earthquakes with MS < 6
序号(震级/MS) 发震时刻(北京时间) 中心解参数 地震类型 日期 时分 节面Ⅰ/(走向，倾角，滑动角)/(°) 节面Ⅱ/(走向，倾角，滑动角)/(°) 1(5.5) 2022-9-17 22∶45 104.4，88.1，168.8 194.7，78.8，1.9 走滑型 2(5.7) 2022-9-18 13∶19 208.6，66.1，24.9 108.0，67.3，154.0 逆走滑型 3(5.1) 2022-9-18 14∶32 200.3，80.7，9.19 108.8，80.9，170.6 走滑型 4(5.8) 2022-9-18 17∶39 169.9，31.1，46.9 37.4，67.9，112.4 逆断型 5(5.7) 2022-9-19 10∶07 195.0，63.8，43.0 82.6，52.3，146.1 逆走滑型
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表 4 应力张量在各类型地震的总体震源机制节面上的相对剪应力和相对正应力统计表
Table 4. Relative shear stress and relative normal stress of stress tensor on the focal mechanism nodal plane of each type of earthquake focal mechanism
总体震源机制节面Ⅰ (走向，倾角，滑动角)/(°) 总体震源机制节面Ⅱ (走向，倾角，滑动角)/(°) 应力张量在节面Ⅰ上的相对剪应力/相对正应力 应力张量在节面Ⅱ上的相对剪应力/相对正应力 走滑型 163.51，85.24，6.27 72.99，83.76，175.21 0.700/-0.551 0.698/-0.302 逆走滑型 200.15，56.94，45.60 81.03，53.22，137.07 0.929/-0.503 0.706/0.220 逆断型 210.02，63.68，88.42 33.58，26.36，93.19 0.819/-0.697 0.806/0.455
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