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Aftershock Sequence and Statistics of the 2017 Mw 5.5 Pohang, South Korea, Earthquake: Implications of Fault Heterogeneity and Postseismic Relaxation
Bulletin of the Seismological Society of America ( IF 3 ) Pub Date : 2020-10-01 , DOI: 10.1785/0120200059
Jeong-Ung Woo 1 , Minook Kim 2, 3 , Junkee Rhie 1 , Tae-Seob Kang 3
Affiliation  

The sequence of foreshocks, mainshock, and aftershocks associated with a fault rupture is the result of interactions of complex fault systems, the tectonic stress field, and fluid movement. Analysis of shock sequences can aid our understanding of the spatial distribution and magnitude of these factors, as well as provide seismic hazard assessment. The 2017 Mw 5.5 Pohang earthquake sequence occurred following fluid‐induced seismic activity at a nearby enhanced geothermal system site and is an example of reactivation of a critically stressed fault system in the Pohang basin, South Korea. We created an earthquake catalog based on unsupervised data mining and measuring the energy ratio between short‐ and long‐window seismograms recorded by a temporary seismic network. The spatial distribution of approximately 4000 relocated aftershocks revealed four fault segments striking southwestward. We also determined that the three largest earthquakes (⁠ML>4⁠) were located at the boundary of two fault segments. We infer that locally concentrated stress at the junctions of the faults caused such large earthquakes and that their ruptures on multiple segments can explain the high proportion of non‐double‐couple components. The area affected by aftershocks continues to expand to the southwest and northeast by 0.5 and 1 km decade−1⁠, respectively, which may result from postseismic deformation or sequentially transferred static coulomb stress. The b‐values of the Gutenberg–Richter relationship temporarily increased for the first three days of the aftershock sequence, suggesting that the stress field was perturbed. The b‐values were generally low (⁠<1⁠) and locally variable throughout the aftershock area, which may be due to the complex fault structures and material properties. Furthermore, the mapped p‐values of the Omori law vary along strike, which may indicate anisotropic expansion speeds in the aftershock region.

中文翻译:

2017年韩国浦项Mw 5.5地震余震序列和统计数据:断层非均质性和地震后松弛的含义

与断层破裂有关的前震,主震和余震的顺序是复杂断层系统,构造应力场和流体运动相互作用的结果。地震序列分析可以帮助我们理解这些因素的空间分布和大小,并提供地震危险性评估。2017年浦项5.5浦项地震序列发生在附近增强的地热系统站点的流体诱发地震活动之后,是韩国浦项盆地重应力断层系统复活的一个例子。我们基于无监督数据挖掘创建了地震目录,并测量了由临时地震网络记录的短窗地震图和长窗地震图之间的能量比。大约4000个重新定位的余震的空间分布揭示了向西南走向的四个断层段。我们还确定了最大的三个地震(ML>4⁠)位于两个断层段的边界。我们推断,断层交汇处的局部集中应力引起了如此大的地震,并且它们在多个段上的破裂可以解释非双耦合分量的高比例。受余震影响的区域继续向西南和东北扩展,分别为十和一公里-1和1 km,这可能是由于地震后的变形或相继转移的静态库仑应力造成的。余震序列的前三天,古腾堡-里希特关系的b值暂时增加,这表明应力场受到干扰。b值通常较低(⁠<1⁠),并且在余震区域中局部变化,这可能是由于复杂的断层结构和材料属性所致。此外,大森定律的映射p值会随着走向而变化,这可能表明余震区的各向异性膨胀速度。
更新日期:2020-09-25
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