Collaborative Research: High-resolution imaging of the Elgin-Lugoff earthquake swarm sequence and subsurface structures in South Carolina using a dense seismic nodal array

2021年12月27日，南卡罗莱纳州埃尔金和卢戈夫之间发生了3.3级地震，这是一个地震活动非常少的地区。自那以后，该地区报告了90多起小地震，最大的3.6级地震发生在2022年6月29日。这些地震至少持续到2023年夏天，这对该地区来说非常不寻常。这个群没有明显的主震，也没有明显的构造或人为原因。到目前为止，标准的地震位置并没有显示出与先前存在的东部山前断层系统的明确关系。该项目将分析研究人员在2022年10月部署的一套仪器的地震数据，以更好地成像承载群的断层结构和潜在驱动力。这个项目自然会为Elgin-Lugoff和南卡罗来纳中部地区的受影响社区提供公众参与的机会，那里的许多居民有生以来第一次感受到地震。该项目还通过资助来自南卡罗来纳州当地社区的本科生在他们自己的州进行研究，提供包容和STEM劳动力发展机会。获得资助的学生将参与外展活动，通过知识转移培养STEM素养，并进一步扩大对地球科学的参与。该项目还将为两所院校的暑期实习提供资金支持。2022年10月，研究小组在Elgin-Lugoff群上部署了85个地震节点，为期4个月，以记录更多的地震。这些仪器由南卡罗来纳地震台网运营的一个宽带站和附近的一个强震站补充。丰富的数据集可用于识别其他较小的地震，这将更好地描绘承载该群序列的断层结构，并阐明其驱动力。这项工作将尝试通过使用机器学习和模板匹配技术，揭示节点部署期间低于部署前检测阈值的小地震。然后，研究小组将利用基于波形的技术重新定位这些新检测到的事件，然后确定震源机制。这些信息将用于构建埃尔金地下地震速度的新模型。利用更完整的地震目录和改进的速度模型，可以对地震群中较早的地震进行相对于较晚地震的重新定位，从而提高了整个地震目录的准确性。这个项目的结果将回答以下问题：1)群体是否发生在一个扩散的或定义明确的区域，2)群体如何在空间和时间上进化，以及3)驱动群体的可能过程是什么。从分析中获得的信息将告知南卡罗来纳正在进行的构造过程，以及东海岸的地震活动和构造。该项目由地球物理学计划和促进竞争性研究的既定计划（EPSCoR）共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

A prolonged earthquake swarm sequence started on December 27, 2021, beginning with a magnitude 3.3 earthquake between Elgin and Lugoff in South Carolina, a region with very low prior seismic activity. More than 90 small earthquakes have been reported in this region since then, with the largest magnitude of 3.6 occurring on June 29, 2022. These earthquakes have continued until at least summer of 2023, making this highly unusual for the region. This swarm has had no clear main shock and has no obvious tectonic or anthropogenic causes. So far, standard earthquake locations do not reveal a clear relationship with the pre-existing Eastern Piedmont Fault System. This project will analyze seismic data from a set of instruments deployed by the researchers in October 2022 to better image the fault structure hosting the swarm and the underlying driving force. This project will naturally provide opportunities for public engagement with the impacted community in Elgin-Lugoff, and the broader midlands of South Carolina, where many residents have felt earthquakes for the first time in their lives. This project also provides inclusion and STEM workforce development opportunities, by funding research by undergraduate students from the local South Carolina community to perform research in their own state. Students on this grant will participate in outreach to build STEM literacy through knowledge transfer and further broaden participation in geoscience. This project will also provide funding to support summer internships at both institutions.In October 2022, 85 seismic nodes were deployed for 4 months by the research team over the Elgin-Lugoff swarm to record further earthquakes. These instruments were complemented by one broadband station and one nearby strong motion station operated by the South Carolina Seismic Network. The rich dataset can be used to identify additional smaller earthquakes, which will better image fault structures hosting this swarm sequence and elucidate the driving forces. The work will attempt to reveal small earthquakes during the nodal deployment period, below the detection threshold before this deployment, by using both machine learning and template matching techniques. The team will then relocate these newly detected events with waveform-based techniques, followed by focal mechanism determinations. This information will be used to construct a new model of the seismic velocities beneath Elgin. Using both the more complete catalog and the improved velocity model, the earthquakes earlier in this swarm can be relocated relative to the later ones thus improving the accuracy of the whole catalogue. Results from this project will answer the following questions: 1) does the swarm occur in a diffuse or well-defined region, 2) how the swarm evolved in space and time, and 3) what is the likely process that drives this swarm. The information gained from the analysis will inform on the ongoing tectonic processes in South Carolina, and on East coast seismicity and tectonics generally.This project is jointly funded by the Geophysics program and the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.