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，南卡罗来纳州埃尔金和卢戈夫之间发生3.3级地震，这是一个先前地震活跃度非常低的地区。自那以来，该地区已报告了90多次小规模地震，最大震级为3.6级，发生在2022年6月29日。这些地震至少一直持续到2023年夏天，这对该地区来说非常不寻常。该震群没有明显的主震，也没有明显的构造或人为原因。到目前为止，标准的地震地点并没有显示出与先前存在的东山前断层系统之间的明确关系。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，以更好地成像容纳震群的断层结构和潜在的驱动力。该项目自然将提供机会，让公众参与埃尔金-卢戈夫受影响的社区，以及更广泛的南卡罗来纳州中部地区，那里的许多居民有生以来第一次感受到地震。该项目还提供包容性和STEM劳动力发展机会，资助南卡罗来纳州当地社区的本科生在本州进行研究。这笔助学金的学生将参加通过知识传授建立STEM扫盲的外联活动，并进一步扩大对地球科学的参与。该项目还将提供资金，以支持这两个机构的暑期实习。2022年10月，研究小组在埃尔金-卢戈夫震群上部署了85个地震节点，为期4个月，以记录进一步的地震。这些仪器还得到了南卡罗来纳地震台网运营的一个宽带台和附近一个强震动台的补充。丰富的数据集可以用来识别更多的较小地震，这将更好地成像承载这个震群序列的断层结构，并阐明驱动力。这项工作将试图通过使用机器学习和模板匹配技术，揭示节点部署期间的小地震，低于这次部署之前的检测阈值。然后，研究小组将使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。这些信息将被用来构建埃尔金下方地震速度的新模型。使用更完整的目录和改进的速度模型，可以将该震群中较早的地震相对于较晚的地震进行重新定位，从而提高了整个目录的精度。这个项目的结果将回答以下问题：1)群发生在扩散或明确的区域，2)群在空间和时间上是如何演变的，以及3)什么可能的过程驱动了群。从分析中获得的信息将为南卡罗来纳州正在进行的构造过程以及东海岸的地震活动和总体构造提供信息。该项目由地球物理计划和既定的激励竞争研究计划(EPSCoR)联合资助。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.