Detecting offshore seismicity by combining back-projection and matched filter analysis

大多数巨型逆冲断层地震涉及近海地区的重大滑动，导致强烈的地面震动和/或海啸危险。更好地了解近海变形对于模型评估和大地震和海啸的减灾至关重要。绝对压力计或海底地震仪等海底仪器可以探测到海底变形事件。然而，由于高成本或难以分离的构造信号从海洋学的影响，这些仪器的近海变形的长期特性仍然是具有挑战性的。在这个项目中，我们建议整合两个最近开发的方法，以提高我们的理解，近海变形及其关系的巨型逆冲断层破裂。该项目充分利用全球密集地震网络日益增加的可用性，包括Earthscope USAray，日本的Hinet，并支持一名女研究生的博士工作。研究成果通过会议和期刊，以及通过推广到公立学校和本科生班级来分享。慢滑在大地震成核阶段的作用仍然是个谜。震前慢滑动的一个重要表现是异常的微震活动，这是陆地地震仪器难以直接探测到的。基于阵列的方法的最新进展允许提高在常规目录中检测缺失事件的能力。为了更好地表征近海地震活动性，本研究提出了联合收割机两种最新发展的检测方法：反向投影（BP）成像和匹配滤波（MF）检测。BP方法对大地震尾波中的近海地震活动性反演是有效的。BP推断的事件，然后可以用作模板，以检测更多的海上微震阵列地震数据的MF处理。这项研究的目的是获得一个更完整的长期地震活动率和重复地震（可用于推断慢滑情节）周围最近的大规模地震破裂带。这些观测结果可能提供小事件和/或慢滑动与大逆冲断层地震相互作用的关键约束。

Most megathrust earthquakes involve significant slip in the offshore area, leading to strong ground shaking and/or tsunami hazards. A better understanding of the offshore deformation is crucial for model evaluation and hazard mitigation of large earthquakes and tsunamis. The episodes of seafloor deformation can be detected by ocean-bottom instruments such as absolute pressure gauges or ocean bottom seismometers. However, due to either the high cost or difficulty in separation of tectonic signals from oceanographic effects, long-term characterizations of offshore deformation with these instruments are still challenging. In this project, we propose to integrate two recently developed methods to improve our understanding of offshore deformation and their relation to megathrust rupture. The project takes full advantage of the increasing availability of global dense seismic networks including the Earthscope USArray, Hinet in Japan and supports the PhD work of a female graduate student. The research results are being shared via conferences and journals, as well as via outreach to public schools and undergraduate classes.The role of slow slip in the nucleation stage of large earthquakes remains enigmatic. An important manifestation of the pre-seismic slow slip is the precursory micro-seismicity, which is difficult to detected directly by landward seismic instruments. Recent advances in array-based methods allow to improve the capability of detecting missing events in routine catalogs. To better characterize the offshore seismicity, this research proposes to combine two recently developed detection methods: Back-projection (BP) imaging and Matched-filter (MF) detection. The BP method is effective in retrieving offshore seismicity in the coda wave of large earthquakes. The BP inferred events can be then used as templates to detect even more offshore microseismicity in the MF processing of array seismic data. The goal of this research is to obtain a more complete picture of the long-term seismicity rate and repeating earthquakes (which can be used to infer slow-slip episodes) around recent megathrust earthquake rupture zones. These observations may provide key constraints of the interaction of small events and/or slow slip with megathrust earthquakes.