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

大多数大型逆冲地震都涉及近海地区的明显滑动，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大地震和海啸的模型评估和减灾至关重要。海底变形的事件可以通过海底仪器，如绝对压力表或海底地震仪来探测。然而，由于将构造信号与海洋效应分离的成本高或难度大，这些仪器对近海变形的长期表征仍然具有挑战性。在这个项目中，我们建议整合两种最近发展的方法，以提高我们对近海变形及其与大逆冲破裂的关系的理解。该项目充分利用包括Earthscope USArray、日本Hinet在内的全球密集地震台网日益增加的可用性，并支持一名女研究生的博士研究。研究结果通过会议和期刊分享，也通过向公立学校和本科班级推广。慢滑在大地震成核阶段的作用仍然是个谜。震前慢滑的一个重要表现是前兆微震活动，这是陆地地震仪器难以直接探测到的。基于数组的方法的最新进展允许提高检测常规目录中缺失事件的能力。为了更好地描述近海地震活动性，本研究提出结合两种最新发展的检测方法：反向投影成像（BP）和匹配滤波（MF）检测。BP方法在大地震尾波中检索近海地震活动性是有效的。然后，BP推断的事件可以作为模板，在阵列地震数据的中频处理中检测更多的海上微地震活动。这项研究的目的是获得更完整的长期地震活动率和重复地震（可用于推断慢滑事件）在最近的大逆冲地震断裂带附近的图片。这些观测结果可能提供小事件和/或慢滑与大逆冲地震相互作用的关键约束条件。

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.