Studies of Seismogenesis and Plate Boundary Structure on Canada's West Coast

板块边界断层危及加拿大西海岸超过 300 万人和社会基础设施。 最大的地震危险来自卡斯卡迪亚俯冲巨型逆冲断层，该逆冲断层从加利福尼亚州北部延伸到温哥华岛北部，位于海岸线以西约 150 公里处。 该板块边界经历过平均间隔为 550 年的 9 级地震，能够产生与最近日本（2011 年）和印度尼西亚（2004 年）发生的大地震相当的海啸和地面震动。第二个值得关注的板块边界是夏洛特皇后断层，它沿着海达瓜伊海岸延伸到温哥华岛以北。 1949 年，这条断层引发了加拿大有记录以来最大的地震（M=8.1）。 2012 年 10 月 27 日，海达瓜依 (Haida Gwaii) 下方发生了 7.8 级地震，引发了一场小海啸，并凸显了莫尔兹比岛下方存在一个向东北倾斜的额外主要逆冲断层。 卡斯卡迪亚巨型逆冲断层曾经被认为在大地震之间处于静止状态。 2001年，加拿大科学家在该断层的较深处发现了一种新的变形形式，其形式是准周期性释放应力，从而产生了阵发性震颤和滑动（ETS）。每隔约 14 个月，25 至 40 公里深度之间的板块边界会在几天内滑动几厘米。这种运动相当于一场“无声”的 6.5 级地震，但由于它无法辐射出明显的地震波，因此不会被公众注意到。 然而，它伴随着微弱的准谐波震动，该震动已被证明由许多重复和干扰低频地震（LFE）的级联组成。 ETS 和 LFE 的研究很重要，因为它们 i）对巨型逆冲断裂的面积（以及最终大小）提供了限制，ii）可能充当巨型逆冲断裂的触发因素，iii）提供了对板块边界如何在深度运行的见解。 该提案寻求资金来调查地震过程，特别是 ETS，其最终目标是更好地为加拿大西海岸的地震灾害评估提供信息。该研究分为 5 个目标：a）利用震颤中的散射波对板块边界断层的精细细节进行成像，并阐明导致 ETS 的物理条件； b) 使用 LFE、常规地震和先前地震实验的数据进行层析成像，以对卡斯卡迪亚俯冲带内的流体生成、迁移和消耗进行成像； c) 对跨多个板块边界（卡斯卡迪亚、日本、圣安地列斯）的 LFE/震颤尺度关系进行比较研究，以约束潜在的生成机制； d) 使用夏洛特皇后断层上的重复地震作为蠕变仪来限制过去 20 年的断层运动和应力状态，特别是 2012 年 10 月事件之前和之后； e) 开发新方法来分析卡斯卡迪亚地震的有限维特征。

Plate boundary faults endanger over 3 million people and societal infrastructure along Canada's west coast. The greatest seismic hazard arises from the Cascadia Subduction Megathrust that extends from northern California to northern Vancouver Island, approximately ~150 km west of the coastline. This plate boundary has experienced M=9+ earthquakes at intervals averaging ~550 years and is capable of generating tsunamis and ground shaking comparable to recent great earthquakes in Japan (2011) and Indonesia (2004). The second plate boundary of concern is the Queen Charlotte Fault that extends north of Vancouver Island along the coast of Haida Gwaii. In 1949 this fault generated Canada's largest historically recorded earthquake (M=8.1). On October 27 2012, a M=7.8 earthquake beneath Haida Gwaii spawned a small tsunami and highlighted the existence of an additional, major thrust fault dipping to the northeast beneath Moresby Island. The Cascadia Megathrust was once thought to be quiescent between major earthquakes. In 2001, Canadian scientists discovered a new form of deformation on deeper portions of this fault in the form of a quasi-periodic release of stress coined episodic tremor and slip (ETS). Every ~14 months the plate boundary between 25 and 40 km depth slips a few cm over a few days. This motion is equivalent to a "silent" M=6.5 earthquake but goes unnoticed by the general public as it fails to radiate significant seismic waves. It is accompanied, however, by a weak, quasi-harmonic tremor that has been shown to comprise a cascade of many repeating and interfering low frequency earthquakes (LFEs). The study of ETS and LFEs is important because they i) afford constraints on the area (and so ultimate magnitude) of megathrust rupture, ii) may act as triggering agents for megathrust rupture and, iii) provide insight into how plate boundaries operate at depth. This proposal seeks funding to investigate earthquake processes, and ETS in particular, with the ultimate objective of better informing earthquake hazard assessment on Canada's west coast. The research is divided into 5 objectives: a) employ scattered waves within tremor to image fine details of plate boundary faults and elucidate the physical conditions leading to ETS; b) perform tomography using LFEs, regular earthquakes and data from previous seismic experiments to image fluid generation, migration and consumption within the Cascadia subduction zone; c) undertake comparison studies of LFE/tremor scaling relations across multiple plate boundaries (Cascadia, Japan, San Andreas) to constrain underlying generative mechanisms; d) use repeating earthquakes on the Queen Charlotte fault as creep meters to constrain fault motions and stress regime over the past 20 years, in particular, before and after the October 2012 event; and e) develop new methods to analyze finite-dimensional characteristics of Cascadia earthquakes.