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300多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，它从加利福尼亚州北部延伸到温哥华岛北部，大约在海岸线以西约150公里处。该板块边界经历了平均约550年一次的9级地震，能够引发海啸和地面震动，可与最近在日本(2011年)和印度尼西亚(2004年)发生的大地震相媲美。第二个令人担忧的板块边界是夏洛特女王断层，该断层沿着海达瓜伊岛海岸延伸到温哥华岛北部。1949年，这条断层引发了加拿大历史上最大的地震(M=8.1)。2012年10月27日，海达瓜伊岛下7.8级地震引发了一场小海啸，并突显了莫尔兹比岛下向东北倾斜的另一条主要逆冲断层的存在。 卡斯卡迪亚巨型逆冲曾被认为在两次大地震之间处于静止状态。2001年，加拿大科学家在该断层较深的部分发现了一种新的形变形式，即由幕式地震和滑动(ETS)引起的准周期应力释放。每隔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。这一运动相当于一次6.5级的“无声”地震，但公众并没有注意到，因为它没有辐射出显著的地震波。然而，伴随而来的是一种微弱的准谐振动，已被证明由许多重复和干扰的低频地震(LFE)组成的级联。对ETS和LFEs的研究很重要，因为它们i)对巨型逆冲断裂的面积(以及最终震级)提供了约束，ii)可以作为巨型逆冲断裂的触发剂，iii)提供了对板块边界如何在深部运作的洞察。 这项建议寻求资金来调查地震过程，特别是ETS，最终目标是更好地为加拿大西海岸的地震风险评估提供信息。这项研究分为5个目标：a)在地震中使用散射波成像板块边界断层的精细细节，并阐明导致ETS的物理条件；b)使用LFE、常规地震和以前地震实验的数据进行层析成像，以成像卡斯卡迪亚俯冲带内的流体生成、迁移和消耗；c)对跨越多个板块边界(卡斯卡迪亚、日本、圣安德烈亚斯)的LFE/地震标度关系进行对比研究，以约束潜在的生成机制；d)使用夏洛特女王断层上反复发生的地震作为蠕变计，以约束过去20年，特别是在2012年10月地震前后的断层运动和应力制度；以及e)开发新的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.