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大逆冲断层，它从加利福尼亚北部延伸到温哥华岛北部，距离海岸线以西约150公里。该板块边界平均每隔550年经历一次M=9+级地震，并有能力产生海啸和地面震动，可与日本（2011年）和印度尼西亚（2004年）最近发生的大地震相媲美。第二个值得关注的板块边界是夏洛特女王断层，它沿着海达瓜伊海岸向温哥华岛以北延伸。1949年，这个断层引发了加拿大有历史记录以来最大的地震（里氏8.1级）。2012年10月27日，海达瓜伊岛发生了里氏7.8级地震，引发了一场小型海啸，并突显了莫兹比岛下方东北方向存在另一条主要逆冲断层。

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.