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

加拿大西海岸地震成因和板块边界结构研究

• 批准号: RGPIN-2016-04239
• 负责人: Bostock, Michael
• 金额: $ 3.5万
• 依托单位: University of British Columbia
• 依托单位国家: 加拿大
• 项目类别: Discovery Grants Program - Individual
• 财政年份: 2016
• 资助国家: 加拿大
• 起止时间: 2016-01-01 至 2017-12-31
• 项目状态: 已结题
• 来源: https://www.nserc-crsng.gc.ca/ase-oro/Details-Detailles_eng.asp?id=614932
• 关键词: Studies; Seismogenesis; Plate; Boundary; Structure

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.

板块边界断层危及加拿大西海岸沿着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断层，该逆冲断层从加州北方延伸到北方温哥华岛，距离海岸线以西约150 km。这个板块边界经历了平均间隔约550年的M=9+地震，能够产生海啸和地面震动，与日本（2011年）和印度尼西亚（2004年）最近的大地震相当。第二个值得关注的板块边界是夏洛特女王断层，该断层沿着海达湾海岸延伸到温哥华岛北部。1949年，该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·冈底斯发生7.8级地震，引发了一场小型海啸，并突出了莫尔斯比岛下方存在一个额外的、向东北倾斜的主要逆冲断层。 卡斯卡迪亚巨型逆冲断层曾被认为在大地震之间是静止的。2001年，加拿大科学家在该断层的较深部分发现了一种新的变形形式，其形式是准周期性的应力释放，称为幕式震颤和滑动（ETS）。每隔14个月，25至40公里深的板块边界就会在几天内滑动几厘米。这种运动相当于一个“无声”的M=6.5地震，但由于它没有辐射出明显的地震波，因此没有被公众注意到。然而，它伴随着一个弱的，准谐波震颤，已被证明包括许多重复和干扰的低频地震（LFE）的级联。ETS和LFEs的研究是重要的，因为它们i）提供了对巨型逆冲断层破裂的区域（以及最终大小）的限制，ii）可以作为巨型逆冲断层破裂的触发剂，iii）提供了对板块边界如何在深度上运作的洞察。 该提案寻求资金，以调查地震过程，特别是ETS，最终目标是更好地为加拿大西海岸的地震危险评估提供信息。该研究分为5个目标：a）利用地震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）利用LFE、常规地震和以往地震实验的数据进行层析成像，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）对多个板块边界的LFE/震颤标度关系进行比较研究（卡斯卡迪亚，日本，圣安德烈亚斯），以限制潜在的生成机制; d）使用夏洛特女王断层上的重复地震作为蠕变计，以限制过去20年来的断层运动和应力状态，特别是在2012年10月事件之前和之后;发展新的方法来分析卡斯卡迪亚地震的有限维特征。