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TitleInvited review paper: Some outstanding issues in the study of great megathrust earthquakes - the Cascadia example
AuthorWang, K; Tréhu, A M
SourceJournal of Geodynamics vol. 98, 2016 p. 1-18,
Alt SeriesEarth Sciences Sector, Contribution Series 20150275
Mediapaper; on-line; digital
File formatpdf
ProvinceWestern offshore region
Lat/Long WENS-128.0000 -124.0000 49.5000 40.0000
Subjectsgeophysics; earthquakes; earthquake studies; earthquake mechanisms; subduction; subduction zones; Cascadia
Illustrationslocation maps; cross-sections; profiles
ProgramWestern Canada Geohazards Project, Public Safety Geoscience
AbstractBecause of a combination of new observational tools and a flurry of large megathrust earthquakes,tremendous progress has been made in recent years towards understanding the process of great subduc-tion earthquakes at Cascadia and other subduction zones around the world. This review article attemptsto clarify some of widely used geodynamic concepts and identify the most important scientific questionsfor future research related to megathrust behaviour. It is important to specify how the megathrust seis-mogenic zone has been defined when comparing data and models. Observations and concepts currentlyused to define the seismogenic zone include: (A) the stability transition in rate-and-state dependent fric-tion; (B) the slip zone of large interplate earthquakes; (C) the distribution of small-medium earthquakes;and (D) the geodetically-determined zone of fault locking. Land-based geodetic measurements indicatethat the Cascadia megathrust is locked to some extent, but the degree of locking is not well constrained.The near absence of detectable interplate seismicity, with the exception of a segment near 44.5?N andnear the Mendocino Triple Junction, is presently interpreted to indicate full locking along most of Cas-cadia. Resolving the locking state requires seafloor geodetic measurements. The slip behaviour of theshallowest segment of the megathrust and its tsunamigenic potential are complex and variable. Struc-tural studies combined with modeling have the potential to improve our understanding of the signatureleft in the structure by the slip history. For several reasons, but mostly because of interseismic viscoelas-tic stress relaxation, the downdip limit of megathrust locking cannot be reliably constrained by geodeticdata. Independent information is needed on the composition and thermal state of fault zone materials.The spatial relationship between the seismogenic zone and the zone of Episodic Tremor and Slip (ETS)remains controversial. Observations from the Nankai subduction zone and the San Andreas Fault suggestthat ETS does not mark a simple spatial transition from seismic to aseismic behaviour and that multi-ple transitions may be present because of petrological and rheological changes with depth. Coseismicrupture in the AD 1700 Cascadia earthquake has been shown to vary along strike, and it is important toinvestigate whether the position of boundaries between high slip and low slip are stationary with time(and therefore probably geologically controlled) and are reflected in current interseismic locking of themegathrust.
Summary(Plain Language Summary, not published)
Over the past three decades, the scientific community has made tremendous progress in understanding megathrust earthquake processes at the Cascadia subduction zone and other subduction zones. However, recent observations, especially data from several great megathrust earthquakes, have raised new scientific questions. In this review article, we clarify some of widely used geodynamic concepts and identify the most important scientific questions for future research related to megathrust behavior. We focus on issues that are the most urgent for improving hazard assessment and risk mitigation.