Strain accommodation on forearc faults: a case study on the Beaufort range fault, an active crustal fault in the northern Cascadia forearc, Vancouver island, BC, Canada

Lynch, Emerson M. (2023) Strain accommodation on forearc faults: a case study on the Beaufort range fault, an active crustal fault in the northern Cascadia forearc, Vancouver island, BC, Canada. Doctoral thesis, Northern Arizona University.

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Abstract

The distribution of permanent deformation on crustal faults in subduction zone forearcs is a product of subduction obliquity, the geometry of the plate interface, and the structure and materials of the overriding plate. Only a portion of the strain accumulated during the interseismic period of the subduction zone seismic cycle is released in large megathrust earthquakes. The remaining accumulated strain is accommodated through permanent deformation of the upper plate, primarily localized on forearc faults. Therefore, it is important to quantify the magnitude and kinematics of deformation on these forearc faults to understand how the upper plate accommodates tectonic stresses and the hazards these faults pose to the millions of people who live in highly populated coastal regions near plate margins. The Cascadia Subduction Zone (CSZ) is located off the west coast of the USA and Canada, where the Juan de Fuca plate subducts obliquely under the North American plate. Many faults have been documented throughout the southern and central CSZ forearc, but no active faults have been documented in the northernmost ~150 km of the forearc. Some propose this reflects a change in strain accumulation near the USA-Canada border, resulting in minimal permanent deformation in the northernmost CSZ forearc. However, this is inconsistent with the largest crustal earthquake in Canadian history, the June 23, 1946 Ms 7.3 Vancouver Island earthquake, occurring in the northernmost CSZ forearc. In this dissertation, I identify the first active fault in this region, the Beaufort Range fault (BRF) on central Vancouver Island, BC, Canada. In Chapter 2, I use lidar and field mapping to identify >35 km of Quaternary fault scarps. Kinematic inversions of offset geomorphic markers show that the BRF accommodates right-lateral transtension along a steeply NE-dipping fault. This fault geometry and kinematics are similar to those modeled for the 1946 earthquake, suggesting that the BRF is a candidate source fault for this event. Through surficial mapping and topographic surveys, I determine that the BRF is a right-lateral transtensional fault that has hosted at least three earthquakes since ~15 ka. In Chapter 3, I refine this chronology using paleoseismic trenching to document five Mw 6.7-7.5 surface-rupturing earthquakes (Event 1 10.9-10.3 ka, Event 2 5.2-4.9 ka, Event 3 4.9-4.2 ka, Event 4 4.2-3.0 ka, and Event 5 2.8-0.7 ka). Recurrence intervals and slip rates on the BRF are non-steady, with shorter recurrence and faster slip rates in the mid-late Holocene. In Chapter 4, I examine these patterns of recurrence and slip rates in the northern CSZ forearc more broadly. I find that recurrence and slip rates are non-steady on most faults, and that faults tend to have long recurrence intervals in the late Pleistocene to early Holocene, and short recurrence intervals in the mid-late Holocene. Furthermore, recurrence and displacement on these forearc faults are inconsistent with periodic, time-predictable, or slip-predictable models of earthquake recurrence, and suggest that regional processes such as glacial isostatic adjustment or Coulomb stress transfer may help promote non-steady strain release.

Item Type:	Thesis (Doctoral)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	fault; forearc; paleoseismology; plate pressure; plate deformation; Cascadia Subduction Zone; CSZ; Vancouver Island, Canada
Subjects:	Q Science > QE Geology
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	24 Jul 2026 17:56
Last Modified:	24 Jul 2026 17:56
URI:	https://openknowledge.nau.edu/id/eprint/6310

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