Kristin D. Morell¹*, Christine Regalla², Lucinda J. Leonard³, Colin Amos⁴, Vic Levson⁵

1 School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada
2 Department of Earth and Environment, Boston University, Boston, Massachusetts 02215, USA
3 School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada
4 Geology Department, Western Washington University, Bellingham, Washington 98225-9080, USA
5 School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada

Abstract

The seismic potential of crustal faults within the forearc of the northern Cascadia subduction zone in British Columbia has remained elusive, despite the recognition of recent seismic activity on nearby fault systems within the Juan de Fuca Strait. In this paper, we present the first evidence for earthquake surface ruptures along the Leech River fault, a prominent crustal fault near Victoria, British Columbia. We use LiDAR and field data to identify >60 steeply dipping, semi-continuous linear scarps, sags, and swales that cut across both bedrock and Quaternary deposits along the Leech River fault. These features are part of an ~1-km-wide and up to >60-km-long steeply dipping fault zone that accommodates active forearc transpression together with structures in the Juan de Fuca Strait and the U.S. mainland. Reconstruction of fault slip across a deformed <15 ka colluvial surface near the center of the fault zone indicates ~6 m of vertical separation across the surface and ~4 m of vertical separation of channels incising the surface. These displacement data indicate that the Leech River fault has experienced at least two surface-rupturing earthquakes since the deglaciation following the last glacial maximum ca. 15 ka, and should therefore be incorporated as a distinct shallow seismic source in seismic hazard assessments for the region.

E-mail: kmorelluvic.ca

Manuscript received 28 March 2016; Revised manuscript received 20 June 2016; Manuscript accepted 6 July 2016; Posted online 10 Feb. 2017

10.1130/GSATG291A.1