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Quaternary Rupture of a Crustal Fault beneath Victoria,
                    British Columbia, Canada

Kristin D. Morell, School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada, kmorell@; Christine Regalla, Department of Earth and Environment, Boston University, Boston, Massachusetts 02215, USA; Lucinda J.
Leonard, School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada; Colin Amos,
Geology Department, Western Washington University, Bellingham, Washington 98225-9080, USA; and Vic Levson, School of Earth
and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada

ABSTRACT                                       detectable by seismic or geodetic monitor-       US$10 billion in damage (Quigley et
                                               ing (e.g., Mosher et al., 2000; Balfour et al.,  al., 2012).
  The seismic potential of crustal faults      2011). This point was exemplified by the
within the forearc of the northern Cascadia    2010 MW 7.1 Darfield, New Zealand                  In the forearc of the Cascadia subduc-
subduction zone in British Columbia has        (Christchurch), earthquake and aftershocks       tion zone (Fig. 1), where strain accrues due
remained elusive, despite the recognition      that ruptured the previously unidentified        to the combined effects of northeast-
of recent seismic activity on nearby fault     Greendale fault (Gledhill et al., 2011). This    directed subduction and the northward
systems within the Juan de Fuca Strait. In     crustal fault showed little seismic activity     migration of the Oregon forearc block
this paper, we present the first evidence for  prior to 2010, but nonetheless produced a        (McCaffrey et al., 2013), microseismicity
earthquake surface ruptures along the          30-km-long surface rupture, caused more          data are sparse and do not clearly elucidate
Leech River fault, a prominent crustal fault   than 180 casualties, and resulted in at least    planar crustal faults (Cassidy et al., 2000;
near Victoria, British Columbia. We use                                                         Balfour et al., 2011). But geomorphic,
LiDAR and field data to identify >60
steeply dipping, semi-continuous linear                  130°W        120°W
scarps, sags, and swales that cut across             A. NA
both bedrock and Quaternary deposits                                             124°W                      123°W  122°W
along the Leech River fault. These features                                                                          B.
are part of an ~1-km-wide and up to            50°N  PA Georgia Strait
>60-km-long steeply dipping fault zone
that accommodates active forearc transpres-          Mw      JF OB                                               DHF BCF   49°N
sion together with structures in the Juan de           44-5                                                 SPF BBF
Fuca Strait and the U.S. mainland.                   65--76                      Vancouver Island
Reconstruction of fault slip across a                >7
deformed <15 ka colluvial surface near the
center of the fault zone indicates ~6 m of                            UCSaAnada  LRF                           DDMF
vertical separation across the surface and                                                                    SWF USPtPFF
~4 m of vertical separation of channels              People per km2              FigJu. 2an de Fuca Strait  SF
incising the surface. These displacement
data indicate that the Leech River fault has               50-100                                                          48°N
experienced at least two surface-                          100-500
rupturing earthquakes since the deglacia-                  500-1,000
tion following the last glacial maximum                     >1,000
ca. 15 ka, and should therefore be incorpo-
rated as a distinct shallow seismic source in                active
seismic hazard assessments for the region.                   fault

INTRODUCTION                                                 20 km

  Unlike plate boundary faults that often      Figure 1. (A) Tectonic setting. White circles—locations of historical earth-
exhibit a strong seismic or geodetic           quakes (USGS NEIC) between AD 1946 and 2015, scaled by magnitude. White
expression (e.g., Rogers, 1988), active        line—boundary between Oregon Block (OB) and North America plate (NA)
faults within the adjacent crust can have      (McCaffrey et al., 2013; Wells and Simpson, 2001). JF—Juan de Fuca plate;
long recurrence intervals (e.g., 5–15 k.y.;    PA—Pacific plate. (B) Population centers (Balk et al., 2006) relative to mapped
Rockwell et al., 2000), and they may not be    active faults in black (Sherrod et al., 2008; USGS, 2010; Kelsey et al., 2012;
                                               Personius et al., 2014; Barrie and Greene, 2015). The Leech River fault (LRF) is
                                               shown as dashed line. BBF—Birch Bay fault; BCF—Boulder Creek–Canyon
                                               Creek fault; DDMF—Darrington–Devil’s Mountain fault; DHF—Drayton Harbor
                                               fault; SPF—Sandy Point fault; StPF—Strawberry Point fault; SF—Seattle fault;
                                               SWF—South Whidbey Island fault; UPF—Utsalady Point fault.

     GSA Today, v. 27, no. 3–4, doi: 10.1130/GSATG291A.1

4 GSA Today | March–April 2017
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