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surface-rupturing earthquakes since the
formation of the surface ca. 15 ka. For a
60–90° reverse fault, the displacements
across the scarp require minimum dip
displacements of 6.4 ± 1.5 m for inter-
fluves (n = 8) and 4.4 ± 1.1 m for channels
(n = 4). The ~2 m difference in displace-
ment between the channels and interfluves
implies multiple episodes of fault activity
and suggests that at least one event with
~2 m displacement occurred after the for-
mation of the colluvial apron but before
channel incision. In addition to this early
event, the ~4 m of displacement of the
channels (Fig. 3B and DR2 [see footnote
1]) requires either one large event with
~4 m of slip, or multiple smaller events
that together sum to ~4 m of slip. Global
empirical relationships suggest that sur-
face displacements on the order of meters
correspond to earthquakes of MW 6 or
greater (e.g., Wells and Coppersmith, 1994).
IMPLICATIONS FOR REGIONAL
SEISMOTECTONICS
Several observations indicate the active
Leech River fault zone is part of a network
of high-angle oblique faults that accom­
modate regional transpression across the
Juan de Fuca Strait and Puget Sound region.
Barrie and Greene (2015) trace the Devil’s
Mountain fault of Washington state, USA,
to within 10–20 km of the fault scarps on
Fig. 2, and their bathymetric and seismic
surveys reveal a steeply dipping oblique
-slip fault zone similar to our observations
of the Leech River fault zone. Both the
Darrington–Devil’s Mountain fault and
the Southern Whidbey Island fault systems
of Washington state (Fig. 1) are likewise
near-vertical fault zones with oblique slip
histories (Sherrod et al., 2008; Personius et
al., 2014) similar to many of the crustal
fault systems throughout the Puget Sound
region (e.g., McCaffrey and Goldfinger,
1995; ten Brink et al., 2006; Blakely et al.,
2014; Nelson et al., 2014; Sherrod et al.,
2016). Considering these similarities in
orientation and slip sense, we suggest that
the Leech River fault is part of this regional
active forearc fault system. Although it
remains possible that the timing of past
ruptures along these fault systems was
influenced by stress loading or release
related to the last glaciation (e.g., Hetzel and
Hampel, 2005), repeated earthquakes on
crustal faults including the Leech River
should be expected in order to accommodate
ongoing tectonic strain in the forearc of the
active Cascadia subduction zone.
IMPLICATIONS FOR SEISMIC
HAZARD
The length of the active Leech River
fault zone (30–60 km; Fig. 2A) and its
history of multiple Quaternary ruptures
suggest it is capable of producing earth-
quakes of MW >6. This active fault zone
lies within tens of kilometers of down-
town Victoria and in close proximity to
three local water dams. One of these dams
is located within 2 km of the active fault
zone and supports the region’s principal
water supply reservoir (Fig. 2A). The other
two dams lie within the active fault zone
and support a hydroelectric power plant.
Thus, our new identification of a signifi-
cant shallow seismic source has consider-
able implications for the seismic risk
exposure of this populated region.
Surface-rupturing earthquakes with shal-
low hypocenters can be highly destruc-
tive, and it is therefore important that the
Leech River fault zone be incorporated
into seismic hazard assessments of south-
western British Columbia and neighboring
regions.
ACKNOWLEDGMENTS
We thank CRD watersheds and BC Hydro for
access to key field sites. This manuscript benefit-
ted from comments by Jack Loveless, Alan Nelson,
Christie Rowe, and two anonymous reviewers.
We thank Steven Whitmeyer for editorial han-
dling. This research was supported by an NSERC
Discovery grant to KM and NSF EAR IRFP Grant
#1349586 to CR.
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