Bulletin of the Seismological Society of America, Vol. 101, No. 1, pp. 1–12, February 2011, doi: 10.1785/0120090360
A Revised Earthquake Chronology for the last 4,000 Years Inferred
from Varve-Bounded Debris-Flow Deposits beneath
an Inlet near Victoria, British Columbia
by Andrée Blais-Stevens, Garry C. Rogers, and John J. Clague
Abstract
A reanalysis of the varve chronology from hydraulic piston sediment
cores was carried out to establish better uncertainty estimates on ages of prehistoric
debris-flow deposits (DFDs) in the last 4000 yr. Saanich Inlet is an anoxic fiord located
in southeast Vancouver Island near the city of Victoria, British Columbia. It contains
annually laminated (varved) marine mud deposited in anoxic conditions. Interlayered
with these Holocene varves are massive layers of coarser sediments deposited by submarine debris flows. It has been previously interpreted that these flows were induced
by earthquake shaking. Two of the DFDs correspond to known earthquakes:
A.D. 1946 Vancouver Island (M 7.3) and the A.D. 1700 Cascadia plate-boundary
subduction earthquake (M 9). Based on varve counts, 18 DFDs (310, 410–435,
493–582, 767–887, 874–950, 1001–1133, 1163–1292, 1238–1348, 1546–1741,
1694–1811, 1859–2104, 2197–2509, 2296–2483, 2525–2844, 2987–3298, 3164–
3392, 3654–4569, 3989–4284 yr ago from A.D. 2010 datum) were correlated among
two or more cores during this time period, suggesting an average return period of
strong shaking from earthquakes of about 220 yr. Nine of the DFDs overlap with
the age ranges for great plate-boundary earthquakes that have been determined by
other paleoseismic studies: coastal subsidence and offshore turbidity deposits. The
remaining nine events give an average return period of about 470 yr for strong shaking
from local earthquakes. The peak ground acceleration calculated from a recurrence
relation based on statistics from local earthquakes for a 470-yr period is 0.30g, which
corresponds to the upper range of Modified Mercalli Intensity (MMI) VII (seven).
Historical data from Vancouver Island and other areas show that this level of shaking
(MMI VII) is sufficient to trigger submarine landslides.
Introduction
Saanich Inlet is an anoxic fiord located in a zone of high
seismic activity on southeast Vancouver Island, British
Columbia (Fig. 1). The uppermost sediments in the fiord consist of annually layered muds (varves) deposited in an anoxic
environment. Interlayered with the varves are layers of coarser
sediments deposited by submarine debris flows. Previous studies have interpreted the debris-flow deposits (DFDs) to be
seismically triggered and have used varve counts and radiometric ages to date the past earthquakes (Blais, 1996; BlaisStevens et al., 1997, 2001; Blais-Stevens and Clague, 2001).
Caution, however, is required when using varves to date
DFDs because of inherent and sometimes unavoidable uncertainties in applying the technique. Some sediment record
may be lost during coring, although this problem can be
mitigated by collecting and correlating several cores.
Another problem is discriminating individual annual layers
where they are indistinct. In such situations, significant errors
may be introduced during varve counting. An alternative is to
estimate varve numbers in distinctly laminated sections of
cores by applying an average sedimentation rate estimated
from adjacent sections of well-varved sediments or by using
radiocarbon ages as stratigraphic reference points. A third
problem is that debris flows may strip some varves (and possibly some DFDs) from the seafloor as they travel across it.
Several studies have suggested that when varves are observed
with sediment gravity flow deposits in a sediment column,
varve chronology may lead to underestimation of the age
of deposits because of potential erosion of varves from sediment gravity flows (Degens et al., 1976; Troften and Mörner,
1997; Trauth et al., 2003). Blais-Stevens et al. (1997) calculated that up to 6% of sediment could be missing from the
sediment column in a core with the highest number of DFDs,
a 137 Cs age at the top of the sediment core, a biohorizon indicating A.D. 1940, and a radiocarbon age at the bottom for
1
2
stratigraphic reference points. A fourth problem is the piston
coring method in water-rich sediment. The top part of the
sediment column is usually missing because the method is
explosive. Other coring methods are used to try to compensate for missing tops of sediment columns, like freeze-coring
and box coring (Blais-Stevens et al., 1997, 2001).
Several studies have examined the level of seismic shaking required to trigger subaerial landslides, but aside from
Keefer (1984, 2002), few have examined the effects of shaking on subaqueous slope failures. A related issue in the coastal Pacific Northwest is discriminating (M 8) subduction
earthquakes at the Cascadia subduction zone from smaller
crustal earthquakes within the North America plate and
earthquakes within the subducting slab. Extensive paleoseismology research over the past 30 yr (e.g., Atwater and
Hemphill-Haley, 1997; Clague et al., 1992, 1997, 1998; Williams and Hutchinson, 2000; Kelsey et al., 2005; Williams
et al., 2005; Goldfinger et al., 2008, 2010) has provided a
record of subduction earthquakes, but the frequency and
magnitude of crustal and interslab earthquakes beyond the
historic period are not well established.
In our previous study (Blais-Stevens and Clague, 2001),
we proposed that extensive DFDs emplaced by single large
failures or many smaller coincident failures were likely of
seismic origin, and we established a preliminary chronology
A. Blais-Stevens, G. C. Rogers, and J. J. Clague
of possible seismic events. We made only a few correlations
with documented seismic events. In this paper, further consideration was given to establishing a more precise chronology
of events by refining varve chronology; therefore, the first
objective of the paper is to quantify uncertainties in the use
of varves for dating DFDs and, by inference, earthquakes
affecting Saanich Inlet. The second objective is to compare
the debris-flow events with ground acceleration and consequent levels of seismic shaking in Modified Mercalli Intensity
(MMI) values that can trigger subaqueous failure. Third, the
timing of paleoearthquakes inferred from the Saanich Inlet
data set is compared with events (e.g., tree-ringed based
age, turbidites and fault trench deposits, coseismic subsidence
stratigraphy and surface faulting, and tsunami deposits) documented elsewhere in southwest British Columbia, coastal
Washington, and deeper Pacific Ocean in the Cascadia subduction zone. Finally, the return period defined by the number
of observed subaqueous landslides is compared with the seismic shaking level expected for that return period, which is
estimated from earthquake statistics.
Methods
Eight piston cores up to 20 m long (i.e., short cores)
were collected in 1989 and 1991 and correlated with longer
piston cores collected in 1996 during the Ocean Drilling
Figure 1. Location map indicating the epicenter for the A.D. 1946 crustal earthquake (star) and the approximate rupture area of the
offshore A.D. 1700 Cascadia subduction zone earthquake (hatched area). Question mark indicates uncertainty in the northern limit of the
1700 rupture. The circle indicates a 100 km radius from Saanich Inlet (British Columbia) and includes sites that have documented paleoseismic events. The locations of these events are Fraser Delta (#1) and southern Vancouver Island (#2) (Clague et al., 1992, 1997, 1998;
Mathewes and Clague, 1994); Lake Creek-Boundary Creek fault (#3) (Nelson et al., 2007); Discovery Bay (#4); Swanton Marsh (#5)
(Williams and Hutchinson, 2000; Williams et al., 2005); north Whidbey Island (#6) (Johnson et al., 2004); south Whidbey Island (#7)
(Kelsey et al., 2004); and, located at the limit of the 100-km circle, Kendall fault scarp within Boulder Creek fault (#8) (Barnett,
2007; Barnett et al., 2007) (sites are also mentioned in Fig. 5).
Revised Earthquake Chronology for the last 4,000 Yr Inferred from Varve-Bounded Debris-Flow Deposits
48°45 '
Cowichan
Bay
0
m
m
15
0
10
m
100
Satellite
Channel
70 m
Sill
m
91-05
100
91-03
89-02
1034
m
91-04
Bathymetric
contour
Shoreline
200
Program (ODP) Leg 169S (Fig. 2). During this leg in 1996,
four holes (A, B, C, and D) with lateral offsets of 10 m were
drilled to a maximum depth of 105.1 m below the seafloor at
ODP site 1033. Five holes (A, B, C, D, and E) with lateral
offsets of 10 m were drilled at ODP site 1034, 4 km north of
site 1033, to a maximum depth of 118.2 m below the seafloor. Coring procedures are described in detail by Blais
(1996), Blais-Stevens et al. (1997, 2001), Bornhold et al.,
(1998), and Blais-Stevens and Clague (2001).
Correlations between cores were made using marker
beds, such as the Mazama ash (ca. 7700 calibrated years
B.P.; Hallett et al., 1997) and a Late Pleistocene outburst flood
deposit (Blais-Stevens et al., 2001, 2003). Seventy-one samples of shell and plant material were AMS (accelerator mass
spectrometry) radiocarbon dated at the Center for Accelerator
Mass Spectroscopy at Lawrence Livermore National Laboratory (Blais-Stevens and Clague, 2001; Blais-Stevens et al.,
2001). Of the 71 organic samples, 22 were located in the upper
40 m of the sediment cores. Calendric ages were calculated
from the radiocarbon ages using the calibration program
CALIB 3.0 of Stuiver and Reimer (1993) and recalculated with
a revised version of the software, CALIB 6.0 (Stuiver et al.,
2005). Most recalculated ages showed the same age range
within less than 100 yr. The datum for calendar ages reported
in this paper is A.D. 2010. Radiocarbon ages were used as
reference points in the stratigraphy along with varve counts.
Sediment samples from the tops of the short cores were
analyzed for 137 Cs and 210 Pb to demonstrate that the rhythmic laminae in Saanich Inlet are varves and to establish age
data in the uppermost part of the sedimentary sequence. In
addition, varves were analyzed for presence or absence of a
diatom species that first appeared in the fiord in A.D. 1940
(McQuoid and Hobson, 1997). Varve counts and couplet
thicknesses were also used for correlation. Precise varve
counts were made for ODP cores 1033B and 1034B and for
the shorter trigger cores A. Varves in the other ODP cores
were not directly counted, but numbers of varves were estimated based on average couplet thicknesses (Blais-Stevens
and Clague, 2001; Blais-Stevens et al., 2001).
3
89-03
1033
ODP core site
91-02
Short core site
89-01
91-01
Kilometers
0
1
2
3
4
5
6
48°30 '
123°28
123°35 '
Goldstream River
Results and Discussion
Stratigraphy
The cored sediments span the past 15,000 yr, the longest
continuous sediment record in Canada. The major stratigraphic units (Fig. 3), from youngest to oldest, are (1) distinctly laminated, olive gray, diatomaceous, marine mud
intercalated with muddy DFDs; (2) indistinctly laminated,
bioturbated, olive gray mud; and (3) gray glaciomarine
mud. Two stratigraphic markers occur within the indistinctly
laminated marine mud unit, a layer of Mazama volcanic ash
and a gray silty clay bed (Blais-Stevens et al., 2003; Fig. 3).
The focus of this paper is on the uppermost 40 m of sediment, which span the past 4000 yr. The reader is referred
Figure 2. Locations of long ODP cores (stars) and short cores
(triangles). The bedrock sill at the north end of Saanich Inlet at 70 m
depth inhibits water circulation, creating anoxic conditions in
deeper waters within the inlet.
to Blais-Stevens and Clague (2001) and Blais-Stevens et al.
(2001) for a complete description of the sediments.
Varves and Debris-Flow Deposits
The main source of freshwater and sediment comes from
Cowichan River, north of the inlet with very little input from
the Goldstream River (Fig. 2; Herlinveaux, 1962). Thus,
varves thin in a southerly direction due to a decrease in
4
A. Blais-Stevens, G. C. Rogers, and J. J. Clague
Figure 3. Schematic stratigraphic summary of ODP cores 1033
and 1034. Ages are in radiocarbon years B.P. This paper focuses
on the upper 40 m of sediments, comprising varved olive gray
diatomaceous mud intercalated with muddy debris-flow deposits.
Below 40 m, the sediments were disturbed by bioturbation.
sedimentation rate in that direction. They range in thickness
from a few millimeters to as much as 20 mm (Blais-Stevens
et al., 1997). Gross et al., (1963), Sancetta and Calvert
(1988), and Sancetta (1989) confirmed that the rhythmites
are varves.
The DFDs are beds of silty clay with erosional basal contacts, diatom-rich caps (usually < 1 cm thick), and basal
zones of deformed varves grading upward into massive mud.
Most of the DFDs have a greater proportion of silt and sand
than the varves and contain broken tests of shallow-water
foraminifera (Blais-Stevens and Patterson, 1998). The beds
range in thickness from a few centimeters to a few decimeters. They were produced by sediment gravity flows from
sidewall failures (Blais, 1996; Blais-Stevens and Clague,
2001; Calvert et al., 2001). The beds are thicker and more
abundant at the southern end of Saanich Inlet (44 in the sediment core at ODP site 1033), where the sidewalls are steep,
than at the northern end (22 at ODP site 1034; see fig. 5 in
Blais-Stevens and Clague, 2001 and fig. 3 in Blais-Stevens
et al., 2001). This observation suggests that a lesser trigger is
needed to produce a subaqueous landslide on the steeper
walls of the inlet. Furthermore, there is no evidence to suggest that some of these failures were triggered subaerially
(Blais, 1996).
Correlation of Debris-Flow Deposits
DFDs were correlated from core to core using radiocarbon, 137 Cs, and 210 Pb dates, varve counts, marker horizons,
sedimentology, and bed thicknesses. The DFDs discussed
here are documented by Blais-Stevens and Clague (2001).
Seventeen debris-flow units, deposited over the past
4000 yr, are observed in the two ODP cores which are
4 km apart (Fig. 2 and 4) and thus have large lateral extent.
Two debris-flow units dating about A.D. 1700, the time of
the last great Cascadia subduction earthquake (Satake et al.,
1996; Atwater and Hemphill-Haley, 1997; Atwater et al.,
2004), were observed in the short cores 89-03 and 91-03
about 6 km apart but not in the intervening two short cores.
A DFD was observed in short core 89-03 dating to
A.D. 1946, the time of a major (M 7.3) earthquake in
central Vancouver Island (Rogers and Hasegawa, 1978).
There are DFDs older than 4000 yr, but they were not included in this analysis because some of the sediment below
the 4000-yr level was disturbed by bioturbation due to frequent oxygenation events in Saanich Inlet during early and
middle Holocene time (Fig. 3).
Once stratigraphic markers were identified, varve counts
were used as the main tool for determining the ages of each
DFD (Fig. 4; Table 1). In this paper, we reexamine the method used by Blais-Stevens and Clague (2001) with respect to
the sources of error mentioned in the introduction. Varves in
cores 1033A, B and 1034A, B were individually counted,
whereas they were estimated in others (1033C, D, 1034C, D,
and E) from average sedimentation rates in sections of the
core with easily recognized varves. Sedimentation rates were
estimated based on the average varve thickness at a specific
ODP site. For example, in the sediment cores at ODP site
1034, above 22 m below the sea floor, the average varve
thickness was 12 mm and thus reflected a sedimentation rate
of 12 mm=yr. Below 22 m below the sea floor, the average
varve thickness changed abruptly to 7 mm, thus the sedimentation rate was 7 mm=yr (Blais-Stevens et al., 2001). The
number of varves was calculated based on the average sedimentation rate between two DFDs. At ODP site 1033, there
was no noticeable change in varve thicknesses throughout
the sediment core; varve thicknesses varied slightly between
4–6 mm (Blais-Stevens et al., 2001).
To use all the data available to us, we calculated the
mean number of varves separating successive debris-flow
beds in all analyzed cores with a 2σ standard deviation (left
side of 1033 in Fig. 4; Table 1). Therefore, we calculated the
horizontal uncertainty as a measure of variability in the
numbers of varves counted and estimated in stratigraphic intervals correlated among the ODP sediment cores. For example, the numbers of varves between DFDs 4 and 5, estimated
from average sedimentation rates in cores 1033A, C, and D
and 1034C–E, range from 124 to 164 with a mean of 150 and
a 2σ standard deviation of 34 (Table 1). Our best estimate
with applied corrections for each event is given in an age
range (right-hand column of 1034 in Fig. 4; Table 1).
Erosion of varves by debris flows introduces additional
uncertainty. Blais (1996) and Blais-Stevens et al. (1997)
estimated that up to 6% of the varves could be missing
due to erosion in the short core (89-03) with the highest number of DFDs (10), the total varve counts compared with a
137
Cs age and a biohorizon indicating A.D. 1940 at the top
of the sediment column and a radiocarbon age at the bottom.
Revised Earthquake Chronology for the last 4,000 Yr Inferred from Varve-Bounded Debris-Flow Deposits
5
We summed the horizontal uncertainty and the erosion
correction and show the total range about the mean values for
all cores (in Fig. 4 and 5; see also Table 1). This approach
provides a larger range of possible ages for each DFD than
was reported by Blais-Stevens and Clague (2001), but we
believe the estimates more realistically capture the inherent
uncertainties in estimating debris-flow age, given the limitations of the data set. It still shows that varve counting offers
greater precision than other dating methods used alone, for
example radiocarbon dating. Hence, our best estimate for the
ages of the DFDs is 310, 410–435, 493–582, 767–887,
874–950, 1001–1133, 1163–1292, 1238–1348, 1546–1741,
1694–1811, 1859–2104, 2197–2509, 2296–2483, 2525–
2844, 2987–3298, 3164–3392, 3654–4569, and 3989–
4284 yr ago from A.D. 2010 datum (Figs. 4 and 5 and
Tables 1 and 2).
Origin of Debris Flows
Saanich Inlet is an ideal environment for preserving
DFDs produced by earthquake-triggered slope failures be-
Figure 4.
Composite correlation of sediments at ODP core sites
1033 and 1034. Numbers on the left of 1033 are the mean number of
varves between all correlated debris-flow deposits (gray boxes) at
both sites. Refer to Table 1 for individual numbers of varves.
Numbers on the right of 1034 are the best estimated age ranges
for correlated debris-flow deposits from both sites which include
the horizontal and vertical uncertainties in calendar years with
A.D. 2010 datum. See Table 1 for calculated age ranges. The event
at 310 yr is not shown, but it is present in the short cores that overlap
with the ODP cores (Blais-Stevens and Clague, 2001). Radiocarbon
ages included are in calendar years from A.D. 2010 and demonstrate good correlation with varve chronology. Mbsf, meters below
sea floor.
This erosional uncertainty is addressed in Table 1 by adding
an erosion correction of 6% to the interevent ages calculated
in the manner just described. Moreover, a similar observation
can be made for DFDs where some could be missing from the
sediment record.
cause it is a semi-enclosed, low-energy setting, protected
from the more open waters of Satellite Channel by a sill at
its north end. Most of the terrigenous sediment deposited in
the inlet is derived from an external source (Cowichan River;
Herlinveaux, 1962). It settles from suspension on the walls
and floor of the fiord and is well preserved in annual laminae
because the anoxic waters inhibit bioturbation. Mud that has
accumulated on the fiord walls is susceptible to failure when
shaken during earthquakes.
We interpret large, extensive DFDs or groups of synchronous DFDs found over a large area in two or more cores to be
the result of earthquake shaking. Seventeen extensive DFDs
are present within the most recent 4000 yr of sediments in the
ODP cores, which are 4 km apart (Fig. 4). Debris-flow units
were observed in two of the short cores dating to about
A.D. 1700 and in one short core dating to A.D. 1946
(Blais-Stevens and Clague, 2001; Fig. 6). These two younger
deposits are less widespread than the other DFDs down core,
likely because the piston coring method is explosive, especially in soft water-rich sediment, which usually explains
why there is sediment missing at the top of the sediment
column (Blais-Stevens et al., 2001).
Some debris-flows might be triggered, for example, by
slumps from small deltas at the mouths of ephemeral streams
during high runoff or by failures related to discharge of
groundwater on the walls of the fiord (Blais-Stevens and
Clague, 2001). The DFDs found in only one core in Saanich
Inlet may not be aerially extensive and generally cannot be
confidently attributed to earthquakes unless they are shown
to be synchronous with historical events, as with the DFD at
A.D. 1946, for which the age is confirmed with 137 Cs and a
biohorizon indicating A.D. 1940.
Other factors such as a change in climate conditions or
fire activity in the area have been ruled out as possible trigger
mechanisms for forming DFDs. There is no evidence in the
164
80
298
126
A
44
647
124
402
304
54
150
B
123
442
31
364
174
103
362
78
180
164
92
C
1079
1033
43
908
140
465
321
53
110
339
74
164
166
86
270
D
85
108
A
20
667
135
230
21
299
234
106
60
B
668
128
284
46
388
235
315
61
150
124
C
1034
23
737
136
36
389
240
59
145
139
80
294
D
897
148
470
34
63
145
144
77
262
98
E
33 26
800 328
133 18
445 62
285 79
37 23
360 85
221 63
107 7
339 48
64 17
156 28
150 34
83 11
281 35
111 28
Mean SD†
4015
3982
3182
3049
2604
2319
2282
1922
1701
1594
1255
1191
1035
885
802
521
410
Mean Age‡
3989–4041
3654–4310
3164–3200
2987–3111
2525–2683
2296–2342
2197–2367
1859–1985
1694–1708
1546–1642
1238–1272
1163–1219
1001–1069
874–896
767–837
493–549
410
Age Range§
4228–4284
3873–4569
3354–3392
3166–3298
2677–2844
2434–2483
2329–2509
1971–2104
1796–1811
1639–1741
1312–1348
1233–1292
1061–1133
926–950
813–887
523–582
435
Age Range∥
3989–4284
3654–4569
3164–3392
2987–3298
2525–2844
2296–2483
2197–2509
1859–2104
1694–1811
1546–1741
1238–1348
1163–1292
1001–1133
874–950
767–887
493–582
410–435
Best Estimate#
Number of varves between each DFD from each ODP core. B (and A) core varves were visually counted. Varve counts from other cores were estimated based on
the sedimentation rate.
*DFD1: Only the erosional uncertainty is added to the age range.
†
Mean varve count with 2 standard deviations (2σ) reflecting depositional (horizontal) uncertainty.
‡Mean age in calendar years from A.D. 2010.
§
Age range in calendar years from A.D. 2010, including depositional (horizontal) uncertainty only.
∥Age range in calendar years from A.D. 2010, including 6% erosional (vertical) uncertainty (Blais-Stevens et al., 1997).
#
Best estimate includes the age range with combined depositional and erosional uncertainties.
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1*
Debris-Flow
Deposits
Table 1
Varve Counts and Age Ranges with an A.D. 2010 Datum in ODP Cores 1033 and 1034
6
A. Blais-Stevens, G. C. Rogers, and J. J. Clague
Revised Earthquake Chronology for the last 4,000 Yr Inferred from Varve-Bounded Debris-Flow Deposits
7
Figure 5. Modified from Williams et al. (2005). Inferred correlations of Saanich Inlet DFD, subduction earthquakes, and local earthquakes established from subsidence records in coastal Washington (Atwater et al., 2004; Hagstrum et al., 2004), deep-sea turbidites (T1–T9,
Goldfinger et al., 2010), Discovery Bay, Swanton tsunami deposits (Williams and Hutchinson, 2000; Williams et al., 2005), north and south
Whidbey Island ground deformation (Johnson et al., 2004; Kelsey et al., 2004, respectively), ground deformation at Kendall fault scarp
within Boulder Creek fault (Barnett, 2007; Barnett et al., 2007), and land-level deformation and liquefaction deposits on southern Vancouver
Island and Fraser Delta (Clague et al., 1992, 1997, 1998; Mathewes and Clague, 1994). Dark gray rectangles indicate the most likely age
range for earthquakes inferred from the coastal subsidence record of Atwater et al., (2004) and Hagstrum et al., (2004). Upward pointing
arrows in Discovery Bay samples indicate ages on detrital material that provide only maximum ages and the downward pointing arrow, a
minimum age (Williams et al., 2005). Saanich Inlet ages are the combined corrected age ranges (i.e., the minimum value from varve counts
and maximum value from including the potential 6% sediment loss; best estimate in Table 1 and 2). Bar indicates best estimate age range.
Numbers 1–17 correspond to the DFDs labeled in Table 1. The DFD corresponding to A.D. 1946 and 1700 events were observed in the short
cores only.
sediments to suggest that there was intense fire activity
during the past 15,000 yr (Blais-Stevens et al., 2001). A
palynological study (Pellat et al., 2001) in the Saanich Inlet
basin sediments indicates that climate and vegetation conditions became established about 4000 yr ago, reflecting modern conifer forests and oak savannas. Hence, climate
conditions have remained the same over the last 4000 yr.
Moreover, one of the arguments of possible turbidity
currents originating from Cowichan River and travelling
down the axis of Saanich Inlet was ruled out (Blais, 1996;
Blais-Stevens et al., 1997). A turbidity current from Cowichan River would travel down the bathymetric lows (150 m
depth) towards Satellite Channel (northeast of Saanich Inlet;
Fig. 2) rather than climb up the sill (70 m) at the mouth of the
inlet. In addition, the DFDs do not possess typical characteristics of turbidites, for example, normal or inverse gradation,
and the deposits are thinner in the north than in the south,
which is the opposite of what would be expected if they were
deposited by a turbidity current travelling from the north
(Blais-Stevens et al., 1997).
Intensity of Seismic Shaking
To test whether it is reasonable to associate the DFDs in
Saanich Inlet with earthquakes, we estimated from the literature the MMI required to trigger subaqueous landslides. In his
8
A. Blais-Stevens, G. C. Rogers, and J. J. Clague
Table 2
Comparison of Saanich Inlet Debris-Flow Deposit Ages with Other Timing
Estimates of Great Plate-Boundary Earthquakes
Saanich Inlet
Debris-Flow Deposit*
Best Estimate
Atwater et al., 2004†
Goldfinger et al. (2010)‡
T1 231–427
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
410–435
493–582
767–887
874–950
1001–1133
1163–1292
1238–1348
1546–1741
1694–1811
1859–2104
2197–2509
2296–2483
2525–2844
2987–3298
3164–3392
3654–4569
3989–4284
T2 436–605
W 820–1230
T3 690–963
U 1289–1324
S 1600–1670
T4 1196–1430
T5 1459–1802
N 2480–2680
L 2905–2985
J 3365–3450
T6 2477–2761
T7 2948–3245
T8 3360–3699
T9 3939–4364
*Saanich Inlet debris-flow number used in Table 1 shown on Figures 4 and 5.
Coseismic subsidence ages along the Pacific Coast (Atwater et al., 2004).
‡Ages of seismically triggered turbidites (Goldfinger et al., 2010). All ages have been
corrected to A.D. 2010.
†
analysis of historic earthquake-triggered landslides, Keefer
(1984, 2002) groups subaqueous landslides with liquefactioninduced failures and concludes that the predominant MMI
required to trigger them is MMI VII, which corresponds to
Figure 6.
and 1034.
a peak acceleration range of 0.18–0.34g (Wald et al.,
1999). He notes that some subaqueous landslides and liquefaction may occur in MMI zones VI (0.09–0.18g) and even
MMI V (0.04–0.09g), but they are not common. He also notes
Modified from Blais-Stevens and Clague (2001). Diagram indicating correlation of the short cores and longer ODP cores 1033
Revised Earthquake Chronology for the last 4,000 Yr Inferred from Varve-Bounded Debris-Flow Deposits
that the duration of shaking is a significant factor in these
types of mass movement: the great 1964 Alaska earthquake
(M 9.2) produced several minutes of strong shaking and triggered more submarine landslides than any other earthquake in
his data set.
We use estimates of peak ground acceleration at Saanich
Inlet for an M 9 earthquake (Fig. 1), like that in Cascadia in
A.D. 1700, which are in the range of 0.13–0.18g. These
values are obtained by applying information contained in
publications describing the Canadian and United States seismic hazard maps (Adams and Atkinson, 2003; Petersen et al.,
2008). Great Cascadia subduction earthquakes should produce peak accelerations at Saanich Inlet typical of MMI
VI, lower than the typical level of shaking suggested by Keefer’s (1984, 2002) analyses, but the duration of shaking is
much longer than that of large local earthquakes (e.g., Olsen
et al., 2008), and that likely enhances the ability of the
Cascadia subduction earthquakes to trigger subaqueous
landslides.
The distribution of subaqueous landslides and liquefaction caused by the A.D. 1946 central Vancouver Island earthquake (Rogers, 1980) is consistent with Keefer’s conclusions
and with the work of Ambraseys (1988), as extended by Papadopoulos and Lefkopoulos (1993). Almost all subaqueous
and liquefaction-induced landslides for this earthquake were
in regions of MMI VII; only a few occurred in MMI VI.
Saanich Inlet is about 180 km from the epicenter, within
MMI VI but near the boundary between MMI VI and V. This
location is just beyond the distance that Keefer (1984, 2002)
argues would experience subaqueous landslides and at the
extreme distance limit of liquefaction according to Papadopoulos and Lefkopoulos (1993). The fact that a debris-flow
deposited sediment in 1946 at only one core site is thus not
surprising. This single DFD is particularly well dated because
of the age constraint provided by the A.D. 1940 biohorizon
(McQuoid and Hobson 1997; Blais-Stevens and Clague,
2001). The report of a person fishing near the core site stating
that an unusual wave about 50 cm high occurred at the time
of the earthquake (Geological Survey of Canada files) may
be related to the submarine landslide. Cores from Effingham
Inlet, another anoxic inlet located 135 km northwest of
Saanich Inlet and about 85 km from the epicenter of the
1946 earthquake, contain a sediment gravity flow deposit
that dates to this event (Skinner and Bornhold, 2003; Dallimore et al., 2005).
In conclusion, information from the A.D. 1946 and 1700
earthquakes indicates that both local and giant subduction
earthquakes are capable of triggering landslides in Saanich
Inlet even though these two events were produced by shaking
near the lower limit of intensity likely to cause subaqueaous
landslides.
Other Seismically Triggered Debris Flows
In Tables 1 and 2, we list DFDs that we attribute to earthquakes, and in Figure 5, we compare plate-boundary and
local paleotsunami and paleoearthquake (Table 3) data from
sites within 100 km of Saanich Inlet (circle in Fig. 1). This
distance is the maximum from the epicenter for subaqueous
landslides and liquefaction based on the empirical relationship developed by Keefer (1984). For plate-boundary events,
we have included both the coastal subsidence record in
Washington State (Atwater et al., 2004; Hagstrum et al.,
2004) and the offshore turbidite record from the Cascadia
subduction zone (Goldfinger et al., 2010). We have used
the nomenclature originally proposed by these authors.
Table 2 shows comparisons in age ranges (corrected to
A.D. 2010 datum) of plate-boundary events to those from
the Saanich Inlet events. The observations from different environments (Williams and Hutchinson, 2000, and Williams
et al., 2005 [Discovery Bay, Swanton tsunami deposits];
Johnson et al., 2004 [north Whidbey Island]; Kelsey et al.,
2004 [south Whidbey Island]; Clague et al., 1992, 1997,
1998, and Mathewes and Clague, 1994 [south Vancouver Island and Fraser Delta]; Barnett, 2007, and Barnett et al.,
2007 [Kendall fault scarp within Boulder Creek fault]) correlate well with the paleoearthquake evidence from Saanich
Inlet. Paleoseismic evidence was also documented along the
Lake Creek-Boundary Creek fault north Washington (Fig. 1).
Ages of events were documented at 6000–200 yr ago and <
5000 yr ago (Nelson et al., 2007). The events were not included in Table 3 or Figure 5, because the ages are presently
less well constrained.
Moreover, based on our best age range estimates,
Tables 1 and 2 and Figure 5 indicate how most widespread
deposits from Saanich Inlet correlate with plate-boundary
Table 3
Paleoseismic Evidence of Local Earthquakes
Location ≤ 100 km of Saanich Inlet
Type of Evidence
Reference
∼900 yr B.P.
1960 cal yr
Kendall fault scarp/Boulder Creek fault
Vancouver Island and Fraser Delta
2860–3260 cal yr
∼3000 yr B.P.
3660 cal yr
South Whidbey Island, Washington
Kendall fault scarp/Boulder Creek fault
Vancouver Island and Fraser Delta
Ground deformation
Rapid sea-level change
and liquefaction
Land-level changes
Ground deformation
Rapid sea-level change
and liquefaction
Barnett, 2007; Barnett et al., 2007
Mathewes and Clague, 1994
Clague et al., 1992, 1997, 1998
Kelsey et al., 2004
Barnett, 2007; Barnett et al., 2007
Mathewes and Clague, 1994
Clague et al., 1992, 1997, 1998
Age of Seismic Event*
9
*Ages in calibrated years corrected to A.D. 2010, but not those in years B.P. because they were approximate.
10
A. Blais-Stevens, G. C. Rogers, and J. J. Clague
Frequency of Strong Shaking
We have documented 18 extensive DFDs in Saanich
Inlet in two or more widespread cores from the past
4000 yr: 17 in the ODP cores and one in two of the short
cores attributed to the A.D. 1700 subduction earthquake.
In addition, a DFD in one short core is associated with the
A.D. 1946 Vancouver Island earthquake. Nine of the DFDs
correspond to suspected subduction earthquake events
(T1–T9). Assuming that all the widespread DFDs are products of earthquakes as we have argued, then the remaining
nine events must be the result of significant local crustal and
intraslab earthquakes. Dividing the estimated number of
years for the oldest event by 9, we obtain an average return
period ranging from 443 to 476 yr with and without the 6%
erosion correction. In Figure 7, we plot the return period of
ground motion from a combined data set of crustal and
in-slab earthquakes at Saanich Inlet following the procedures
used for the Canadian National Building Code (Adams and
Atkinson, 2003). A return period of 470 yr corresponds to a
peak acceleration of about 0.30g, which is in the upper part
of the range for MMI VII (Wald et al., 1999). This is the level
of shaking that Keefer (1984, 2002) suggests is most typical
for triggering subaqueous landslides. This result gives us
additional confidence that the identified simultaneous widespread DFDs in the sediment cores from Saanich Inlet are
earthquake-induced and can be used as earthquake proxies.
Conclusions
We have documented 18 widespread submarine debrisflow events in Saanich Inlet during the past 4000 yr: 17 in the
ODP cores and one in the short cores, which we interpret as
having been caused by strong earthquake shaking. This reflects a return period of about 200 yr (mean 220 and standard
deviation 187) for strong earthquake shaking at Sannich
Inlet, which is adjacent to Victoria, British Columbia. We
conclude that nine of the events correspond to great earthquakes at the Cascadia subduction zone, and an additional
nine correspond to significant local earthquakes. The average
recurrence interval for strong shaking at Saanich Inlet from
local earthquakes is about 470 yr.
Return Period (yrs)
1000
500
200
100
100
50
VIII
VII
M.M. Intensity
IX
Peak acceleration (%g)
earthquake evidence (Atwater et al., 2004; Hagstrum et al.,
2004; Goldfinger et al., 2010).
Event T2 identified in the offshore turbidite deposits
(Goldfinger et al., 2010) was not widely identified in early
paleoseismic investigations in the coastal environment of the
Cascadia subduction zone (Atwater, 1992; Atwater and
Hemphill-Haley, 1997; Kelsey et al., 2005). It was one of
the smaller coastal subsidence events (Leonard et al., 2010)
and seems to correlate with tsunami deposits in Discovery
Bay and on the Olympic Peninsula south of Vancouver
Island (Williams et al., 2005), in Oregon (Darienzo and Peterson, 1995), and on Vancouver Island (Clague et al., 2000).
Saanich Inlet event 14 does not clearly overlap with
event L. There is a discrepancy of two years (varves). DFD
14 in Saanich Inlet revealed a best estimate age range of
2987–3298 calibrated years before A.D. 2010 and the coseismic subsidence age for event L was precisely dated at 2905–
2985 calibrated years (Tables 1, 2 and Fig. 5; Atwater et al.,
2004; Hagstrum et al., 2004). We infer that because the discrepancy is only two years, it is likely that both events
represent the same plate-boundary earthquake. Both events
14 and L overlap with event T7 from Goldfinger et al.
(2010). Tables 1, 2, and Figure 5 indicate that for each documented plate-boundary event, there is a widespread DFD in
Saanich Inlet.
Comparison of seismic events compiled in Figure 5
shows that the total number of inferred earthquakes in Saanich
Inlet is slightly larger than the sum of subduction events and
other identified local paleotsunami and paleoearthquake
events. If all widespread debris flows are products of strong
earthquake shaking, Saanich Inlet would appear to be superior
to other environments as a recorder of seismic events.
Notwithstanding the potential errors in dating, we conclude
that all the widespread debris-flow events in Saanich Inlet
are likely related to strong earthquake shaking.
Another perhaps more useful way to use the information
in Figure 5 is to compare the number of events in Saanich
Inlet with the predicted number of times that strong shaking
should occur using analysis of the historical earthquake
catalogue.
20
VI
10
0.001
0.002
0.005
0.01
Probability of exceedence
(per annum)
Figure 7. Probablility of exceedence of levels of acceleration
and MMI for Saanich Inlet, based on historical local earthquake
data. Widespread debris flows in Saanich Inlet that are thought
to be triggered by local earthquakes occur, on average, once every
470 yr. Shaking from local earthquakes with this return period is
MMI intensity VII (seven), which is sufficient to trigger subaqueous
landslides.
Revised Earthquake Chronology for the last 4,000 Yr Inferred from Varve-Bounded Debris-Flow Deposits
Strong shaking at Saanich Inlet with a 470 yr return period is expected to have a peak ground acceleration of about
0.30g typical of MMI VII. This is sufficient shaking to trigger
subaqueous landslides. This observation supports previous
arguments that the extensive debris flows in Saanich Inlet
are triggered by moderate to large earthquakes and not by
nonseismic processes.
We note that these calculated earthquake recurrence
intervals are maximum estimates because several factors
can affect the deposition (or lack thereof) of a debris-flow
deposit: (1) Availability of sediments on the side wall. If
large earthquakes affecting Saanich Inlet occurred close in
time, there may not have been enough time for sediment
to accumulate on the side wall; (2) Some of the DFDs and
varves may have been eroded by subsequent DFDs; and
(3) Sediment may be lost during coring. However, even with
possible sources of uncertainty documented in this paper,
varves offer advantages over other dating methods for dating
DFDs from subaqueous landslides.
Data and resource
All data used in this paper came from published sources
listed in the references.
Acknowledgments
We thank Kim Conway for technical support and advice. Judith Baker
and Randy Enkin are thanked for helpful discussions. Heidi Pass, Anastasia
Ledwon, and Kevin Robertson assisted with laboratory analyses. Critical
reviews by John Cassidy, Brian Atwater, Brian Sherrod, and an anonymous
reviewer have greatly improved the manuscript. We also thank Chris
Goldfinger for providing revised deep-sea turbidite ages.
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Manuscript received 18 November 2009