Paleoseismicity of the Cascadia Subduction Zone: Evidence From Turbidites Off the Oregon-Washington...

8/19/2019 Paleoseismicity of the Cascadia Subduction Zone: Evidence From Turbidites Off the Oregon-Washington Margin

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TECTONICS, VOL. 9, NO. 4, PAGES 569-583, AUGUST 1990

PALEOSEISMICITY OF THE CASCADIA SUBDUC-

TION ZONE: EVIDENCE FROM TURBIDITES OFF

THE OREGON-WASItINGTON MARGIN

John Adams

GeophysicsDivision, Geological Survey of Canada,

Ottawa, Ontario

Abstract. Cores from Cascadia deep-seachannel contain

sequencesof turbidites that can be correlated and dated

by the first occurrence of volcanic glass from the Mount

Mazama eruption 6845 4- 50 radiocarbon rBP). Turbid-

ity currents fi'om the tributaries appear to have occurred

synchronously o form single deposits n the main channel,

there being only 13 turbiditc deposits in the lower main

channel since the Mazama eruption, instead of the twice

as many expected if the tributaries had behaved indepen-

dently. In addition to the Cascadia Channel, 13 post-

Mazama turbidires have been deposited in the Astoria

Canyon and at two sites off Cape Blanco, sample loca-

tions that span 580 km of the Oregon-Washingtonmar-

gin. Pelagic intervals deposited between the turbidites

suggest hat in each place the turbidity currents occurred

fairly regularly, every 590 4- 170 years on average. The

best explanation of the spatial and temporal extent of the

data is that the turbidity currents were triggered by 13

great earthquakeson the Cascadiasubductionzone. The

variability of turbiditc timing is similar to that for great

earthquake cycles. The thickness of the topmost pelagic

layer suggestshe last eventwas300 4- 60 yearsago (from

three placesalong he margin), but this numbermay be

a biased underestimate. It is, however, consistent with

the youngestsudden-subsidencevent on the Washington

coast. The turbiditc data demonstrate that the near-term

hazard of a great earthquake on the Cascadia subduction

zone s of the order of 2 - 10% in the next 50 years.

Copyright 1990

by the American Geophysical Union.

Paper number 89TC03759.

0278-7407/90/89TC-03759510.00.

INTRODUCTION

Earthquakes have long been known to causesubmarine

disturbances sufficient to break submarine cables. The

true nature of these events was not explained until the

classicpaper by Heezenand Ewing [1952] analyzed he

sequentialbreakageof telegraph cables ollowing the 1929

Grand Banks earthquake off Canada's eastern margin

and showed that a submarine debris flow and subsequent

turbidity current had travelled down the Laurentian Fan

at speeds p to 65 km/hr and deposited turbidireon the

deep sea floor. Oceanographers n the 1950s and 1960s

cored many turbidites in all the world's oceans, showed

that they were chiefly associatedwith canyonsand chan-

nels on submarine fans, and speculated on their rates of

occurrence. Very high rates - every fe•v years - off deltas

such as the Magdalena and Congo suggested hat a prime

causeof the turbidity currents was sediment instability due

to rapid sedimentation; the role of infrequent earthquakes

was less easy to assess.

A classicstudy in turbidites and deep sea-sedimentation

was carried out in the late 1960s by a group of students

(Griggs, Nelson,Carlson,and Duncan) at OregonState

University under the supervision of L. D. Kulm. The

most spectacular results came from the Cascadia Channel

off the Oregonand southernWashingtonmargin [Griggs,

1969; Griggs and Kulm, 1970]. Cores rom the Cascadia

Channel contained sequencesof turbidites that could be

correlated and dated by the first (lowest) occurrence f

volcanic glass rom the Mount Mazama eruption. The pre-

dominantly muddy sedimentation in the channel and the

similarity of the coressuggests hat they represent a com-

plete record of turbidity currents in the Holocene. Here,

20 years later, the results are used to address the problem

of great-earthquake hazard in the Pacific Northwest.


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570 Adams: Paleoseismicity f the CascadiaSubductionZone

THE EARTHQUAKE PROBLEM IN THE PACIFIC

N O RTHWES T

About a dozen years ago, perceptions of the nature of

the subduction zone beneath southern British Columbia,

Washington and Oregon began to change [e.g., Riddi-

hough and Hyndman, 1976]. Earlier wor.• had established

that the Juan de Fuca plate was converging oward North

America. However the lack of seismicity on the plate in-

terface lead some to consider that the subduction was oc-

curring extremely slowly or had stopped.

More recent studies of the deformation front at the base

of the slope [e.g., Barnard, 1978], of geodeticdeforma-

tion rates on land [Ando and Balazs, 1979; Reilingerand

Adams, 1982; Savageand Lisowski, 1988], and of on-

shore ectonic deformation uch as warped terraces see

for example,Adams [1984],but also Vest and McCrumb

[1988a,b]and Atwater [1988a])have confirmed hat the

Oregon-Washington margin is being deformed at rates as

rapid as those above other active subduction zones. The

conundrum s this: with suchhigh deformation rates, why

have there been no large thrust earthquakeson the plate

interface?

In a stimulating paper that addressessimilarities be-

tween the Cascadia subduction zone and other zones

worldwide,Heaton and Kanamori [1984]show hat physi-

cal conditions mply that the subduction zone could gen-

erate earthquakesof magnitude 8.3 q- 0.5. Although the

magnitude range they obtain is large, their study strongly

suggests hat infrequent great earthquakes are a distinct

possibility,even though none have occurredhistorically.

Independently of Heaton and Kanamori, I suggested

[Adams,1984;1985] hat the simplestway to reconcilehe

discrepanciesbetween long-term and short-term evidence

for deformation, strain accumulation, and stressdirections

was if there was a great-earthquake cycle of long dura-

tion. I discussedhow an analysis of landslides, andslide-

dammed lakes, drowned trees below sea level, and uplifted

beachescould be used to determine evidence or past great

earthquakes, and mentioned the turbidites in the Cascadia

Channeldescribed y Griggsand Kulm [1970]as provid-

ing a minimum recurrence nterval for such earthquakes.

The present paper analyses he turbidite record further,

and shows hat it providesgood evidence or earthquakes

about every 600 years. Such conclusions are important

for the current debate [Heatonand Hartzell, 1987]on the

level of seismic hazard in the Pacific Northwest, and are

an important complement to the large body of onshore

research nto paleoseismicity hat has occurred since 1986

[e.g.,Atwater, 1987a,b].

SEDIMENT DISPERSAL OFF THE CASCADIA

MARGIN

To understand sedimentation off Oregon and Washing-

ton, it is necessary to understand the sediment sources,

the processes y which sediment s carried to the deep-sea

floor, and the temporal changes hat have affected these

processes. The chief source of sediment is the Columbia

River, with lesser amounts of sediment coming from the

Klamath Mountains in southern Oregon. Much sediment

from the Oregon and Washington coastalrangesand from

Vancouver Island is trapped in coastal inlets, and sedi-

ment from the Fraser River has been trapped in the Strait

of Georgia for the last 8000 years.

At the present time, sediment from the Columbia River

is carried north along the shelf and is depositedon the shelf

and the edgeof the slope Figure 1). Sternberg 1986] e-

views and cites the literature and infers that of the 21 x 106

t yr x carried by the Columbia, 17% is deposited n the

upper slope, most of it in the upper reachesof the Willapa,

Grays, and Quinault canyons, which are tributaries of the

Cascadia Channel, and some in the head of the Astoria

Canyon. The substantial storage n the fluvial system and

on the shelf s able o average ut the short-term10x -

102yr) climatic ariationsn supply.

The sediment accumulates on the slope until submarine

failure occurs and the newly-accumulated sheet of sedi-

ment then sweeps do•vn the channels as a density flow or

turbidity current Figure2). Griggsand Kulm [1970] ug-

gest that each major turbidity current takes about two

days to travel the 735 km length of the Cascadia Channel

and carriesabout 525 1106 ms of sediment n a flow up

to 17 km wide and 100 m high. Low deposition rates in

the middle channel indicate that most of the sediment is

carried as far as the Blanco Fracture zone, where it ponds,

forming individual turbidite layers up to several meters

thick [Griggsand Kulm, 1970].

The present dispersal pattern described above has lasted

only for the past 6000-7000 years. Before about 7000 years

ago, sea level was lower and still rising as water from the

melting ice sheets was added to the oceans. During lower

sea levels, the sediment was not carried north along the

shelf to the same degree, but travelled directly to the deep

sea down the Astoria canyon and channel. Cores on the

middle Astoria Channel show ittle or no turbidite deposi-

tion in the last 7000 years Nelson,1976],althougha core

from the mouth of the Astoria Canyon 6502-PC1)shows

many thin turbidites, presumablygenerated rom the sed-

iment spill-over rom the Columbia shownon Figure 1.

A typical turbidite in the Cascadia Channel system con-

sists of an erosional base, a fine sandy basal layer and

an upper layer of silt and clay that fines upwards. Be-

cause his sediment was derived from the biologically-rich

outer shelf and upper slope, it contains shallow-water shell

fragments and is high in carbon from partially decayed

organic debris [Griggset al., 1969, p. 168]. Once trig-

gered, deposition from the turbidity current takes a few

hours to days. Even the clay fraction must be deposited

quite rapidly becausedeep-water currents would move any

remaining suspendedclays away from the newly-deposited

turbidite. After all deposition has ceased, pelagic or

hemipelagic sedimentation resumes, which is partly bio-

logical- foraminifera and radiolaria tests - together with

small amounts of clay carried in suspension rom the con-

tinent. In contrast to the turbidity currents which deposit

300 - 3000 mm in a matter of days, the pelagic sediment

may accumulate t less han 0.1 mm yr 1, slower arther

away from the continent. Between turbidity currents there

is colonisation of the newly-formed sea-bottom by burrow-


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Adams: Paleoseismicity f the CascadiaSubductionZone 571

126 ø

x Scale

0 50 t00 150kin

I i I • I • I

124øW

I

122 ,=,

IøN

53-18

•.2 ø--

6705

6508-KI

6705-9

6705-5

6609-27

ß 6509-15

509 27 j

24 j

ß -

//

• •/609-1

6502-PC 1

:APE

BLANCO

124 ø

12.2ø

MOUNT

ST. HELENS

I•ORTLAND

• CRATER LAKE/

• MT.MAZAMA

%/////

__.. ORE• .___-

Fig. 1. Map of the Oregon-Washingtonmargin showing the extent of the air-fall Mazama

tephra [Fryxell,1965], he pattern of sediment ispersal orth from the ColumbiaRiver (with

amountsof sediment s percentage f the river input, after Sternberg 1986]), canyons n the

Washington ontinental lope (J, Juan de Fuca, Q, Quinault, G, Grays, W, Willapa, and A,

Astoria), submarine hannelseading o the deep-sealoor, the locationof coresmentioned all

from OregonState Universityexceptcore53-18 which s Universityof Washington), nd (within

large circles) he numberof post-Mazama urbidiresas discussedn the text. The baseof the

continental lope dashed ine) and the 200-m sobath,marking he edgeof the shelf,are also

shown.


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572 Adams: Paleoseismicity of the Cascadia Subduction Zone

Fig. 2. Perspectiveview showingschematically he genesis

of turbidity currents in the Cascadia Channel. Sediment

is carrieddown he ColumbiaRiver (1), drifts north along

the shelf (2), and accumulates t the top of the continen-

tal slope n the headsof deep-sea anyons 3). According

to the hypothesis presented n this paper, a great thrust

earthquakeon the subduction one 4) causes trongshak-

ing of the shelf and slope. The shaking causessediment

liquefaction and slumping simultaneously at many places

along the margin. The resultant massiveunder-sea debris

flows mix with the water to become a series of turbidity

currents ravelling synchronously own the channels 5).

At junctions, the tributary turbidity currents coalesce o

travel down the Cascadia Channel as one large turbidity

current (6).

ing organismsthriving on the high organiccontentof the

turbidite silts) which causebioturbationof the upper part

of the turbidite and of the slowly accumulating pelagic

sediment Griggs t al., 1969].

The turbidites off Oregon and Washington have been

sampled s 2 - 8 m longpistoncores e.g., Figure3) taken

in 2.0 - 2.5 km of water. For the purposesof this study

the detailed core ogs (which describepiston and associ-

ated pilot coresseparately)compiledby studentsand staff

at Oregon State University have been used to determine

the turbidite and pelagic thicknesses hown on Table 1.

Most of the coresstill exist in storageat the university, but

the logs have the advantage hat they were written when

the cores were freshly recovered, and were described by

people with considerableexperience. In almost all cases,

the original colorsand other evidence ecorded n the logs

are sufficient to determine the base and top of each tur-

bidite, the thicknessof the overlying pelagicsediment, and

the amount of bioturbation; key parameters used in the

present analysis.

CORRELATION AND DATING OF THE TURBIDITES

BY TtIE MAZAMA TEPtIRA

Because arge turbidity currents travel down the length

of the Cascadia Channel, each can leave a single deposit,

and turbidite layers along the length of the channelshould

correlate thoughsomemay be missing ocally n parts of

CORE

6609-24

Fig. 3. Sketch of core 6609-24, made from the core log,

showing the even thicknessesof the turbidite and pelagic

(shaded) ayers. Only the top 6.4 m of the core s shown.

The x's in the 13th turbidite show the position of the ear-

liest Mazama glass.

the channelwhere erosiondominates).A uniqueevent n

the last 10,000 years was the eruption of Mount Mazama

at 6845 4- 50 radiocarbon rBP [Bacon, 983].During he

brief eruption bout 34 kma of tephra volcanic sh )

was deposited on Oregon and Washington. Sites on land

show that prevailing westerly winds allowed little if any

of the airfall tephra to fall as far west as the coastal

ranges Figure 1). Much, however, ell in the catchment

of the Columbia River and this loose material was easily

washed into the sea and deposited on the shelf. Experi-

ence with the much smaller Mount St. Helens eruption in

1980 showed that only about 18 months were needed for

the tephra to travel to the headof Quinault Canyon e.g.,

Sternberg,1986],and there is no reason o think that the

Mazama tephra would have behaved differently.

When the shelf-edge deposits slumped, the turbidity

current carried the tephra-rich sediment down into chan-

nels. Cores taken along and near the channelscontain the


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TABLE 1' Pelagic and Turbidire Thicknesses in Selected Oregon-Washington Cores

Turbidire

sequence

number #

6705-2 6705-6 6705-5 6508-K1 6509-15 6509-27 6609-24 6509-26 6502-PC1 6604-12

20 25 ? ? ? 15 ? 15 70 50

1 55 >60 >670 >160 > 20 750 >230 470 250 230

50 80 15 20 15 50 60 25 180 150

2 90 60 660 >240 25 730 >420 630 70 240

10 35 40 25 10 25 70 30 120 160

3 285 40 530 570 40 760 300 620 20 660

20 40 10 50 15 20 55 55 130 60

4 110 55 680 120 50 780 445 640 130 500

35 120 10 35 20 10 55 40 180 40

5 170 180 540 65 50 1070 565 720 130 290

85 80 25 45 10 25 65 60 110 30

6 140 170 180 55 60 1280 455 515 120 160

50 50 30 40 15 25 20 55 130 130

7 110 170 290 60 85 1530 620 720 120 190

40 50 45 25 30 40 40 25 80 240

8 110 75 180 75 80 >1630 550 890 170 350

90 40 40 30 20 30 30 180 60

9 100 80 120 880 95 480 730 150 620

110 40 35 30 10 75 50 210 50

10 180 280 130 900 75 345 >690 170 730

25 45 50 35 15 85 150 130

11 100 230 110 125 130 685 220 370

25 30 60 30 15 10 170 5

12 60 240 130 430 70 340 50 550

160 60 30 10 10 15 140 10

13 50 110 30 640 90* 385* 100' 590*

80 40 30 20 5

14 75 430* 120 750* 70

70 10 5

15 75 690* 75

16 125'

Pelagic (left-hand) and turbidire (right-hand column of each pair) thicknesses (in mm) are given to the nearest 5mm.

# 1 represents the topmost turbidire in each core,

* Lowest post-Mazama turbidire (see text).

the tephra, but coresdistant rom channels o not [Nelson,

1976], confirming hat air-fall tephra did not fall offshore

and that deposition was via river, shelf , and channelled

turbidity current.

By examining the coarse raction at the base of each

turbidite for freshvolcanicglass, t is possible o determine

the first (lowest) turbidite that containsabundant resh

glass. Deeper layerscontainsparse,weatheredglass rom

eruptions older than the Mazama, but the first turbidite

containing Mazama glass s very distinctive, and in some

cores he sandy fraction is 25% glass e.g., in core 6508-

K1).

The first presenceof the redepositedMazam• glass n

the deep-sea urbidites provides ime control with two im-

portant consequences: t allows the first turbidire with

abundant glass to be traced down the Cascadia Channel,

so confirming he nature of the event, and it allowscalcula-

tion of the mean period between urbidity currents. Griggs

and Kulm [1970]used he Mazamaglass o compute ecur-

rence ntervals of 410 - 510 years or the turbidity currents

in the Casc•dia Channel.

In this p•per I invert Griggs and Kulm's approach n

order to discuss irsfly the number of turbidity currents

since the M•zam• eruption, and then the mean recurrence

interval. A typical core is shown n Figure 3. Turbidire

and pelagic thicknesses nd the lowest layer containing

Mazama glass have been determined from the core logs,

in almostall cases sing he describer's oundariesTable

1). In CascadiaChannel cores6509-15 and 6609-24 the

Mazama glass occurs n the 13th turbidite from the top;

in cores6705-6 and 6508-K1 it is in the 14th; for core 6705-

5 there are no marginal notes on the core log indicating

the position of the Mazama glass,but from the recurrence

intervals ivenby Griggsand Kulm [1970,Table3] it is in

the 15th turbidite; and for core 6705-2 it is probably the

16th, but might be the as low as the 19th, as someof the

sedimentary units are poorly defined.

The position of the Mazama glass n the three cores rom

the lowerCascadiaChannelsupportsGriggs' 1969]asser-

tion that not only must the lowest turbidite containing

Mazama glasscorrelate, but so must each of the 12 over-

lying turbidites i.e., the 13 turbidites n the cores epre-

sent the same 13 turbidity currents). Griggs'assertions

further supported by correlatable vertical variations in the

degreeof bioturbation in the top eight layersof cores6509-

27, 6609-24, and 6509-26 [Griggset al., 1969]. Although

the pattern of bioturbation is distinctive in these three

cores and allows their correlation over 65 km of the lower

channel, it is unlikely that similar correlationscould be ex-

tended o the coresn the tributary channelsdiscussede-


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574 Adams: Paleoseismicityof the Cascadia Subduction Zone

low), becausehosecores re moreuniformlybioturbated.

Also, until the reasons or the degreesof bioturbation are

fully understood Griggs et al. [1969]suggested ariable

time between events as one cause, but also noted that no

correlation exists between pelagic thickness and depth of

burrowing- it is not possible to correlate the Cascadia

Channel coreswith those off southern Oregon.

Upstream from the correlated cores, core 6508-K1 con-

tains 14 post-Mazama turbidites (from the core logs;

thoughTable 3 of Griggsand Kulm [1970] mplies13),

and evidently one of the 14 was a small turbidity current

that did not travel as far downstream as core 6509-15.

Downstream from core 6509-26, only 4 of the 13 turbid-

ity currents since the Mazama eruption passed through

the gap in the Blanco Fracture zone and onto the Tufts

Abyssal plain.

Besides he coresdiscussed bove, there are many others

off Oregon and Washington that contain Mazama glass.

These cores contain varying numbers of turbidites over-

lying the earliest occurrenceof Mazama glass, although

none, to my knowledge,has more than 13 overlying tur-

bidites. n the middle CascadiaChannelsomecores e.g.,

core6705-13,Griggs ndKulm [1970]) ave ewor nopost-

Mazama turbidites because he channel s actively eroding

and little sediment is being deposited. Because he tur-

bidity currents must have passed hrough this section to

deposit the 13 turbidites in the lower channel, these cores

are seen to provide an incomplete record. Other cores on

the channel levees are high above the channel floor and

contain 8-10 post-Mazama urbidites (e.g., cores6705-7

and 6705-9). As noted by Griggsand Kulm [1970], hey

represent only the largest turbidity currents, and so may

provide interesting nformation about the size distribution

of the turbidity currents, but only a minimum estimate of

the total number. Away from the CascadiaChannel, there

are many cores hat contain a few post-Mazama turbidites

or someHolocene urbidites e.g., cores hownby Duncan

[1968]and Nelson 1976]). They recordsomeHolocene

turbidity currents not necessarilyhe sameones n differ-

ent places),but add ittle information bout he maximum

numberof turbidites hat may be found n a given egion.

SYNCHRONOUS TURBIDITY CURRENTS FROM

INDIVIDUAL TRIBUTARIES

Core 6508-K1 lies in the Cascadia Channel downstream

of the confluence of two main tributaries, the north-

ern from the Juan de Fuca Canyon and nearby canyons,

and the southern from the Willapa, Grays, and Quinault

canyonsFigure 1). In the northern ributary, core6705-6

has 14 turbidites since the Mazama eruption, the same as

below the confluence. Upstream from core 6705-6, core

6705-2 contains16 post-Mazama urbidites according o

Griggsand Kulm [1970]),and by inference wo small ur-

bidity currents did not flow down the channel as far as core

6705-6. In the Quinault Canyon, core 53-18 contains at

least 14 post-Mazama urbidites Barnard,1978,p. 111].

Downstream, in the main Willapa Canyon, core 6705-5 has

15 turbidites since he Mazama eruption, and the presence

of only 14 post-Mazama turbidites in core 6508-K1 farther

downstream is probably becauseone small turbidity cur-

rent did not flow downstream to the confluence and to core

6508-K1.

The cores n the tributariessuggest hat a few (2 or 3

out of a maximumof 16) small urbidity currentsmay have

been generated that were smaller than the 13 turbidity

currents recorded in the lower channel and did not travel

as far. With this in mind, the remarkable inference can

be made that pairs of turbidity currents were generated

synchronously n the two tributaries. This arises because

there have been 15 post-Mazama turbidites in the Willapa

Channel and 14 turbidites in the northern channel, but

only 14 turbiditesbelow heir confluencewhereup to 29

mightbe expectedf the lowshad occuredndependently).

An alternative, though less plausible explanation, is that

exactly 15 of the turbidity currents, some rom the Willapa

Channel and the rest from the northern channel, died out

before the confluence.This would require a higher rate of

attrition than is evident in either the upper tributaries or

in the lower Cascadia Channel.

Therefore turbidity currents were generated synchron-

ously in two independent channels, the headwaters of

which are a minimum of 50 km and a maximum of 150 km

apart, and each pair of small turbidity currents merged

to form one large turbidity current. That the number in

the main channel is only 13 implies that every turbidity

current in the two tributaries occurred synchronously,.e.

that synchronous occurrence is the rule rather than the

exception. A similar argument can be made for other trib-

utary channels that join the main channel farther down-

stream. Several of these tributaries contain 8- 10 Holocene

turbidites e.g., cores 609-27and 6705-10),and yet the

number of turbidites in the main channel does not increase

beyond the 13 already present. Either all these turbidity

currents predate the Mazama or at least some were syn-

chronouswith the 13 turbidity currents in the main chan-

nel.

How closely in time did the tributary turbidity currents

occur? If the turbidites xvere separated by 50 years or

so, a thin pelagic drape (5 mm, at rates derived ater in

Figure 5) between he pair of eventsmight be expected.

For separations of a decade or so, there need be no per-

ceptible pelagic layer, but some bioturbation in the lower

turbidite could have occured. For separationsof a day or

greater, each turbidite would still have left an individual

fining upwards sequenceso each event would be a doublet:

coarse base + fine top, coarsebase -I- fine top, pelagic

layer. Furthermore these doubletswould persist down the

Cascadia Channel. Examination of core 6508-K1 and the

cores downstream does not reveal any double events, so

I conclude that the tributary turbidity currents occurred

less han a few days, and perhaps ess han hours, apart.

My rough analysis of the time needed for turbidity

currents to travel downstream from the canyon heads

mapped on Figure 1 suggestshat only a few tens of hours

might separate currents rom different, but synchronously-

triggered sources at the canyon heads, so that the trans-

port and deposition event in the main channel would be al-

most continuous.Using realistic estimatesof flow velocity

and sediment concentration, some tens of hours would be


7/15

Adams' Paleoseismicity of the Cascadia Subduction Zone 575

needed to transport the sediment in an average Cascadia

turbidire, which is consistent with xvith the above. Thus

the coalescing lows combine o form the deposit described

as a single urbidire. Similar coalescing f channelled ur-

bidity currents to form a single turbidire deposit has been

documented from the Mediterranean and from the Puerto

Rico Trench [Pilkey, 1988],and has alsobeen nterpreted

to represent he effectsof a synchronousegional trigger.

SPATIAL EXTENT OF THE 13 TURBIDITY CUR-

RENTS SINCE THE MAZAMA ERUPTION

Figure 1 shows three other cores that lie outside the

Cascadia Channel system but contain the Mazama tur-

bidite, one from Astoria Canyon, and two from the Blanco

Valley physiographic rovince [Duncan, 1968] off Cape

Blanco. In all three, the Mazama occurs in the 13th tur-

bidire from the top.

The Astoria Canyon was a major conduit for sediment

up until 7000 yearsago. While Nelson 1976]shows hat

the lower Astoria Fan has been almost inactive since about

the time of the Mazama (presumably ue o the changen

sediment dispersal that accompanied he sea level change

at that time), core6502-PC1sho•vs 3 thin (averagehick-

ness130mm) post-Mazama urbidity currents ccurredn

the upper channel. Apparently there is enoughspill-overof

the Columbia River sedimentcarried north along he shelf

(Figure 1) for small turbidity currents o be generated,

currents that do not travel all the way down the Astoria

Channel. The thin turbidires are interbedded with unusu-

ally thick hemipelagic sediments,again due to closenesso

the Columbia River source.

Both the coreoff Cape Blanco 6604-12)and the oneoff

the RogueRiver (6609-1) are remote rom the Columbia

River mouth, and heavy mineral analysis shows that at

least since the Mazama eruption their sediment has come

from the Klamath Mountains, inland and south of Cape

Blanco Duncan,1968]. Despite heir remotenessrom the

Cascadia and Astoria Channel systems they too contain

13 post-Mazama turbidites.

It is at least 50 km from the Juan de Fuca to the

Quinault Canyon, 100 km across the Quinault-Willapa

canyon system, 30 km more to the Astoria Canyon, 350

km farther to Cape Blanco, and then 50 km to the Rogue.

From all five sites along the margin there have been 13 tur-

bidity currents since the Mazama. I consider the simplest

explanation for the synchronousevents in neighbouring

tributaries, and the same numbers of events at sites that

span 580 km, is a seriesof great earthquakes.

REGULAR RECURRENCE OF TIIE 13 TURBIDITY

CURRENTS

Up to this point the paper has discussedonly the num-

bers of turbidires without regard to their timing. As noted

by Griggs [1969] and Griggs and Kulm [1970], the tur-

bidires in the Cascadia system have generally similar thick-

nesses nd are separated by pelagic intervals also of gen-

erally similar thickness. The left half of Figure 4 plots

turbidire and pelagic hemipelagic lus pelagic) hick-

CORE 6609- 24

mm

loo

5o

Pelogic

mm Turbidite

O

200tBOTTOMTOP

• • I I I I I

8C

E 60

>,,40

ro 20

ß 0

o 80

•60

'F 4O

2O

ß

ß ß ß

ß

OVERLYING

I I I I

[ x

x

x

x

x x

x x

I i i I I I I I i 0

TURBIDITE UNIT 2oo 4.oo 6oo

- x

x

UNDERLYING x x

i I I

Turbidite Thickness(ram)

Fig. 4. Pelagicand turbidite thicknessesn core6609-24. (Left) Plots of unit thickness gainst

sequence umber. (Right) Plots of relationship etweenpelagic hickness nd the thickness f

the overlying top) and underlying bottom) turbidite.

I

800


8/15

576 Adams: Paleoseismicity f the CascadiaSubductionZone

nesses or core 6609-24, in the lower Cascadia Channel.

Both the turbidite (440 q- 130 mm each)and pelagic 55

q- 19 mm) layersare surprisinglyegular n thicknessthe

standard deviation is 29% of the mean for the turbidite

units and 35% of the mean or the pelagicunits). For the

pelagichicknesses,hemeasurementrrorsabout -¬ o

q-« ofeach alue.

Some correlation between the thicknesses of adjacent

pelagic and turbidite units might be expected. For exam-

ple, an unusually ong period betweenevents would give

both a thick pelagicunit (because f the longer ime avail-

able for the pelagic sediment to accumulateon the deep

sea loor) and a thick overlying urbidite unit (because f

the longer time available or the slope sediments o accu-

mulate n the canyonheads). However his positivecorre-

lation is not evident on the upper right diagram of Figure

4. For someother cores e.g., 6604-12) there is an in-

verse relationship between pelagic and overlying turbidite

thicknesses, erhaps suggesting he thicker turbidites were

deposited rom larger turbidity currents hat eroded more

of the underlying pelagic sediments.

The variation of pelagic hicknessesor severalcorescan

be seenconvenientlyby plotting cumulative pelagic hick-

nessagainst urbidite sequence umber (Figure 5). For

this analysis o work, the rate of pelagic sediment accu-

mulation shouldbe steady,and the amountsof pelagicsed-

iment eroded by subsequent urbidity currents should have

been negligibleor constant. The cumulative curvesshow

similar linearity and scatter, supportingpreviousqualita-

tive assertionshat the eventswere fairly regular n time,

and n particular that they did not clusterat the beginning

or end of the sequence.

If turbidites n many coreswere he result of single,spa-

tial extensive events, there should be a correlation of devi-

ations between he core plots, reflectingsystematicdiffer-

ences in the intervals between the events. Correlations are

not obvious,suggestinghe signal/noise atio is low, i.e.,

variability of timing (signal) s not very large relative to

the imprecision n using the measuredpelagic hicknesses

to estimate the intervals.

MEAN INTERVAL BETWEEN THE EVENTS AND

VARIABILITY OF THE MEAN

The above argument suggestshat 13 large eventshave

shaken he margin. In addition to providing a coprelatable

horizon, the Mazama eruption is well-dated by multiple

•4C datesat 6845q- 50 radiocarbonrBP (Bacon 1983],

thoughBrownet al. [1989]suggesthat 6480q- 60 might be

a better estimate),and provides reliabledate to compute

the mean recurrence nterval. The Bacon radiocarbon age

is equivalento 7620 159 alibratedears efore 950 at51

the 2rr confidence evel using version 2.0 of the computer

programof Stuiver and Reimer [1986]). The difference

between the upper and lower errors is small relative to

the stochastic error discussed elow, so adjusting for the

39 years since the 1950 reference date gives 7660 q- 100

calibrated years before 1989.

There is an unknown delay between the eruption and

the deposition of the first turbidite containing Mazama

glass. Because the St. Helens tephra was carried to the

head of Quinault Canyon in 18 months, it is reasonable

to suppose that Mazama tephra travelled similarly fast

and was available for slumping almost immediately after

eruption. When did the next turbidity current occur? For

13 turbidity currents the mean recurrence nterval is about

600 years, so the next one would probably have been 300

q- 300 years ater. Therefore, 7360 q- 400 yearsbefore 1989

is adopted as the age of the 13th turbidite.

One might speculate hat the large volume of tephra

must have caused more rapid accumulation on the shelf

edge than normal, thus possibly reducing the time to the

next sediment ailure (which need not have been seismi-

cally induced). This might be the explanation or the

'14th' turbidite in the upper CascadiaChannel e.g., cores

6705-6 and 6508-K1). Futhermore, f the Mazama erup-

tion were triggered by one great earthquake, he likelihood

of the next would be reduced for about 600 years. Neither

of these extreme adjustments o the age of the first post-

Mazama turbidite can yet be proven.

Since the 13th turbidite there have been 12 turbidites,

12 inter-turbidite intervals and a period of time since the

last turbidite. A priori, the length of the last period is

notknowno s aken s« q- « intervalora totalof 12.5

q- 0.5 intervals. The mean interval between turbidires is

then 7360 q- 400 years 12.5 q- 0.5 intervalsor 590 q- 50

years for comparison,he Broxvn t al. [1989]age or the

Mazama eruptiongives570 q- 60 years). Not used n this

analysis s the information that if the last turbidity current

was earthquake-triggered, t must have occuredmore than

150 years ago, for there has been no great earthquake on

the Cascadia subduction zone in historical times.

An independent check comes from core 6509-27 from

which otal carbon mostlyplant fibers) n the eighth ur-

bidite from the surface was dated at 4645 q- 190 radiocar-

bon yrBP [Griggset al., 1969], or about 5400 q- 500 cali-

brated years before 1989. The carbon had accumulated in

the canyon heads in the •600 years prior to the dated tur-

bidity current,so applyinga correction f-300 years half

the inferred accumulationime) givesan interval of about

680 q- 150 years and by extrapolation beyond the base of

the core, about 11 q- 2 turbidites since the Mazama. This

is in good agreement with sedimentological orrelations

made by Griggset al. [1969] on the basisof the degree

of bioturbation, and the presenceof 13 post-Mazama tur-

bidites in the channel upstream and downstream of the

core.

It is important to realize that the above error on the

mean recurrence interval does not measure the variabil-

ity of turbidity current timing. Such an estimate might

be made directly by examining the thickness variations

of the pelagic intervals. The thicknessesare not simple

to determinebecause 1) the grainsize nd color of the

uppermost turbidite layer and of the pelagic sediment is

similar, and (2) bioturbationof the uppermost urbidite

layer and the pelagic sediment confuses he lower bound-

ary of the pelagic sediment. In addition, the present hick-

nessmay not record he original hickness ecause3) the

pelagic sedimentmay have been partially eroded during

deposition f the overlying urbidite, and (4) bottom cur-


9/15


- ..'. /' : ': ' •///// ' ' :: '

E ....'.'..'?' :ss:'

E ...... ,'.:'..::/. '

....?.•::.'.. .5: ..[O ::i•...

•,•:..',•%•' ' '•':. ;• ' ' :'

.... .'•?',:•:¾'... olo

ANNEL

2X ..:•i?::'

...:..'.. - - :

/ / .....'::'

..:

./, /' ••• •6/o PELAGICCCUMULATIONATE

• mm/EVENTMEAN_17')

'U.BD'rE EeU.

Fig. 5. Cumulative pelagic sediment deposition versus urbidite number for seven cores rom the

Oregon-Washingtonmargin. The plots are lined-up on the turbidite containing he first Mazama

glass star), and penultimateand top pelagic ayersare labelledby P and T respectively.f the

pelagicsedimenthas accumulated teadily,and if the turbidity currentsoccurred egularly,each

plot wouldbe a straight ine. The data (dots oinedby heavy ines)shows goodapproximation

to the expectedelationshiplight ines). Note that the verticalscalehasbeenadjusted y factors

of 2 for three coresso that all the curveshave a similar slope. The slopeof the fitted line represents

the mean pelagicsedimentaccumulation ate, and the deviationsof the data about the line gives

information on the variability of turbidity current timing, here expressed s a percentageof the

mean pelagic accumulationper event.


10/15


rents may have eroded or winnowed the pelagic layer as

it accumulated Harlett and Kulm, 1972]. All of these

factors would tend to reduce the thicknessof the pelagic

layer, but are difficult to quantify unlessgood estimatesof

the pelagic accumulation ate can be applied. The rather

constant thickness of the pelagic layers suggests hat the

losses, f any, are rather constant. In any event, the vari-

ance from the four sources above must be added to the

variance due to irregular event occurrence, so that the

events must have been more regular than is implied by

the variability of the pelagic thicknesses.

For example, the numbers against the plot for each core

on Figure 5 represent he best-estimate mean pelagic accu-

mulation per event and the standard deviation expressed

as a percentageof the mean. Both accumulation ates and

their variability are generally argest near shore, while the

deepercoresare the most regular, 4-26% or 6508-K1 and

4-36% or 6509-15 not shownon Figure 5). If the cores

are considered o represent the same 13 events, those with

the more variable pelagic thicknessesmust include a larger

amount of variance due to measurement errors and ero-

sion, as discussedabove, and so provide poor estimates

of the variability of event timing. Therefore, the average

variability for the three lowestcores let = 28%) is chosen

as representative. For a 590-year recurrence nterval, this

translates o a variability let) of about 170years,and an

implied standard error on the mean of 50 years. There-

fore, if the intervals are normally distributed there is one

chance in two that one of the 12 intervals would lie outside

it 4- 1.7rr, or outside the range 300 to 900 years.

DO THE TURBIDITES REPRESENT GREAT EARTH-

QUAKES, OR NOT?

Although G. Griggsoriginally believed hat earthquakes

were the causeof the turbidity currents L. D. Kulm,

personalcommunication, 988), the lack of large earth-

quakesnearby made this position difficult to support in the

middle-to-late 960s.Griggs nd Kulm [1970] oted hat if

sediment s supplied by the Columbia River at a constant

rate, it would accumulate in the canyon heads until there

is enough o trigger collapse,whereupon he cycle would

start again. Such a self-triggeringsystem would tend to

repeat itself, and would generate similar-sized turbidites

in the channel system; to borrow a term from seismol-

ogy, t would generate the characteristic urbidite in the

channel system.

Of course, riggering by external eventsof a cyclic na-

ture (suchas great earthquakes)would also end to dis-

place similar masses f sedimentand producesimilar-sized

turbidites. Perhaps the strongest argument against the

self-triggeringhypothesis or the Cascadiamargin is that

similar numbers of events are found at widely separated

places along the margin; it would be highly improba-

ble that every canyon had the same temporal response

to the spattally-varying rates of sedimentation hat occur

along the margin. It is simpler to conclude hat, while

self-triggering might occur in the absenceof an external

trigger, the triggers occur more frequently than the time

needed to reach the critical mass in most canyons, and so

usually short-circuit the endogenic lumpingprocess.

For a similar reason, events local to one canyon head

(e.g., a magnitude6.5 earthquakewithin 50 km) do not

provide a sufficient explanation for synchronous urbidity

currents in separated tributaries and the same numbers

of events along the. margin. Therefore the cause must

be exogenic to the canyon heads and affect the Oregon-

Washington margin as a whole. Three such external trig-

gers with the required spatial extent are: tsunamis, wave-

induced slumping during large storms, and great earth-

quakes on the Cascadia subduction zone.

Tsunami. The most recent damaging tsunami on

the Oregon-Washingtoncoast was generated by the 1964

Alaska earthquake, the second argest earthquake of this

century. It did not trigger one of the 13 great turbidites in

the CascadiaChannel, becauseboth the amount of pelagic

sediment n top of the cores seebelow)and the amount

of bottom life on the channel floor during the 1965-1967

cruises e.g., Griggset al., 1969] are too large for sucha

recent turbidity current. Tsunami-generatingearthquakes

are relatively commonaround the Pacific, and if the large

tsunami in 1964 did not trigger a turbidity current, it

seems unlikely that a coupling of multiple independent

tsunami sourceswith the time-dependentstability changes

in the canyon heads would give both long recurrence n-

tervals and the samenumber of eventsalong the margin.

Wave-induced slumping. The case for wave-induced

slumping on such a large scale s poorly documented, nd

computations suggest hat for all but the steepestslopes

wave loading does not influence slope stability in water

depthsgreater han 120 m [Moran and Hurlbut, 1986],

i.e., shallower than most of the canyon heads. Physical

parameters constrain the maximum size of storms and

storm-generatedwavesso that the rate of size ncrease alls

off rapidly with decreasing robability e.g., the 1000-year

wave may be only 30% larger than the 100-yearwave, and

not ten times the size). Therefore he 100- and 1000-year

storms would have rather similar effects, t is likely that

the sedimentswouldneed o be close o failure anyway,and

this would happen at different times for different places.

Like tsunamis, each large storm would trigger some tur-

bidity currents when individual canyon heads were ready,

and so the processwould be unlikely to give both long re-

currence intervals and the same number of events for the

whole margin.

Earthquakes. Earthquakes are an unusual natural phe-

nomenon in that even for quite rare events the expected

ground shaking ncreasesdramatically as the probability

level drops. Thus a great earthquake is so overwhelm-

ingly large that it would trigger both marginally stableand

stable canyon head deposits. Great earthquakes,should

they occur on the Cascadia subduction zone, would proba-

bly have a long return period, and indeedreturn periodsof

several hundred to a thousand years have been estimated

by Adams [1984]and Heaton and Hartzell [1986],based

on the rate of geodeticstrain accumulation. As a first ap-

proximation, plate-boundary earthquakesoccur fairly reg-

ularly (a consequencef the steadybuild-upof strain due

to plate movement and the constant physical parameters

of the fault zone), and so tend to have a characteristic

size. Very short intervals are unlikely because nsufficient

strain is available to be released,and very long intervals


11/15

Adams: Paleoseismicityof the Cascadia Subduction Zone 579

are unlikely because he strength of the fault zone limits

the amount of strain that can be stored. For such char-

acteristic earthquakes there may be a scale-independent

variability n timingamountingo aboutq-•-of the mean

recurrencenterval Nishenko nd Buland, 1987],120years

for a 590-year recurrence nterval. This is close o the 20-

30% variability in pelagic thicknessesound in the cores

(which ncludes ot only variabilitydue to the earthquake

cycle but also that due to variable partial erosionand mea-

surementerrors n determining he thickness f the pelagic

layers).

Evidence that the 13 turbidity currents indeed repre-

sent earthquakes s circumstantial and proof is unlikely for

several years. Nevertheless,becausea great thrust earth-

quake is an extremely large and relatively rare event, it

should cause synchronouseffects throughout the coastal

Pacific Northwest. Such confirmatory effects would in-

clude: sudden coastal subsidenceor uplift, landslides,sed-

iment liquefaction and sand volcanoes, edimentslumping

in large lakes, and submarine debris flows. Other less di-

rect evidence would be: abnormal sedimentation events,

deformed ree growth, abandonmentof Indian settlements,

and secondary aulting on crustal faults. It should also be

noted that while the turbiditc record may well be com-

plete becauseof the muddy nature of the sediments, he in-

ferred earthquake recordmight be incompletebecause ub-

sequent ear-byearthquakesa secondmainshock r large

aftershock) oonafter a great earthquakewould probably

not generate their own turbidity current; all of the unsta-

ble sediment having already slumped.

The first onshore vidence nferred o indicate past great

Cascadiaearthquakeswas reported by Atwater [1987a]

who ascribed buried Holocene marshes in coastal Wash-

ington to sudden coseismicsubsidences f the coast. His

most ecentwork [Atwater, 1988b]suggestsive subsidence

events and one shaking event in the last 3100 years. Cor-

respondingevidencecomesalso from eight buried soils n

about 5000 years from another coastal Washington site

[Hull, 1987], sevenburied marshes n about 3550 years

from northern Oregon Peterson t al., 1988], and eight

buried marshes n about 5000 years rom southernOregon

[Nelson t al., 1989]. At the southern nd of the Cascadia

subduction one,Carver and Burke [1987]deduced ecur-

rence intervals of about 600 years for thrust faults in the

accretionary prism ust north of Cape Mendocino. In each

place the implied recurrence interval is about 500 - 600

years, in good agreement with the mean recurrence nter-

val and spatial extent established or the turbidity currents

(Figure 6a).

B. Atwater [personalcommunication, 988] has also

drawn my attention to the coincidence etween he age of

4290 4- 80 radiocarbon yrBP for the 8th peat at Willapa

Bay (sampleH1-B of Atwater [1988b]) nd the ageof 4645

4- 190 radiocarbon yrBP from the 8th turbidite from core

6509-27 (discussed reviously). Applying the correction

of-300 years to the core date brings the two ages into

remarkableagreement.This agreement s all the more sig-

nificant becauseeach date is on the eighth event - thus

suggestinga one-to-one correspondence etween the tur-

bidite and subsidence vents,and reinforcing he argument

made from the mean recurrence intervals above.

I --- .... II I

•o --- •'• I/

44 ø 44 ø

I I -

•6 o •4 o

400 I

,

I•6 o t•4 o

Fig. 6. Spatial distribution of derived values or (a) Re-

currence intervals for turbidites and onshore subsidence

events,and (b) Age of the last turbidite, submarine lump-

ing, or onshoresubsidence vent. Data taken from sources

discussed in the text.

While I do not claim that there is yet any completely

convincing proof that the 13 turbidity currents do rep-

resent 13 great thrust earthquakes on the Cascadia sub-

duction zone, I believe that it is the best interpretation

available at present.

IMPLICATIONS FOR GREAT TIIRUST EARTH-

QUAKES ON THE CASCADIA SUBDUCTION ZONE

The occurrence of 13 great thrust earthquakes, their

mean recurrence nterval of 590 years, a variability of 170

years, and the assumed580 km extent of triggered turbid-

ity currents and hence nferred strong ground motion that

are deduced above place some constraints on the slip, the

rupture dimensions,magnitude, and style of rupture of the

subductionzone. Rogers 1988]has divided the Cascadia

subduction zone into plausible segments. The Juan de

Fuca plate extends 900 km from the Nootka Fault Zone

off Vancouver Island to a boundary with the South Gorda

plate off northern California. An alternative segmentation

stops the Juan de Fuca plate at the Blanco Fracture zone.

Subduction f the Juan de Fucaplate at 45 mm/yr and

the mean recurrence nterval of 590 yearsgives26 m for the

average slip per earthquake, provided no slip is aseismic.

A 26 m slip for the 750 km long Juan de Fuca plate north

of the Blanco Fracture Zone and a fault width of 100 km

[Rogers, 988]gives maximummomentmagnitudeMw)

of 9.1, which is in accordwith the maximum size proposed

by Rogers [1988]. A variability of 28% in event timing

would imply slip displacements f between 19 and 33 m,

but a variability in magnitude of only 4-0.1 magnitude

unit.

Would a rupture of the Juan de Fuca subduction zone

stop at the Blanco Fracture Zone? Key evidence comes

from core 6604-12, which lies on the extension of the frac-


12/15

580 Adams: Paleoseismicityof the Cascadia Subduction Zone

ture zone. This core - as does core 6609-1 to the south, on

the subducting Gorda plate - contains 13 post-Mazama

turbidires, here presumed o representgreat earthquakes

on the Juan de Fuca subduction zone. If the Gorda plate

had a historyof independent ubductione.g., in Mw 8.3

earthquakes; ogers 1988])core6604-12 s closeenough

that additional turbidity currents might have been gener-

ated by earthquakes to both north and south. Therefore

the presenceof only 13 events n these two coressuggests

that either every Juan de Fuca rupture extends past the

BlancoFractureZone i.e., a Mw 9.2 earthquake), r the

earthquakeson the Gorda segmentare synchronizedwith

thoseon the Juan de Fuca (i.e., zipper effect n which an

earthquake on one segment riggers earthquakeson adja-

cent segmentsn succession few hours o years ater).

In the latter case insufficient sediment would have accu-

mulated between the paired earthquakes for a turbidity

current to have been generatedby each earthquake.

WHEN WAS THE LAST EVENT?

The age of the last event can be estimated roughly

from the thicknessof the topmost pelagic sediment in the

cores relative to the mean accumulation rate. As noted by

Griggsand Kulm [1970,p. 1367] A lack of recognizable

hemipelagicsediment at the surfaceof these coresand the

absenceof extensive burrowing in the surficial layer com-

pared to the extensive reworking in lower layers indicate

that the last flow was recent , although the fact that all

their cores have thinner last turbidires than penultimate

turbidiressuggestshat the very soft pelagicsedimentand

the top, bioturbated part of the most recent turbidire were

often washed away. In addition, their use of recent is in

the context of Holocene time, and might be taken to in-

clude the last 1000 years.

The thickness of the topmost pelagic sediment, from

the available core logs which include Phleger trigger-

weight cores aken with the piston cores,has been used

together with the long-term pelagic accumulation rates

(someshownon Figure 5) to compute he time since he

last turbidire. From six cores Table 2) an age of about

300 4- 60 years before1989) s estimated.This is consis-

tant with the lack of a great earthquake in the 150-year

historical record, but may be a biased underestimate if

some sediment washed out from the top of each core. It

is noteworthy that both the Astoria and the Blanco cores

givesimilar to within the poor esolution) ges o the four

Cascadia ores Figure6b), thereby howinghat they did

not occur at greatly different imes (though this is not

proof that they occurredat the same ime), and perhaps

suggesting regional correlationof the last event.

A date for the last turbidite could also be derived from

the amount of sediment that has accumulated in the sub-

marine canyons since the sediment last slumped away.

In Quinault Canyon piston core 68-18 only 200 mm of

hemipelagic sediment has been deposited since the last

flushingof the canyon Barnard, 1978, p. 111], repre-

senting400 years usingBarnard's ates) or 100 yearsof

sediment ccumulationusing ates of Thorbjarnarson t

al. [1986]). Cores53-19 and 63-08 contain200 mm and

360 mm of post-flushing ediment Barnard, 1978], epre-

senting200-400yearsaccumulationusing atesof Thorb-

jamarson et al. [1986]). While improving he precision f

these estimates clearly requires a critical assessment f the

sedimentation ates at the coresites n question, he above

rates generally support the inference that m300 years or

so have elapsed since the last event.

Tree-ring dates on drowned trees at six sites in coastal

Washington suggest the last synchronous, apid submer-

genceevent was about 300 years ago [Yamaguchi t al.,

1989]. Comparabledates on the youngestburied marsh

soil suggest he last subsidence vent occurred about 300

yearsago n southwestWashington Atwateret al., 1987],

less han 400 4- 200 yearsago n northernOregon Peter-

son et al, 1988], ess han 380 4- 60 yearsago n southern

Oregon [Nelsonet al., 1989], and 270 4- 30 yearsago in

northern California (G. Carver, personalcommunication,

1989); dates hat are consistent ith the preceeding nal-

ysis Figure6b).

TABLE 2. Age of the Last Turbidite From Thickness

of the Topmost Pelagic Sediment

Core Pelagic hickness Accumulationate Ageof Last

(mm) (mm/event) Turbidite years)

6705-2 23 4- 6 62 220 4- 60

6705-6 25 4- 3 55 270 4- 30

6509-26 17 4- 5 41 245 4- 70

6509-27 16 4- 3 28 335 4- 60

6502-PC1 70 4- 20 148 280 4- 80

6604-12 47 4- 10 105 260 4- 60

meanage 2704- 60

aThickness4- possible rror) in 1965/1967.

bFromTable 1 and Figure5 andsimilarplots.

CRelative o 1965/1967; add 25 years o get yearsbefore 1989.

dOr about300 yearsbefore1989. May be biased owardbeing oo

young becauseof washoutsat top of core; see text.


13/15

Adams: Paleoseismicity of the Cascadia Subduction Zone 581

TABLE 3. Age of the Last Five Events in Core 6508-K1

Pelagic Turbidite At a Age

(lrtm) (mm) (years)

Date

? 300+60 c

160+ 3004-60 A.D. 16904-60

224.3 3904.70

240 6904-130 A.D. 13004-130

284-3 5004-70

570 11904-200 A.D. 8004-200

554-12 9804-250

120 21704-450 1804-450 B.C.

344-3 6104-70

65 27804-520 7904-520 B.C.

aTime interval represented t 334-1 mm/590-year event

assuming no loss of sediment.

bCalendar earsbefore1989.

See Table 2.

Working from the pelagic thicknesses n core 6508-K1

(chosenbecause ts pelagic hicknessesre the most reg-

ular), assuminghat no sedimenthas beenerodedby the

overlying urbidite, and adopting 300 years or the age of

the last event, the followingdates for the last five earth-

quakes re deduced Table 3): 1690 AD, 1300AD, 800

AD, 180 BC, and 790 BC. Although the slow accumu-

lation rates of the pelagic sediment and the disturbance

by the corer mean these dates are nos very precise and

in particular, the errors compound for the older dates as

shown n Table 3), they shouldproveof interest o other

researchers eeking to match their onshoreevents to the

turbidite record.

The analysis presented here underlines the need to col-

lect new cores from the Cascadia Channnel and the re-

mainder of the margin. A profile of closely-spaced ores

across the channel, somewhere around core 6508-K1 on

Figure I would reveal much about the history of the tur-

bidity currents. Griggs and Kulm [1970] obtained sev-

eral cores from the levees of Cascadia Channel banks that

contained only 8 post-Mazama turbidites; presumably the

largest ones. At some point between the levees and the

channel floor, there should be a complete section of the

13 turbidites, from which each subsequent urbidity cur-

rent has eroded ittle or none of the pelagic sediment. Such

coreswould be invaluable for establishing he relative sizes

and timing of the 13 turbidity currents. It might also

test whether the amount of bioturbation is an indicator of

elapsed time or relates to the size of the previous turbidity

current. In addition, radiocarbon dating of detrital car-

bon or even of individual pelagic foraminifera in the cores

would place independent ageson the events and avoid the

compounding errors intrinsic in an analysis like that in

Table 3. In the upper channel and on the slump scars

of the canyon heads, box cores, or other non-disturbing

sampling methods, should be used to determine the thick-

ness of the topmost sediment in order to establish the time

since the last event.

PROBABILITY OF THE NEXT EVENT AND SEISMIC

HAZARD IMPLICATIONS

From the mean interval between events (590 years),

the standard deviation of the mean (•170 years), the

time since the last event (•300 years), and a normal-

distribution model for simple recurrent ruptures without

clustering, it is possible to estimate the likelihood of the

next great Cascadiaearthquake Figure 7). At present

there is about a 5% chance that the next earthquake

should have already happened. For the future, the con-

ditional probabilities are crudely 0.1% in the next year,

5% in the next 50 years, 10% in the next 100 years, and

25% in the next 200 years.

These valuesare probably good to only a factor of 2 and

100-

80-

60-

40-

:• 2o-

o

o

•2-10

5O%

i

•1 590ñ170 r

a l /-/..• o

I

I

•o 4•o •o s•o •6oo'

TIME SINCE PREVIOUS GREAT CASCADIA EVENT(yr)

Fig. 7. Graph showing he cumulative normal probability

distribution for a mean of 590 years and a standard de-

viation of 170 years as inferred for great earthquakes on

the Cascadia subduction zone, an estimate of where the

present ies relative o the last event (thick bar on the ab-

scissaat 300 4- 60 years), and the range of conditional

probabilities of the next event for the next 50, 100, and

200 years.


14/15


need to be refined using a revised date for the last earth-

quake and perhapsa lognormaldistributionmodel [fol-

lowingNishenko nd Buland, 1987];however omeof the

uncertainty arises rom the possibility that the eventsclus-

ter in time (say hreeeventsn onemillenium ollowed y a

long quiescence)nd from the still-unknownmodeof fail-

ure of the subduction zone. While the same number of tur-

bidites in the Cascadia Channel, Astoria Channel, and the

sites off Cape Blanco strongly suggests ynchronous ur-

bidity currents, t is not yet possible o rule out a zipper ef-

fect whereby smaller earthquakes rupture the plate bound-

ary in a short-term sequence. Such multiple modes of

rupture -sometimes single great earthquakes,sometimes

sequencesof smaller earthquakes - would complicate the

palcoseismichistory and are probably beyond the resolv-

ing powerof the turbiditc record, houghcouldbe resolved

through onshore nvestigationsusing dendrochronologyor

other precisedating of earthquake effects.

Even if the rupture mode is complex, the net effect on

hazard estimates may not be great because circumstan-

tial evidence uggestshat the (hypothetical) uptureseg-

ments would need to 'stay in step' and the time since the

last event is already close to half the mean recurrence in-

terval. In addition, the damage implications for any place

along the margin are not greatly different for the single

M9 or multiple M8 earthquakescenarios; hough clearly

should one M8 earthquake happen somewhereon the sub-

duction zone the likelihood of others •vould be increased

becauseof the temporal clustering mplied by the alternate

hypothesis.

CONCLUSIONS

Turbidites in the tributaries of the Cascadia Channel

and at other places along the Oregon-Washingtonmar-

gin provide circumstantial evidence for the occurrence of

13 great Cascadia subduction zone earthquakessince the

Mazama eruption. My analysissuggestshat magnitude

•9 earthquakesoccurredevery 590 years on average.The

pelagic ntervalsdepositedbetween he turbidites suggest

that the earthquakesoccurred airly regularly, with a stan-

dard deviation of 170 years or lesson the recurrence nter-

val, similar to the variability found for great earthquake

cycles elsewhere.

Rhythmic triggering of turbidity currents by great

earthquakes may be a much more common phenomena

than hitherto realised, and might be expected at conti-

nental margins such as Alaska, Japan, New Zealand, and

Chile where great thrust earthquakeswith a long return

period are combined with a moderate supply of sediment

to the edge of the shelf. If sampled correctly, the turbiditc

record can provide a quick estimate of the palcoseismic-

ity of a margin and so provide evidence ndependent of

onshore palcoseismicitystudies.

The thickness f the topmostpelagic ayer suggestshe

last earthquakewas 300 4- 60 years ago, but this number

may be a biased underestimatedue to washout at the top

of the core during the collectingprocess. t is, however,

consistent with the youngest subsidence episode on the

southwest Washington coast. The pelagic accumulation

between the turbidites has the potential to determine the

timing of previous eventsby working backwards;however

in the absenceof absolute dates on the turbiditc layers in

the cores like those available rom the submerged eats

onshore), he errors compoundand suchdatesshouldbe

used with caution. The current state of great earthquake

expectationor the PacificNorthwest Figure7) demon-

strates that the near-term hazard of a great earthquake is

appreciable.

Material capable of precisely dating the past earth-

quakes has already begun to be studied at onshore sites

where coseismic deformation and tsunamis can be dated

by radiocarbon or dendrochronology.With turbidites to

provide an independent record of shaking, the concordance

of diverse data may ultimately reveal the history of great

thrust earthquakes n the Pacific Northwest.

Acknowledgments.This work would not have been pos-

sible without the meticulouswork of G. B. Griggs, his con-

temporaries, and their supervisor La Verne Kulm. Their

work, which I. have reinterpreted in this paper, is proof

that good sciencehas ramificationswell beyond the aspi-

rations of the individual scientist. I thank Mitch Lyle and

the College of Oceanography or their cooperationand for

making the core logs available. I apologize o my col-

leagueswho have heard me present this work at the 1984

Chapman Conference on Vertical Crustal Motion and at

subsequentmeetings,who have only now a written version

to examine critically, but who have nevertheless arefully

cited and attributed my work. D. Piper assistedwith tech-

nical advice regarding the behaviour of turbidity currents

and B. Atwater pointed out the importance of the bias in

the uncalibratedradiocarbonages. thank P. W. Basham,

M. J. Berry, D. J. W. Piper, and G. C. Rogers or valu-

able critical commentson early versionsof this manuscript,

and B. Atwater and L. D. Kulm for constructive reviews.

GeologicalSurvey of Canada contribution number 33288.

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(ReceivedSeptember 0, 1988;

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