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CrustalUplift in the SouthcentralAlaskaSubductionZone:A NewAnalysisandInterpretationof TideGaugeObservations StevenC. CohenI and Popular

measures

sea level height seismically

the ocean

can be of either

active

tectonic

at appropriately

have examined

height crustal

relative

located

the tide gauge

the long term tidal gauge

indicates

provide

at the seven

rates

origin.

are often

tide gauges records

to a land datum;

or oceanic

plate boundaries

by the massive (magnitude 9) 1964 Prince records to determine the rate of postseismic pared

T. Freymueller

2

Summary:

A tide gauge

ments

Jeffrey

therefore,

Because

more rapid

insight

changes

the rates

in apparent

of crustal

uplift

than sea level changes,

into crustal

at

measure-

deformation

processes.

Alaska

that were affected

sites in south-central

We

William Sound, AK earthquake. We have used these (after the earthquake) crustal uplift and have com-

with more

recent

GPS rates

at nearby

sites. Our analysis

the following:

(1) The ongoing

the coseismic

(earth-

quake) motion in the sites that subsided during the earthquake are now moving visa-versa. In detail, however, the correlation between the amount of coseismic

up and motion

and the current (2) At Kodiak earthquake. at Valdez uous.

pattern

there

There

mirrors

is a distinct

decrease

to some

extent

in the rate of crustal

is also a mathematically

and Cordova,

rapid,

motion

rate of uplift is poor.

(3) On the western is quite

of vertical

although

the behavior

side of the Kenai

-1 cm/yr.

Such

significant

change

at Valdez,

Peninsula,

uplift

in particular,

i.e. at Seldovia

a rapid uplift cannot

with time since the

in the rate of vertical

be maintained

is somewhat

and Nikiski

motion ambig-

the uplift

over the several

rate

hundred

year recurrence interval between great earthquake; however, it is consistent with the "anomalous" southeast motion of GPS sites on the western side of the Kenai Peninsula that we have previously reported. (4) Vertical motion at some other boundary. Peninsula, strain

In particular, is consistent

accumulation

I Geodynamics -Geophyslcal

Branch, Institute,

sites is consistent

with strain

accumulation

at a plate

the vertical motion at Seward, on the eastern side of the Kenai with our interpretation of the horizontal GPS velocities in terms

due to steady-state

NASA's University

Goddard

plate convergence.

Space

of Alaska,

Flight

Fairbanks

Center

of

Created March, 1999. Current version November 3, 1999 2:20 pm

Crustal

Uplift

in the Southcentral

New Analysis

1Geodynamics

(phone:

Branch,

C. Cohen

Goddard

301-614-6466;

2Geophysical

Alaska

and Interpretation Steven

Space

Institute,

University

of Alaska,

T. Freymueller

Center,

FAX: 301-614-6522;

Subduction

of Tide Gauge

1 and Jeffrey

Flight

l

Greenbelt,

email:

scohen

Fairbanks,

Zone: A

Observations 2

MD 20771

@carnoustie.gsfc.nasa.gov)

AK 99709

Abstract

We have examined

the sea level height

southcentral

Alaska

the history

of crustal

behavior

that were affected

depending

uplift

ern side of the Kenai lation

while

uplift

of about

quake,

uplift,

at Seldovia 1 cm/yr

but which

of several

hundred

which

Peninsula

on the western

likely

be sustained Further

is still several

mm/yr,

represents

William There

earthquake

is considerable

side of the Kenai a long term transient

to the southwest,

sites in the region

Sound

that is consistent

over the expected

has slowed

tide gauge

to the 1964 rupture.

we find a slow uplift

that most

years.

to the earthquake.

of the site relative

and Nikiski

cannot

at seven

by the 1964 Prince

subsequent

on the location

tide records

recurrence

at Kodiak,

significantly

At Seward,

with elastic

response

interval

strain

for a great

we find evidence

accumu-

over the past three

earthquake

and a half decades.

and an uncertain

Valdez.

sites there

is a mathematically

significant

time-dependence

are not as convincing

rapid

that the rate of

at Cordova

this time dependence

on the east-

to the earth-

we find subsidence

but the data confirming

in the

we find a persistent

Peninsula

behavior,

to determine

variation

To the east of the Kenai At both of these

of

behavior

at

to the uplift

as at Kodiak.

At

Created March,1999. Current version November 3,19992:20pm

2

Anchorage,to the north thereis little evidenceof verticalmotionsincethe earthquake.Wecomparetheselong termtide gaugerecordsto recentGPSobservations.In generalthereis reasonable consistencyexceptat AnchorageandCordovawheretheGPSmeasurementindicatesomewhat morerapid uplift andsubsidence,respectively.

Created Ma,'ch, 1999. Current version November 3, 1999 2:20 [_m

3

Introduction

Among

the several

W_liam record

geodetic

data sets that have

Sound,

AK earthquake

of ongoing

postseismic

first researchers

to exploit

to 10 yrs long collected Seward,

Kodiak

uplift

occurred

postseismic

(M w - 9.2), tide gauge and interseismic

these

(St. Paul Harbor),

of postseismic

motion

partially

during

the earthquake

showed

uplift

the magnitude The

uplift

rate of 88 + 38 mm/yr

Plafker tions

be sustained

[1991],

herein

and others

slower although between

(sometimes

rates

(Figure

referred

and visa-versa.

to as SP91,

from

the entire

a decade

derived

make

In detail,

(through motion

than

in the uplift rates

since

only the post-1973 argument the SP91

1988).

that

The spatial

the correlation uplift

except

rate was

with a maximum

at Cordova, Savage

speeds and

at these

In general

did Brown

loca-

they found

et al. [1977],

to distinguish

to 1973 (i.e. -9 years

data were

Having

showed

observations

for re-examining

analysis.

up

at Anchorage,

however,

of years.

data set and from data subsequent using

the

records

analysis

10 mm/yr,

the tide gauge

of vertical

analyzed

of postseisrnic

of hundreds

duration

variation

a compelling

has passed

cycle

were

in that sites that subsided

of - 12 + 2 mrn/yr

re-examined

rates

any temporal

The rates

At least two factors

slower)

They

at Cordova.

motion

at all sites exceeded

earthquake

et al. [1977]

1). Their

and the magnitude

the most continuous

sites located

occurred

the coseismic

and a minimum

over the entire

considerably

derived

at this time. First,

at Kodiak

rates.

and Cordova

motion rates

Brown

tide gauge

thereafter

of the inferred

they did not report

movement. uplift

of the 1964 Prince

provide

at permanent

with a data set that was of longer

after the earthquake). slower.

value

observations

postseismic

mirrored

of the coseismic

weak.

that cannot

absolute

in the aftermath

at the first 4 sites, but subsidence

pattern

between

crustal

data to extract

after the earthquake

Seldovia,

been studied

found

to be slightly

the tide gauge data spanning

record

another

ten

Created March, 1999. Current version November 3, 1999 2:20 pm

years

is important

deduced

because

from annual

long fluctuations

due to non-tectonic

is now sufficiently

period

and (at somesites)

Data

robust

for comparing

to determine

sites located

to both year-to-year

As the data presented both average

variations.

Second,

near to the tide gauge

locales

short term to long term

vertical

monthly

mean

in south-central

web. These

sea level Alaska

sites are: Anchorage,

Paul Harbor

and Women's

Bay).

and Juneau)

below.

data were supplemented

The

data we formed

annual

following

the procedure

outlined

effects

at the individual

east Alaska

locales,

ence between

and several-yearindicates,

for the elapsed

contemporary

GPS

and provide

and the rates

in this paper

uplift rates

the

postseismic

data are becoming

a complementary

set of obser-

movements.

based

a region

our procedure

to correct

means

well removed

means, values.

to minimize

Seward,

for three

southeast

between

the residuals

residuals

from

corrects

whereas Since

some

the possibility

From

any annual

at the three

that the analysis

means

means

southdiffer-

that were

in SP91

oscillation would

and

tide gauge

The only significant

annual

these

sea level change

to the detrended

of the annual

there is a strong

sites (Sitka,

oceanographic

rates

(St.

as discussed

of apparent

for local

zone.

is that we rejected

Alaska

tide gauge.

from detrended

1964 rupture

and Kodiak

effects

rates

.tide gauge

over the world-wide

Nikiski,

for the Seward we derived

permanent

available

for local oceanographic

The procedure

and that of SP91

from less than six monthly

records

records

and the average

derived

Valdez,

from which

in SP91.

for the seven

data archives

Cordova,

by paper

from less than nine monthly

we sought

the NOAA

on the coherence

stations

determinations

We also extracted

derived

readings,

from

that are used

monthly

atmospheric

height

Seldovia,

Ketchikan,

gauge

occurred

Analysis

We extracted

rates

since the earthquake

are subject

processes.

temporal

at sites located

vations

has passed

sea level determinations

record

available

only 35 years

4

were

in the tide

be biased

by sea-

Created March,1999. Current version November 3,19993:25pm

5

sonaleffectsevenif it meantsomedecimatingthedatasetsomewhat.Nevertheless,the apparent sealevel rateswe obtainedby analyzingthe datathrough1988closelyreproducedthe rates reportedin SP91.

We estimated

eustatic

sea level change

change,

whereas,

mm/yr

is somewhat

SP91

uplift rates

and postglacial added

arbitrary

seven

are summarized

Constant Figure

2 shows

the uncorrected corrected

and corrected

observations.

is given

Rate

the apparent

in Table

is 85, 80, 42, 47 and 82 percent but nearly seven

zero at Kodiak

locales.

1998),

suggesting

correlated apparent

that the southeast

at Kodiak

Valdez,

to the apparent

are several

Valdez

the data set is too sparse)

the corresponding

than linear)

uncertainty

the value

of 2

uncertainty

in

at each of the

the residuals

correlated

shape

at Kodiak,

sea level correction

Anchorage,

Valdez,

respectively, fit at each

(e.g. between

of the

1996 and

in removing

all

are the systematic

residuals

of these residuals

suggests

a least squares

regression

in time for each site except

and found

on the

rate of sea level change

is not fully successful

The

both

regression

to the linear

residuals

great importance

polynomial

a linear

Alaska

Seldovia,

sea level rate. We performed

Anchorage above

Seward,

and Cordova.

(rather

from

by the southeast

sea level correction

data using a quadratic (for which

any additional

line, i.e. the apparent

3 shows

Of potentially

and possibly

Although

of the data record

and the line derived

achieved

Figure

that there

sea level fluctuations.

time-dependence

reduction at Cordova,

It is noteworthy

rate of sea level

1998 for all 7 sites with each plot showing

of each regression

and Nikiski.

for

Analysis

sea level heights

2. The variance

aspects

(to account

1.

sea level through

The slope

for the same effects.

The site-specific

2 mm/yr

of the apparent

level, we did not attribute

in Table

and Time-Dependent

to the negative

to account

at the mm/yr

due to this choice.

tide gauges

rebound)

2.5 mm/yr

our rate estimates

at each of the sites, by adding

that the rate change

and Cordova,

Nikiski

a

on the and

was formally

but was indistinguishable

Credited March,1999. Current version November 3,t9992:20pm

6

from zeroat SewardandSeldovia.The apparentsealevel ratesderivedfrom thequadraticregressionsarealsosummarizedon Table2. In Figure4 we comparethe linearandquadraticfits to the data.Sincetheerrorsin the tide gaugeobservationsandmonthly andannualmeansarenot known,we estimatedrateandratechangeerrorsfrom the misfit of theregressioncurveto the data,asoutlinedin Press

et al. [1986].

may underestimate

the true error.

subject

of oceanographic

to a variety

addition

it is well known

yet fully understood. yr even

when

Thus

are likely

robustness

in the tide gauge

There there

this question

questions

provided

to a variety

Conversely,

in several

observations

of error,

are also

which

to rate differences differences

data sets spanning

this consistency

in this manor

in the data processing.

the data set is sufficiently

requires

a more

results

with other

motions

in terms

Apparent

sea level change studies

mation

following

leveling

surveys

there

we wish to discuss

in the rate of crustal

by taking

observed

In previous

tide gauge

significance error.

obtained

robust.

a decade

In

are not

of a few mm/ exceeding

sev-

Achieving

such

or longer

whereas

years.

Interpretation

any change

the tide gauge

data is subject

the reported

observations

can achieve

are two important

has been

exceed

from

that are not fully removed

little physical

to be significant

the GPS measurements and

noises

we ascribe

these differences

that error estimates

The rate determinations

that the GPS vertical

eral mm/yr

Discussion

It is well known

critical

data. The second

of the underlying

tectonic

uplift

has been evidence

the 1964 earthquake. and concluded

Brown

that there

The

first is the whether

since the 1964 earthquake.

look at the tide gauge

geodetic

and crustal

uplift

in this section.

data itself question

processes

We will examine

and then we will compare

is how to interpret

the

of the region.

rates both for and against et al. [1977]

time-dependent

and Cohen

was a time dependent

decay

[1998] over

crustal examined

a period

deforseveral

of several

Crealed March, 1999. Current version November 3, 1999 2:20 pm

years

in the rate of crustal

papers

[Cohen

cumulative process

postseismic

as well.

uplift

However,

used than when larger

recently Kenai

might

1998)

the results

recurrence

interval

edge. Where

of the analysis

Since

be operating.

we have

argued

rates

when

postseismic

were

et al., [1999])

of the western

to much

prohave

portions

of the

slip on deep portions

and not subject

was

that a relaxation

We [Freymueller

postseismic

uplift

all the data

of the rates they observed

motion

of

that the

change;-i_n the postseismic

they speculated

horizontal

In a series

with a transient

slower

some

averages,

that is unlocked

in Freymueller

moment

rate of 9.7 _+0.5 ram/yr.

years;

hundred

following

regression

+ c (At) -, where

the SP91 analysis

cannot

gives

At = 1998.5

(data

analysis

The nearly be sustained

thus it is not entirely analysis

shows

a rate change,

- t, of2c

the site located

to other

they are from

et al. [1999]

from the linear

of several

at Cordova,

and progressing

we cite GPS results,

change

The quadratic

sites starting

rupture,

of sea level height

data for 10 years

h = a + bAt

for some

or delayed

at the individual

to the SP91-determined estimate

change.

be due to continuing

1997]

is consistent

1988 produced

trenchward

the rate of sea level change

the earlier

porating

of 102 might

a region

and Freymueller,

evidence

through

end of the 1964 coseismic

At Cordova

pares

some

the earthquake.

of the release

earthquakes.

from the rupture

data from

Cohen

and interseismic

that lie below

We will discuss

tance

data analyzed

Arm following

the late 1990s

found

that the unexpected

Peninsula

the downdip

SP91

on a time scale

argued

great

1996,

data prior to 1973 was excluded.

plate boundary during

Turnagain

through

than the preseismic

cess occurring

along

et al., 1995, Cohen,

rate as their tide-gauge

much

uplift

7

sites at increasing

an update

through

1 cm/yr

of

1997). This com-

subsidence

over the entire surprising

to

dis-

(inclusion

is 6.7 _+0.4 mrn/yr.

a deceleration

closest

implied

by

earthquake

that our data incorin the sea level height

2c, in the sea height

equation,

= 0.39 + 0.06 mm/yr 2. However,

an

Created March, 1999. Current version November 3, 1999 2:20 pm

examination

of the height

dependent

signal

skeptical

comes

as to whether

ically

real. Nevertheless,

uplift

rates

height

shown

since

residuals passes

residuals

rate solution

from the early

and later epochs

the mathematically

significant

we must

in Figure

the F test [Mikhail,

that there

We applied

with the quadratic

curve 1976]

is meaningful.

at the i% level

(SSR 1 - SSR2)

SSR is the sum of the squared

number

of parameters.

The subsidence

the average quadratic

rate from

analysis

surveys.

that can increase

the actual

whether

in apparent

sea level

the improvement

in the

The improvement that the

et al., 1995]

(1)

/ (DF 2)

provide

than the nearly

a weak

GPS vertical

of data points

argument

which

against

is in better

measurements

well beyond

the formal

error

11 years

after the Prince

the

a rate change. with

predicted

bars because are subject

minus

agreement

zero rate of 1.8 + 1.0 mm/yr

rate has large error

uncertainty

rate is phys-

of the corrected

significant.

and DF is the number

The GPS-derived

In addition,

evaluation

that the 99% probability

the GPS data is - 13.2 _+7.1 mm/yr

for 1996.5.

we are somewhat

/ (DF 1 - DF2)

GPS observations

rate of - 4.7 + 0.4 mm/yr

only a few recent errors

Recent

residuals

of the time

in the uplift

little if any change

indicating

is Zhao

Thus

variation

case is statistically

( SSR2)

where

temporal

has been

over the linear

that most

of the record.

the F test to determine

The F statistic

F --

shows

point out that even a qualitative

2 shows

the late 1980's.

improvement

to the constant

8

by the

it is based to a variety

estimates

on of

[Mao

et al.,

1999] At Valdez

the data record

quake.

The rate of apparent

cantly

different

mathematically

dates

back

to 1975,

sea level change

since

from zero,

but is less than the SP91

significant

change

that time,

William

- 0.2 + 0.7 mm/yr,

rate of 5.5 + 0.9 ram/yr.

Sound

Earth-

is not signifi-

There

is a

in the sea level rate of 0.77 + 0.14 mm/yr 2 that produces

a

Created March, 1999. Current version November 3, 1999 2:20 pm

rapid

transition

from crustal

subsidence

the F test for an improvement rate in 1996.5 3 again

result

feature

in many

found

particular

away, do marginally

about

Our qualitative

in view of the residual

confirm

the recent

and yields

ascribing

of the plots However,

The quadratic

an estimate

assessment

a great offset

between

albeit

of the residuals

passes uplift

in Figure

significance

to the

1995 and 1996 is a correlated

that come

with a large

solution

for the crustal

deal of physical

GPS measurements

uplift

sea height

from a site about

uncertainty.

The

5 km

GPS-derived

rate is 8.7 _+8.2 ram/yr.

At Seward

nificantly

the apparent

different

cent TI9, installed

At Kodiak

appears

lines of argument

height

residuals

i.e., it appears

9.5 + 2.7 mm/yr

located

shows

by SP91

from measurements

from measurements

change

the entire

variation domain

that the improvement

the rate that we derived from the data through

from

from the linear

is more

since

begun Marine

analysis

in the rate of - 0.66

over the linear

the data through

1998

First,

less

1993 at the adja-

in 1995 at the newly Facility

in Seward.

an examination

than in the previous solution

fit is statistical

is slightly

The crustal

sea level (for 1996.5,

is slightly

+ 0.11 mm/yr 2. Several

is real.

extensive

is insig-

is - 14.5 + 0.7 mm/yr;

of the data set. The quadratic

1988, - 17.5 + 0.8 mm/yr.

in apparent

of Alaska

that the time dependence

that the temporal

change

obtained

and the time dependence

uplift rate of 3.2 + 0.7 mm/yr

obtained

sea level change

also suggest

to span almost

the time-dependent

crustal

at the pier of the University

to be a significant

test with a 99% probability thermore,

The implied

the rate of apparent

there

is - 1.2 _+0.7 mm/yr

one, 7.6 + 3.5 mm/yr,

and the other, site UAMF,

however,

sea level change

from zero.

than two GPS results,

other

skeptical

uplift.

solution

9.2 _+ 1.6 ram/yr.

us somewhat

mathematical

uplift

over the linear

that is a rapid

makes

to crustal

9

see Table

lower

uplift

of the cases,

passes

significant.

the F Fur-

than that derived

rate we predict

2) is 7.4 mm/y

which

from is

Created March,1999. Current version November 3,19992:20pm

10

goodagreementwith the GPSobservedvalueof 9.2 _ 1.6 mm/yr.Thesevaluesare lessthanthe 14.8_+7.2 mm/yr observedby VeryLong BaselineInterferometry(VLBI) in the late 1980'sand 1990[Ryan,et for the period

al., 1993], (1985-1998)

- 8.1 _+ 1.1 mm/yr, 1981 when

between

rate change change

uplift.

estimated

in Cohen

and elsewhere

rate in about

[1998]

rate of - 7.2 _ 1.4 mm/yr.

between rently

installing

crustal

12 years.

the somewhat

confirms

a "permanent"

Bay is

was associated there

of sites along

to suggest

change

in the speed

faster

decreased

Turnagain

Arm,

the eastern

in good

agreement

GPS site at Seldovia.

the GPS uplift

side of the Kenai comparison

Kenai,

sites.

with SP91's

0.16 + 0.18 mrrdyr 2, is not significant.

a quantitative

of

than the 3-6 year decay

is - 9.3 _+0.8 mnfyr

uplift on the western

rate

to half its

of the various

rapid

that the

of

rate there

Although

in

check

the observed

settings

25 km to the northeast.

that the

with the change

tectonic

term,

1967 to

was a consistency

evidence

physical

This is somewhat

from

to the suggestion

In fact, we consider

that the uplift

about

is to far to make

credence

rate at Kodiak

different

The sea level rate

for the period

no substantive

for an actual uplift

we estimate

The rate change

GPS site is in Homer,

the two locales

candidate

at Seldovia

derived

1 indicate,

of the tide gauge.

for an amalgamation

and may reflect

of 18.7 ± 2.4 mm/yr

likely

- t) mm/yr,

The rate of sea level change

nearest

in Table

significant.

at Women's

that the rate change

of the relocation

the time dependent

postseismic

located

This adds further

as the comments

to be the most

From

has been

at the two sites so we have

r = 6.1 + 0.66 ( 1998.5 immediate

the possibility

However,

is an artifact

at Kodiak

crustal

was at St. Paul Harbor.

the observations

is not statistically

than the rate of - 21.1 ± 1.4 mm/yr

but raises

location.

the difference

that the tide gauge

slower

the gauge

rate has changed, observing

although

Peninsula,

of the two rates.

There

rate at Homer the distance We are cur-

Created March,1999. Current version November 3,19992:20pm At Anchorageweobtaineda sealevel rateof 0.8 + with the SP91

result

at Anchorage.

Our analysis

incomplete.

However,

data set goes back 1980's.

southwest

uplift

for the next nearest

suggested

rate of 12.4 + 5 ram/yr.

The apparent

sea level rate at Nikiski

The GPS crustal

uplift

located

was derived

Given

obtained

set of monthly

heights

indicate

about

again

by observations

movement

records

are quite

tidal heights.

This

and early

little

vertical

located

only 8 km to the

crustal

uplift

be dismissed

10 km southeast

from mean

consistent

crustal

suggesting

a rapid

of

as a single

site

of the tide gauge,

tide heights

rate of - 18.7 _+1.7 mm/yr

rate has been determined

is quite

the early mean

the fact that no data was collected

a higher

which

for most of the 1970's

by the GPS data cannot

GPS site, 1000,

and is - 9.9 + 0.8 ram/yr.

in 1984 since

at site VAN DUSEN

The GPS measurements

The rapid

1997, the fact that SP91

robust

yearly

GPS result

an uplift

level

more

a result

little, if any vertical

a rate of - 0.7 + 0.9 mm/yr,

is the recent

of the tide gauge.

very

on data beginning

is a somewhat

produces

1.3 mm/yr,

and suggests

to 1965 but still is missing

An enigma

16.1 + 6.5 ram/yr. anomaly

was based

there

The tidal heights

movement.

gives

of 1.9 + 1.9 mm/yr

11

rather

at Nikiski

is probably

that were begun

than mean from

sea

1979 to

not significant. in 1993 and is

13.3 + 3.2 mm/yr.

As we discussed have experienced

above,

postseismic

died off near Seward any similar

a portion

relaxation

to the south,

short term relaxation

puted

the quadratic

tions

were strongly

regression effected

of the Kenai

curves

and the Turnagain

with a characteristic

and perhaps influenced

Peninsula

time of a few years.

near Anchorage our estimates

excluding

by this perturbation.

Arm

to the north.

of changes

1973. None

At Kodiak

the rate change

That

To determine

in uplift

data through

region

rate,

may

relaxation whether

we recom-

of the quadratic

solu-

with the decimated

Created M:u'ch, 1999. Current version November 3,19992:20pm

12

datasetwas- 0.59_+0.16mm/yr2 which agreesvery well with theratechangeof - 0.66 +

0.11 mm/yr 2 derived

from

the entire

change

is 0.33 + 0.09 mm/yr 2 compared

Valdez

the datd'set

begins

data set. Similarly

to the 1965-1998

at Cordova,

rate change

in 1975, so it does not contain

the post-1973

rate

of 0.39 + 0.11 mm/y_ 2. At

any observations

from the first decade

after ihe earthquake. Physical

Significance

We now turn to a discussion apparent

sea level data. Table

uplift

rates

sites,

with the exception

during

deduced

perhaps,

motion

in the NE portion

et al., 1996]. The 55 mm/yr

yr at N28W rupture

observations.

lie landward

velocity

in infrequent

where great

the Kodiak

Island

region

yrs) large

earthquakes

Given

the considerable

vertical

As previously

coseismic To some

varies

Plate relative

is less than 5 degrees

earthquake

with recurrence

variation

both in infrequent and Jacob, in trench

at Kodiak

America

obliquity. intervals

great

1990; Perez orientation

1, all of the

lies over

pattern

except, N44W

increases

there

The eastern

region

earthquakes and SchoIz,

hundred

and more

a

Johnson

et al., 1994]

of 25 degrees

of several

in the from

et al, 1994;

[Demets

a por-

of postseis-

uplift

but is about

[Brocher,

and

that slipped

Cordova

plate boundary

with an obliquity

there

[Nishenko

with locale,

to North

1971]

in Figure

the observed

locked

of the figure to 10 degrees

ruptures

shown

subsidence.

by these

[Plafker,

with all the sites exhibiting

trench

at Seward,

implied

of the megathrust

extent

of the interseismically

N17W

rates

motion

of that portion

pattern

of the offshore

of the Pacific

in a direction

at Kodiak

uplift.

the coseismic

The dip angle

of the uplift

both the coseismic

and thus underwent

The strike

of the figure.

significance

and thus experienced

mirrors

Cordova.

few degrees

about

of Cordova,

the 1964 earthquake

mic vertical

3 shows

from the tide gauge

tion of the slip plane

center

of the physical

is

and 59 mrrd tends

years, frequent

to

while (50-60

1997].

and slab dip in the region

under

study,

we

Created March,1999. Current version November 3,L9992:20pm

13

havefoundit moreinstructiveto

plot the uplift data as a function

maximum

1964 earthquake

coseismic

tion of distance detic

from the trench.

data for it lies directly

dislocation cally

subsidence

model

locked

the locked

region.

parameters,

The data

1985,

the effects rates

Valdez,

where

and Kodiak

agreement

tectonic

and Frohlich, Kodiak

because

ate for the great coseismic observed buttressed

crustal

sustained

The

an uplift

of less than a meter for the several

for Seward,

period

there

is consistent

GPS measurements

in Figure

cautiously,

too great

accumulation.

and Freymueller

of 5nor-

of these the

since

it lies

here [Pulpan

is sustainable

recurrence

thought

5 and Table

strain

of Cohen

For all three

rate. At Kodiak

earthquake

be much

depth

for Cordova,

sites considered

year interval

would

with elastic

value.

disloca-

to the trench

except

rate of 16.5 mm/yr

for a large

hundred

shown

of distance

representative

relative

particularly

than the other

1996]. However,

results

by the contemporary

vector

end of

of an elastic

assuming

than the 1996.5

must be viewed

to the east, then the uplift

movement

as a function

postseismic

better

of the plate boundary

and interseismi-

the downdip

are the results

velocity

of .geo-

slip, elastic

ruptured

both the mean rate and the 1996.5 model

than as a func-

dip angle of 5 deg, and locking

for the entire

dislocation

it give a net uplift

subsidence.

values

in a uniform

uplift,

the axis of

in the interpretation

motions

in the figure

of the plate

rate with the model

earthquakes

vertical

rate of 55 mm/yr,

we show

1985; Lu and Wyss,

50 yrs. If such a rate were

the interseismic

from [1971])

this axis also lies over

of the rate of interseismic

are mean

segment

interest

that the coseismically

are the same,

of the obliquity

rate fits the elastic

of the average

in a different

estimate

by Plafker

end of the rupture

5. Also shown

convergence

mal. The tide gauge

sites the mean

downdip Assuming

showing

in Figure

1992]

i.e. a plate

25 km. We ignore

0ve_.!he

of the megathrust

from the axis are plotted tion [Okada,

This axis is of considerable

of the earthquake.

portions

(as estimated

of distance

at

time of

to be approprito be relieved

3, suggest

that the

This argument [1997]

by

and

is

Created March, 1999. Current Freymtteller

et al. [1999]

of the Kenai

Peninsula,

may have

been rapid

version November 3, 1999 2:20 pm

that show that the pattern including

transient

gain Arm in the decade

agreement

and Valdez. expected

for elastic

et al. [1999] throughout

show

motion

rust conditions forward steeply

slip on the shallow

tonically release deep

results

different

downdip

slip model

ory results

segment.

during

of the parameters

observations

on the western

do not claim

this interpretation

Conclusions

that Seldovia

side of the Kenai

depths

peninsula

is the only possibility.

constrained

than would

the locked

that the megath-

with continuous slip on a more

do in fact lie in a tec-

matter 6 shows

coseismic

moment

to find a transient the dislocation

of 40 km [Oleskevich geometry

so we can claim

are consistent

be

of sites

speculated

by the small

fit to the plate interface

are uniquely

motion

postseismic

Figure

are also is

in Freymueller

a model

then it is a simple

between

faster

from

and Nikiski

sites (as suggested

at 25 degrees

is a reasonable

southeast

proposed

is such

are used for Cordova

are much

and transient

there. For example,

and Anchorage

velocities

different

Turna-

of that uplift

et al. [1999}

quite

interface

the 1964 earthquake),

1999] and 80 kin. The geometry

cm/yr)

They

pattern

side

There

and along

GPS measurements

Freymueller

Peninsula.

If we assume

dipping

(-2

conditions

of the plate

than fits the observations

but none

rapid

than the other

for slip on a plane

et al. [1999],

the horizontal

Peninsula.

side of Kenai

environment

in this region

and Seldovia

accumulation.

Peninsula

Valdez,

at Nikiski

from tectonic

portion

at Cordova,

on the eastern

strain

but the spatial

the average

Again

side of the Kenai

there

with elastic

provided

is an anomalously

on the eastern

dipping

uplifts

deformation

side of the Kenai

motions

model

accumulation.

that there

the western

horizontal

The vertical

the crustal

strain

are consistent

the 1964 event,

with the elastic

By contrast,

of horizontal

uplift on the eastern

or so following

that it died out near Seward. reasonable

Seward,

14

found

theet al.,

by Doser

only that the

with deep transient

slip. We

Created March, 1999. Current version November 3, 1999 2:20 pm

The tide gauge temporal Sound

records

distribution Earthquake.

in southcentral

of vertical

crustal

The longer

data

and we find considerable decay

time on the order

the western L

span has enabled

data

to the great

us to search

the record

10 years

and

William

of observations

and recent

GPS mea-

for time-dependent

behavior

in the rate of crustal

has persisted

the spatial

1964 Prince

these obser.-y, ations

We also find that the rapid

Peninsula

set for studying

incorporates

between

for a decrease

of a decade.

side of the Kenai

subsequent

of the records

a comparison

evidence

are a useful

motion

This new examination

since the last study and provides surements.

Alaska

15

uplift

uplift

at Kodiak

at Seldovia

with a

and Nikiski

on

for at least three

and a half decades.

There

is

on the eastern

side of the rupture,

to deter-

.

a need for further mine

whether

observations

the apparent

at Cordova

rate of change

tent with the idea that postseismic dependent quake

on both location

at these

relaxation

relative

sites is real. The data presented

may be a multiphase

to the coseismic

process

here

are consis-

with the details

rupture

features

and time since

for her insights

through

an ongoing

the earth-

occurrence.

Acknowledgments: Alaska

and Valdez,

tectonics.

and Kodiak.

We thank

Jeanne

We also thank

We also express

leveling

records

NASA's

Solid Earth

Sauber

her for providing

appreciation

that were not available and Natural

Hazards

some

of the GPS data, particular

to Scott Duncan through

the internet.

Program.

dialogue

for providing This research

tide gauge

on

from Cordova records

was funded,

and

in part, by

Created March, 1999. Current version November 3, 1999 2:20 pm

16

References

Brocher,

T.M.G.S.

megathrust Res.,

Fuis,

beneath

M.A.

the northern

99, 11663-11685,

Brown,

L.D.,

Cohen,

Alaska,

J.J. Tabor,

Gulf of Alaska

using

and N.J. Chfistensen,

wide-angle

seismic

Mapping

the

data, J. Geophys. ;_

S.R. Holdahl,

J. Geophys.

S.C., Time-dependent

Alaska

G. Plafker,

1994

R.E. Reilinger,

Anchorage,

Fisher,

Res.,

uplift

since the 1964 Prince

and E.I. Balazs,

82, 3369-3378,

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William

Sound

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crustal

and adjacent

areas

uplift

near

1977.

Peninsula

Earthquake,

J. Geophys.

of southcentral

Res.,

101,

8595-8604,

1996.

Cohen,

S.C., On the rapid

Prince

William

Cohen,

S.C.,

insula,

S.Holdahl,

Alaska

2038,

Sound

since

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uplift

along

Turnagain

Earthquake,

Geophys.

Res. Lett.,

D. Caprette,

S. Hilla,

R. Safford,

the 1964 Prince

William

Sound

Arm,

Alaska

following

25, 1213-1215,

1998.

and D. Schultz,

Uplift

Earthquake,

J. Geophys.

the 1964

of the Kenai Res.,

Pen-

100, 2031-

1995.

Cohen,

S.C.,

and J.T. Freymueller,

102, 20,479-20,487, Demets,

on the Kenai

Peninsula,

Alaska,

J. Geophys.

Res.,

1997.

C., R.G. Gordon,

reversal

Deformation

D.F. Argus,

time scale on estimates

and S. Stein,

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plate

Effects motions,

of recent

revisions

to the geomagnetic

Geophys.

Res. Lett.,

of the Prince

William

21, 2191-2194,

1994.

Doser,

D.I., A.M.

over thirty Freymuetler, insula,

years

Veileux,

and M. Velasquez,

following

J.T., S.C. Cohen,

Alaska,

the 1964 Great

Seismicity Alaskan

and H. Fletcher,

and their implications,

earthquake,

Variations

J. Geophys.

Res.,

PAGEOPH,

in present-day (submitted

Sound

region

in press,

1999.

deformation,

to JGR,

3/23/99).

Kenai

for

Pen-

Created March,1999. Current version November 3,19992:20pm

17

Johnson,J.M., K. Satake,S.R.Holdahl,andJ. Sauber,The 1964PrinceWilliam Soundearthquake:Jointinversionof tsunamiandgeodeticdata,J. Lu, Z., and M. Wyss, estimates

based

Mao, A. C.G.A.

on fault

E.M.,

Nishenko,

and Least

J. Geophys.'Res,

Am.,

75, 1135-1054,

boundary

Res.,

101,522-532,

derived

from

101, 803-816,

in GPS coordinate

1996.

stress

direction

1996.

time series,

J. Geophys.

Res.,

D.A.,

tion earthquake:

deformation

Seismic

pp26,

potential

Harper

and Row, New

of the Queen

York,

1976.

Charlotte-Alaska-Aleutian

Seis-

1990.

due to shear

and tensile

faults

in a half-space,

Bull Seismo.

Soc.

due to shear

and tensile

faults

in a half-space,

Bull Seismo.

Soc.

1992.

R.D. Hyndmann, Thermal

J. Geophys.

Squares,

1985.

Am., 82, 1018-1040,

Res.,

during

and K. Wang,

and structural 104,

O.J., and C.H. Scholz,

earthquakes

Noise

95, 2511-2532,

deformation

Y., Internal

Oleskevich,

J. Geophys.

and T.H. Dixon,

Observations

Y., Surface

Perez.

solutions,

plate

Res.,

1999,

Okada,

Chile,

of the Aleutian

S.E, and K.H. Jacobs,

mic Zone,

Okada,

plane

Harrison,

104, 2797-2816, Mikhail,

Segmentation

Geophys.

models

14965-14991, Long-term

the time between

The updip

and downdip

of Cascadia,

south

limits

Alaska,

to great

SW Japan,

subducand

1999.

seismic

behavior

successive

shocks,

of the focal J. Geophys.

adjacent Res.,

102,

regions

of great

8203-8216,

1997.

Plafker,

G. Tectonics,

Academy Pulpan, Inlet,

in The Great Alaska

of Sciences,

Washington,

H., and C. Frohlich, Alaska,

determined

Geometry

DC,

Earthquake

Geology,

pp 47-122,

National

1971.

of the subducted

from relocated

of 1964,

earthquake

plate near Kodiak hypocenters,

Bull.

Island Seismo.

and lower

Cook

Soc. Ant., 75,

Created March, 1999. Current version November 3, 1999 2:20 pm

791-810, Press,

Ryan, NASA

Flannery,

Computing,

S.A. Teukolsky,

Tech. Memo.

104572,

J.C. and G. Plafker,

Geophys.

University

NASA

space

Press, geodesy

Numerical

Cambridge,

Recipes,

The Art of

1986.

program-GSFC

data

analysis-1992,

1993. Tide gage measurements

Res., 96, 4325-4335,

D., D. Christensen,

Geophys.

and W.T. Vetterling,

pp. 507, Cambridge

J.W., C. Ma, and D.S. Caprette.

Savage,

Zhao,

1985.

W.H., B.E

Scientific

18

along

the south

coast

of Alaska,

J.

1991.

and H. Palpan,

Res., 100, 6487-6504,

of uplift

1995.

Tomographic _-

imaging

of the Alaska

subduction

zone, J.

Created March,1999. Current version November 3,19992:20pm Table

1: Tide

19

Gauge

Site

Sites

Comment

Cordova

Record middle

is among the most complete to late 1960's.

Valdez

No data

Seward

Some

Kodiak

The operational

with significant

temporal

gaps

only in the

< prior

to 1975, but from

missing

moved

data, but the record tide gauge

a few kilometers

sites and the leveling gauges

was located

to Women's result

postseismic

at St. Paul Harbor Bay. A leveling

was verified

at both sites for two months to within

over the entire

complete.

until

survey

the two

operation

in 1984. The tide gauge analyses

1984, then was

connects

by the simultaneous

1.5 cm. Separate

interval.

of tide

and leveling

data

in the offset

(1967 faster

- 1982) and Women's Bay (1985-present) records reveal a substantially uplift rate when the gauge was located at St. Paul Harbor, than at

of the St. Paul Harbor

Bay.

Strong record with only a few missing data points. There is a significant aly in the late 1970's which is not removed by the correction procedures employed

Anchorage

extends

is fairly

agree

Women's Seldovia

then on the record

herein;

Interpretation tors. First, lesser obvious annual

the subsequent

of the tide gauge as Brown

extent

eral years

however

et al. [1977]

Seward)

may exhibit

in the first decade at Anchorage

means

signal

record

is well behaved.

at Anchorage

is handicapped

and SP91 point out Anchorage a sinusoidal

following

oscillation

the earthquake.

on less than 9 monthly

by two fac(and to a much

with a period This oscillation

in our data set that of SP91 because

that are based

anom-

means.

we have Second,

of sevis less

deleted there

the are

very few data available from the early 1970's until the mid 1980's. After that the record becomes more robust. An alternative sea level record has been derived from Nikiski

the more complete

The tide gauge 1997.

operated

set of monthly through

much

mean tide data. of the 1970's,

then

was reactivated

in

Created March,1999. Current version November 3,1999 2:20 Table

2: Rate

of Apparent

Sea Level

Change,

Tide Gauge

Time Period

sea

(mm/yr) 1965-98

"

Valdez

Seward

Kodiak

Seldovia

Anchorage

Nikiski

Recent

6.7 + 0.4

analysis,

r = b + 2cAt

b(mm/yr);

2c (mm/yr2);

-0.6+1.0

to Oldest

Determinations

sea level

level analysis, At

Cordova

20

r - Most

r, quadratic r, linear

pm

Ref.

Comment

= 1998.5-t ; 0.39+0.06

(1)

1974-88

9.4

(2)

1964-88

9.7 + 0.5

(2)

1964-74

12.7 _+2.4

(3)

1975-98

- 0.2 _+0.7

1974-88

5.5 + 0.9

1965-98

- 1.2 + 0.7

1974-88

2.9

(2)

1964-88

0.1 + 1.0

(2)

1964-73

- 11.3 +3.7

(3)

1985-98

-8.1+

(1)

Women's

1967-98

- 14.5 + 0.7

(1)

WB and SP

1974-88

-14.8

(2)

1967-98

- 17.5 +0.8

(2)

1967-81

-21.1+_1.4

(1)

St. Paul Harbor

1965-74

- 86.8 + 38.4

(3)

SP

1966-98

-9.3+0.8

1974-88

-7.0

(2)

1964-88

-7.2+1.4

(2)

1964-74

-28.1_+8.1

(3)

1984-98

0.8+

(1)

1965-98

-0.7+0.9

1964-88

1.9 _+ 1.9

(2)

1964-74

- 14.3+7.6

(3)

1972-98

- 9.9 + 0.8

(1)

- 8.7 + 1.6 ; 0.77 + 0.14

(1) (2)

- 2.7 _+2.7 ; 0.09 _+0.15

1.1 -4.1_+2.1

; -0.66+0.11

-11.8+2.9;

0.16+0.18

1.3

(1)

(1) This study;

(2) Savage

(2)

- 18.7 + 1.7 andPlafker

[1991];

(3) Brown

et al. [1977].

(SP)

(1)

mean mean

sea level tide level

no data from through

1971-79

Bay (WB)

no data

1981

1996 after

1984

Created March,1999. Current version November 3,19992:20pm

Table

Tide Gauge Site (TGS)

3: Tide Gauge 1 and GPS Rates

TGS Mean

TGS 1996.5

Uplift

Uplift Rate 2 ._(mm/yr)

Rate

(mm/yr)

21

of Uplift

GPS Uplift

GPS Site

Rate

(mm/yr)

comment

Cordova

- 4.7 +0.4

1.8 + 1.0

4050

- 13.2 + 7.1 8

GPS: dates

Valdez

2.2 + 0.2

9.2 + 1.6

POWE

8.7 + 8.2 8

GPS: dates

Seward

3.2 + 0.7

T19

7.6 + 3.5 5

GPS:

1993-98

UAMF

9.5 + 27 5

GPS:

1995-98

KDK1

9.2 + 1.6 8

GPS: dates

VLBI6

14.8 + 7.2

VLBI:

2.7+0.9

VANDUSEN

16.1 + 6.5 5

GPS:

1995-97

11.9_+0.8

NIK/NIK2

13.5+3.2

GPS:

1993-98

Kodiak 3

Anchorage

16.5 + 0.7

4

Nikiski

1. Assumes combined derived rates. 2. Cordova, 3. Combined 4. Mean

Valdez,

effects

sea level rise and post-glacial

assumed

to have time-dependent

rebound

5

= 2 mm/y

rate.

Bay and St. Paul Harbor.

Analysis.

5. Update of Freymueller 6. VLBI measurement. 7. Ryan

of eustatic

and Kodiak

Women's

Tide Level

7.4 _+2.1

et al. [1999]

to include

1998 data.

et al. [1993].

8. Previously

unreported

GPS data processed

with update

to Freymueller

et al. [1999].

1984_90

for the tide gauge-

Created March,1999. Current version November 3,19992:20pm

22

Figure Captions Figure 1:Map of southcentralAlaskashowingmajorcities,tide gaugelocations(n.b.Nikiski is immediatelynorthwestof Kenai),andcoseismic.upliftpatternfor 1964PrinceWilliam Sound Earthquake.From Plafker Figure squares

2: Uncorrected

3: Sea level

Figure

4: Corrected

Figure

5: Uplift

dislocation Figure creep

(circles)

line with slope

Figure

at depth

as given

height

++

and corrected

(corrected

sea level heights

annual

sea level heights

sea level heights

and linear

and quadratic

from axis of maximum

for elastic

deformation

rate of western

Kenai

in an elastic

(squares)

and linear

least

on Table 2.

residuals

rate vs distance

model

6: Uplift

[1971].

halfspace

due to a locked

Peninsula (solid

line).

minus

least

least squares

coseismic megathrust

sites (data points)

squares

line).

fits.

subsidence'(data (dashed and predicted

points)

and

line). uplift

rate due to

/

Figure

1

Cordova

Valdez

4.1

4.3

E4.0

99

oo ,'

o ,,',

E4.2

,

> _J

-u3.9

-

,_ Q

"4.1

CO

P9

co

E3.8

-E 4.0.

P

o

o

_

_.3.7
4)

"4.1

[]

4) CO

_4.0 _3.9

3.8 1960

= 1970

, 1980 Time, yr

i 1990

i 2000

Figure

4

Elastic

Dislocation

Model:

vel=55

mm/yr,

dip=5

deg; depth=5-25

20

_- Kodiak-avg _Seldovial_ _;J_likiski

!.__

//"

10._.

_.

/

{ Kokiak-199_.5 eward

#: ¢::

F Valdez-1996.5

4-"_/_nch°rage

" Valdez-avg

% "_ _ _"-

_---

.

Cordova-av,

I

-50 Distance from Axis

I

0 of Maximum

I

I

50 Coseismic

Figure

100 Subsidence,

5

150 km

k

Deep Slip Model: vel=50 mm/yr, dip=25 deg; depth--40-80 km Updip Tip of Fault Midway between Seldovia and Nikiski

20 ._

// _'kiski 10 E E

\\

Seldovia o_\

\ \ \ \ \

e_

_

0

-100

-50 0 50 Distance from Axis of Maximum Coseismic

100 Subsidence,

Figure 6

150 km