Tidal Modulation of Stick-Slip Ice Stream Motion

R. A. BindschadlerM.King

P. VornbergerS. Anandakrishnan

I. JoughinD. Voigt

R. A. Alley

We acknowledge NSF OPP for their financial and logistic support

2003-2004 Field Season

• 21 GPS stations – 5 reference (for

differential sol’n)– 4 floating (for tides)– 15 sec. sampling

• 7 seismic networks (triggered)

+ 1 tiltmeter+ 1 winterover station

70+ DAYS OF DATA

Whillans Ice Stream Stick-slip

Only Ross ice stream that exhibits stick slip

5 min positions

B010

Spring and NeapTides

Spring tide intervals (shaded) defined as cycles of single component and high amplitude tide

73-day record has multiple spring-neap cycles

Event and Interval Definition

•~12-hr intervals defined by midpoints between events

138 events picked from 30-min. running avg.

Slip vs. Total Motion

~90% of motion occurs

during slip events

80

82

84

86

88

90

92

94

96

Sli

p M

oti

on

(%

)

Neap

Spring

High position errors

Spring vs. Neap Speed

~15% faster during spring

tides

180

190

200

210

220

230

240

Sp

eed

(m

/a)

Spring

Neap

Tidal Correlation

• Maximum 5-min displacement correlates with tidal magnitude (spring vs. neap)

B010

BFLT

BFLT

Motion

Tide

Slip Characteristics

Time

Position

12 hourtotal

30 minuteslip

slope=max. speed

Spring vs. Neap Slips

Time

Position

Higher maximum

speed

Higher average speed during spring tides

Spring tide slip

Neap tide slip

Actual example

Springslip

Neapslip

Preliminary FindingsIn

crea

sing

ups

trea

m d

ista

nce

BFLT

B010

B090

B140B190

B320

• Maximum slip magnitude decreases upstream

• Tidal modulation occurs at least as far upstream as B190

How does B090 compare with B010?

BFLT

B010

B090

Slip vs. Total Motion (B090)~70% of B090 motion occurs

during slip events

(90% for B010)

62

64

66

68

70

72

74

76

Slip

Mo

tio

n (

%) Spring

Neap

High position errors

Spring vs. Neap Speed (B090)

~6% faster during spring

tides(15% for B010)

350

355

360

365

370

375

380

385

390

Ave

rag

e S

pee

d (

m/a

) Spring

Neap

Event propagation

0

2

4

6

8

10

12

14

Ave

rag

e E

ven

t D

elay

(m

in)

Spring

Neap

Longer delays during neap

tide

Slip Origin?

B090

B140B190

B010

Simple Stick-slip Model

thL

Ht

L

HVE tideob 10

2

Stress drop = .3 kPa

H = 750 m

L = 100 km

yield = 4 kPa

Ice stream pushes Tide resists

Re-examine Model Parameters

thL

Ht

L

HVE tideob 10

2

tghkt tideob

k/g is relative importance of ice stream loading vs. tidal influence

Model has the form

tidehgtkb )(Can examine stress buildup between events

to find temporal variation of stress drop for each event and relative strength of ice stream loading

Previously:Stress drop = 0.3

F = .51

Revised:Stress drop = 0.25

F = .49

Model Prediction

model

observation

Spacing is generally correct, but timing is sometimes wrong

Continuing Research

• Next field season (2004-2005)

• Detailed study of Whillans Ice Stream

– 30+ GPS sites– Seismic

experiments on ice plain

• Final field season (2005-2006)• Detailed study of Ice Stream D

2003-2004

Both seasons

2004-2005

• Examine 2003-2004 data from other Whillans Ice Stream sites

Conclusions• Whillans Ice Stream is the only Ross ice stream

that exhibits stick-slip motion• Slip origins appear to have a persistent location

at or near north margin • Slip accounts for majority of motion

– ~90% at B010; ~70% at B090

• Spring-tide speed faster than neap-tide speed– ~15% faster at B010; ~6% faster at B090

• Spring-tide slips more impulsive than neap-tide slips

Thank you

Outline

• Only Whillans Ice Stream

• Further characteristics of stick-slip motion

Ross Ice Streams

A

Whillans

C

D

E

SipleDome

Transantarctic

Mountains

RooseveltIsland

CraryIce

Rise

Byrd

RossIce

Shelf

Preliminary Findings

Slip Initiation

Incr

easi

ng u

pstr

eam

dis

tan

ceF

loat

ing

site

BFLT

B010

B090

B140B190

B320

• Timing– Slips not evident at

B320– Events occur at very

similar times– Events take <15

minutes to propagate to all stations

– Slips last 5-20 minutes

Movie of Stick-Slip Events

Simple Stick-slip Model

thL

Ht

L

HVE tideob 10

2

tL

V: stored elastic

strain

: minimum

basal stress

h: tide amplitude:1 meter =10 kPa stress

E: Young’sModulus

EHV

L2

yield – o)

P2

=

2 events per tidal period, P

H = 750 m

L = 100 km

yield = 4 kPa

Calculatestress drop

where

Ice Stream pushes