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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
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
Spring vs. Neap Slips
Time
Position
Higher maximum
speed
Higher average speed during spring tides
Spring tide slip
Neap tide slip
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
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
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
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
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