Tidal Modulation of Stick-Slip Ice Stream Motion Update Tidal Modulation of Stick-Slip Ice Stream Motion R. A. Bindschadler M.King P. Vornberger S. Anandakrishnan I. Joughin D. Voigt R. A. Alley We acknowledge NSF OPP for their financial and logistic support

2003-2004 Field Season Old 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 Old Whillans Ice Stream Stick-slip Only Ross ice stream that exhibits stick slip 5 min positions B010

Spring and NeapTides Update 73-day record has multiple spring-neap cycles Spring tide intervals (shaded) defined as cycles of single component and high amplitude tide

Event and Interval Definition Update Event and Interval Definition 138 events picked from 30-min. running avg. ~12-hr intervals defined by midpoints between events

~90% of motion occurs during slip events Update Slip vs. Total Motion ~90% of motion occurs during slip events High position errors

~15% faster during spring tides Update Spring vs. Neap Speed ~15% faster during spring tides

Old Tidal Correlation Maximum 5-min displacement correlates with tidal magnitude (spring vs. neap) Motion BFLT Tide B010 BFLT

Slip Characteristics Position 30 minute slip slope= max. speed 12 hour total Time

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

Update Actual example Spring slip Neap slip

Preliminary Findings Old Maximum slip magnitude decreases upstream Tidal modulation occurs at least as far upstream as B190 Increasing upstream distance BFLT B010 B090 B140 B190 B320

How does B090 compare with B010? BFLT

Slip vs. Total Motion (B090) Update Slip vs. Total Motion (B090) ~70% of B090 motion occurs during slip events (90% for B010) High position errors

Spring vs. Neap Speed (B090) Update Spring vs. Neap Speed (B090) ~6% faster during spring tides (15% for B010)

Longer delays during neap tide Update Event propagation Longer delays during neap tide

Update Slip Origin? B190 B140 B090 B010

Simple Stick-slip Model Old Simple Stick-slip Model Ice stream pushes Tide resists Stress drop = .3 kPa H = 750 m L = 100 km tyield = 4 kPa

Re-examine Model Parameters Update Re-examine Model Parameters k/g is relative importance of ice stream loading vs. tidal influence Model has the form 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 Update model observation Spacing is generally correct, but timing is sometimes wrong

Continuing Research Examine 2003-2004 data from other Whillans Ice Stream sites Next field season (2004-2005) Detailed study of Whillans Ice Stream 30+ GPS sites Seismic experiments on ice plain 2003-2004 Both seasons 2004-2005 Final field season (2005-2006) Detailed study of Ice Stream D

Update 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 Ross E Ice Shelf Transantarctic Mountains Whillans Byrd C Crary Ice Rise D Siple Dome Ross Ice Shelf E Roosevelt Island

Preliminary Findings Old 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 Increasing upstream distance Slip Initiation BFLT B010 B090 B140 B190 B320 Floating site

Movie of Stick-Slip Events

Simple Stick-slip Model Old Simple Stick-slip Model : stored elastic strain to : minimum basal stress h: tide amplitude: 1 meter =10 kPa stress E: Young’s Modulus Ice Stream pushes E HV L2 (tyield – to) P 2 = 2 events per tidal period, P H = 750 m L = 100 km tyield = 4 kPa Calculate stress drop where