Changes in Glacial Seismicity in Response to Terminus Floatation

Fabian Walter, Shad O’Neel, W. Tad Pfeffer, Jeremy Bassis, Helen Fricker

Introduction

Definition of Problem:• Calving controls over 50% of mass loss from Greenland Ice Sheet• Estimates for some Alaskan tidewater glaciers: 15 times surface ablation• Physical base of calving poorly understood• Yet important for sea-level rise prediction

Variability in Calving:• Floating / grounded termini• Variation in proglacial mélange• Water depth• Water temperature• Tidewater/freshwater

Columbia Glacier

• 66km long (pre-retreat)• Max flow velocity ~10km/ year• 7km^3/year discharge• 1100km^2

• 16km retreat since 1982• 75km^2 loss in area• Thinned by over 400m (35%) at terminus• Recent (~2006) flotation of glacier tongue accompanied by rifting• Measurements from before and after floatation

Investigating Calving Activity with Seismic Monitoring

2004/2005• 9 rock-based geophones• 2 ice-based geophones• 1 rock-based broadband seismometer• 100Hz sampling frequencies

2008/2009• 1 rock-based broadband seismometer• 100Hz sampling frequencies

Investigating Calving Activity with Seismic Monitoring

Calving-Related Seismicity• 1-3 Hz• Detect/measure calving activity with frequency detector • Radiated seismic energy representative of size of calving event

Fracture-Related Seismicity• >10 Hz• Englacial or within mélange (?)• Often embedded in calving seismicity

Calving-related Seismic Activity (1-3Hz)

• Decrease in calving activity:• Decrease of events per hour• Decrease of total detection time

• Activity spikes

Total detection time:• 2004/2005: 4.74 %• 2008/2009: 1.35 %

Hourly Calving Activity (1-3Hz)

‘quiet’ hours

‘active’ hours

• Empirical CDF’s normalized by area under curve• More ‘quiet’ hours in 2008/2009• More ‘moderately active’ hours in 2004/2005• More ‘very active’ hours in 2008/2009→ Less but bigger calving events (?)

Fracture-related High-Frequency Icequakes 2004/05 vs. 2008/09

• Higher activity in 2004/2005 • Lower coherence in 2004/2005• Swarms in Jan/Feb 2009• Highly coherent signals during spikes

High-Frequency Swarms

• 2-20 second intervals• Can last several hours• Usually only high frequency

Columbia Glacier During Swarms: Closely Packed, Consolidated Mélange

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Iceberg Harmonic Tremor (MacAyeal et al., 2008)

• Related to iceberg scraping• Even spacing harmonic character• Can last several hours• High frequencies (>5Hz) seismoacoustic phase through water and ice• low frequencies (<5Hz) seismic phase through ice and bedrock

Summary/ConclusionsChanges in seismicity as terminus went afloat

1-3Hz seismic events • Decrease in detections per hour• Increase in ‘quiet’ hours• Decrease in ‘moderately active’ hours• Increase in ‘active’ hoursExplanation: Less smaller calving events, more large ones due to rifting

10-20Hz seismic events • Decrease in activity• Highly coherent high-frequency ‘tremors’ during winterExplanation:

• Decrease of calving event frequency• Presence of large icebergs in a consolidated mélange scraping of icebergs iceberg harmonic tremors (MacAyeal et al., 2008)

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Instrument Performance and Response

Ross Ice Shelf, Antarctica

• MacAyeal et al. (2008)• Seismometers deployed on icebergs, ice shelf and bedrock (Aster et al., 2004)• GPS for iceberg motion

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• 10km calving retreat / ice acceleration initiated in 1990’s • Loss of floating tongue• Change in calving style: Tabular icebergs only produced when tongue was floating• Amundson et al. (2008, in press):

• Winter: advance, buildup of floating tongue• Summer: retreat, main calving activity• Controlling role of mélange

• 16km calving retreat / >400m thinning initiated in 1980’s • Tidewater glacier cycle• Recent flotation of terminus (~2006) Rifting Calving of larger icebergs• Comprehensive data on dynamics, geometry and seismic activity• Data acquired when terminus was grounded and floating • Data complements Jakobshavn data