changes in glacial seismicity in response to terminus floatation fabian walter, shad o’neel, w....

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