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 km^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/ rock-based geophones 2 ice-based geophones 1 rock-based broadband seismometer 100Hz sampling frequencies 2008/ 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 January 21, 2009

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/Conclusions Changes 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’ hours Explanation: Less smaller calving events, more large ones due to rifting 10-20Hz seismic events Decrease in activity Highly coherent high-frequency ‘tremors’ during winter Explanation: Decrease of calving event frequency Presence of large icebergs in a consolidated mélange  scraping of icebergs  iceberg harmonic tremors (MacAyeal et al., 2008)

thank you

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 MacAyeal et al., 2008

Source: Columbia Glacier, Alaska Jakobshavn Glacier, Greenland Source: www. wikimedia.org 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