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					 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
                             • 16km retreat since 1982
• 66km long (pre-retreat)    • 75km^2 loss in area
• Max flow velocity ~10km/   • Thinned by over 400m
  year                        (35%) at terminus
• 7km^3/year discharge       • Recent (~2006) flotation
• 1100km^2                     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:
Total detection time:
                           • Decrease of events per hour
• 2004/2005: 4.74 %
                           • Decrease of total detection time
• 2008/2009: 1.35 %
                        • Activity spikes
  Hourly Calving Activity (1-3Hz)




    • Empirical
‘quiet’ hours CDF’s normalized by area under curve
    • More ‘quiet’ hours in 2008/2009
    • More ‘moderately active’ hours in 2004/2005
       ‘active’ hours
    • 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




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




                                                                                             Source:
                                  Source:
Jakobshavn Glacier, Greenland                               Columbia Glacier, Alaska
  • 10km calving retreat / ice acceleration            • 16km calving retreat / >400m thinning
    initiated in 1990’s                                 initiated in 1980’s
  • Loss of floating tongue                            • Tidewater glacier cycle
  • Change in calving style:                           • Recent flotation of terminus (~2006)
    Tabular icebergs only produced                        Rifting
    when tongue was floating                              Calving of larger icebergs
  • Amundson et al. (2008, in press):                  • Comprehensive data on dynamics,
       • Winter: advance, buildup of                     geometry and seismic activity
          floating tongue                              • Data acquired when terminus was
       • Summer: retreat, main calving                   grounded and floating
          activity                                     • Data complements Jakobshavn data
       • Controlling role of mélange

				
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posted:2/19/2013
language:English
pages:16