An Analysis of Glacial Earthquakes

1
An Analysis of Glacial Earthquakes

• Victor C. Tsai
• Göran Ekström
• Department of Earth and Planetary Sciences
• Harvard University

2
What are Glacial Earthquakes?

• Ekström et al., 2003
• MS 4.6 5.1
• Away from plate boundaries
• Lack high freq. radiation
• Radiation patterns incompatible with standard
  earthquake focal mechanisms
• Seasonality more events in summer months

1993-2004 Events
3
A Simple Model
Force
Symmetric Double Boxcar
Kawakatsu 1989
4
Fitting Seismic Data
Select Seismograms to Use in Inversion
Guess Initial Location, Time
Calculate Double Boxcar Synthetic Seismograms
Revise Parameters
Invert for best Source Parameters by Minimizing
Misfit

• Results
• Improved location and time
• Force amplitude and direction
• Massdistance 0.12 2.04 1014 kgm
• 10 km3 moving 10 m

5
Glacial Earthquake Locations
Before Relocations After Relocations
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Comparison with Icesheet Velocities
Bamber et al. 2001
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Detailed Location
Rignot et al. 2004
Region 1 - Kangerdlugssuaq
10 km
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Force Directions
Rignot et al. 2004
Region 1 - Kangerdlugssuaq
10 km
9
Force Directions
Region 2 - Helheim
Rignot et al. 2004
10 km
10
Why backwards forces?

• Modeled Force Actual
  Force
• Modeled Seismogram Data Seismogram

Easy to have 180 phase shift
11
Size Distribution
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Size Distribution Comparison
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Conclusions

• 182 Glacial Earthquakes
• Occur only in regions of very high ice flow, but
  not in all of these regions
• Force directions consistent with the sense of
  glacial motion
• Amplitudes correspond to
• 10 km3 sliding 10 m