An Analysis of Glacial Earthquakes

							    An Analysis of
 Glacial Earthquakes
          Victor C. Tsai
          Göran Ekström

Department of Earth and Planetary Sciences
           Harvard University
What are Glacial Earthquakes?
                         Ekström et al., 2003
             1993-2004    MS ~ 4.6 – 5.1
             Events
                          Away from plate
                           boundaries
                          Lack high freq.
                           radiation
                          Radiation patterns
                           incompatible with
                           standard earthquake
                           focal mechanisms
                          Seasonality: more
                           events in summer
                           months
                 A Simple Model




        Force


                  Symmetric Double Boxcar
Kawakatsu 1989
              Fitting Seismic Data
Select Seismograms      Guess Initial       Calculate Double
to Use in Inversion    Location, Time       Boxcar Synthetic
                                             Seismograms
                Revise Parameters

                                    Invert for best Source
                                       Parameters by
Results:                              Minimizing Misfit

 Improved location and time

 ‘Force’ amplitude and direction

 Mass*distance: 0.12 – 2.04 * 1014 kg*m

     ~ 10 km3 moving 10 m
      Glacial Earthquake Locations
Before Relocations      After Relocations
Comparison with Icesheet Velocities


   4
                                     4
   4-5 NW Greenland
     5
                                      5
       6             0
                                                          0
   6 Rinks Isbrae                         6
       7         1
               0 Daugaard Jensen                      1
   7 Jakobshavn Isbrae                        7
           2
                                                  2
           3
                                          3
               1 Kangerdlugssuaq
               2 Helheim
               3 SE Greenland


                                   Bamber et al. 2001
        Detailed Location      Rignot et al. 2004


                  10 km




                          Region 1 -
10 km
                          Kangerdlugssuaq
        Force Directions      Rignot et al. 2004


                 10 km




                         Region 1 -
10 km
                         Kangerdlugssuaq
        Force Directions

                    Region 2 - Helheim

                            Rignot et al. 2004




10 km
          Why backwards forces?
      Modeled Force       Actual Force




    Modeled Seismogram   Data Seismogram




Easy to have 180°
phase shift
Size Distribution
Size Distribution Comparison
             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