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					Glacial Earthquakes




      Huda Elasaad
      March 28, 2007
                                Overview
I.     What are glacial-quakes? (Characteristics)
II.    Where do they occur?
III.   How has the science progressed? ( the field of Glacio-seismotectonics)
IV.    What causes them? (Mechanisms)
V.     When and how often do they occur? (Seasonality/Frequency)
VI.    Case Example: West Antarctica




                                    http://www.climateaudit.org/?p=1010
      Glacial Earthquakes- Two Sided Coin
1.   Seismic Activity within Glaciers- “Icequakes”

•    Definition: seismic events that occur within glacial landforms or
     microseisms that occur due to ice cracking

•    Detection: overlooked by the frequencies that are monitored at most
     seismic stations, seismic radiation is depleted at high frequencies

•    Duration: long period vibrations, ~15-30 times longer than an
     ordinary tectonic events of similar magnitude

2. Post-glacial Seismicity

•    Definition: an earthquake caused by glacial landforms (post glacial
     rebound by glacier recession)

•    Detection and Duration: similar to tectonic event though different
     mechanism
        Icequake Seisomogram




Glacial (M=5.0) and Tectonic (M=4.2) earthquake in Alaska (Sept.1999)

Seismograms aligned at predicted P-wave arrival time
    Documented Locations of Icequakes




Current Seismic activity
Previous record of
seismic activity
                           Glacio-Seismotectonics
Previous strains of thought –

•   Ice sheets suppress seismicity:
    Overburden of the ice sheet
    counteracts tectonic stresses that
     would otherwise promote
    seismic activity (Johnston, 1987)


•   Cryoseisms mistaken for intraplate
    earthquakes (only distinguishable by
    meteorological and geological setting)



•   Tectonic vs. Postglacial?

     Evidence for Postglacial
     – High spatial correlation between microseismicity, boulder caves and maximum curvature of uplift
        (Ekman)
     – Steep gradients in post glacial rebound contours correlate well with band of intense seismic activity
        (Basham)
     – Supported by models of glaciation/deglaciation cycles and historic earthquakes
     – Mechanisms that are consistent with presumed ice-movement directions
     – Orientations of postglacial thrust faults indication maximum horizontal principle stress consistent with
        direction of ice retreat (i.e.. Southeastern Canada)
  Icequake Mechanisms – Seismicity within Glaciers
Sub glacial Volcanic Activity
   Volcanic systems have the ability to increase faulting in seismic zones. One example
   occurred in Iceland, where the largest historical eruption (that of Laki in 1783) increased
   shear stress in the South Iceland Seismic Zone and almost certainly triggered the largest
   (M 7.1 in 1784) historical earthquake in Iceland.




 Fissure on ice sheet generated by 1996 eruption           Volcanic activity in Iceland


        http://arjournals.annualreviews.org/doi/pdf/10.1146/annurev.earth.28.1.107?cookieSet=1
Ice Cracks and Stick-Slip Behavior
                                   Cryoseisms

                                   Also known as frost quakes, occur close
                                   to lakes and rivers, when the ambient
                                   temperature drops below -20°C, caused
                                   by cracking ice and movements of ice
                                   blocks one against another. Such ice
                                   cracks can sometimes be detected by a
                                   seismograph if it is located close to the
                                   body of water.

                                   Can have intensities up to IV on Mercalli
                                   scale

                                   http://earthquakescanada.nrcan.gc.ca

           Stick-Slip at Ice-Rock Interface

           Surface meltwater seeps through cracks to bottom of glacier to
           form a layer, lifting glacier up from rock and slips

           Solar panel, battery and GPS receiver on an ice stream, one
           of the ways scientists monitor stick-slip motion of glaciers.
           (Image courtesy of University of Washington)
  Icequake Mechanisms – Glacier Motivated
Post Glacial Rebound
              Removal of ice sheets at the ends of glaciation periods cause non-
              isostatic compressive stresses with the ability to trigger earthquakes
   The Model –
   Mohr Circle: tool used to visualize relationships between normal and shear
   stresses
   Consider: loading and unloading of the earth by ice sheets and melted water loads, as
   well as relaxation of stress associated with creep of mantle rocks
              State of stress inside earth constantly changes even regardless of
              tectonic and all other stresses held constant
              as state of stress changes, Mohr circle moves closer to and/or away from the
              line of failure according to time dependant quantity dFSM


                where,

      б (1,2,3) = maximum, intermediate and minimum (compressive) principle stress respectively
      to = time before the onset of glaciation
      dFSM = change in time of the Fault Stability Margin
During glacial loading – increase
in mean compressive stress
moves circle away from failure




After ice-removal – vertical
stress return to initial value,
horizontal stress relaxes with
time. Increase in deviatoric
stress (radius of circle) moves
closer to failure
Seasonality and Frequency of Glacial Earthquakes
Temporal patterns suggest link to hydrological cycle
Glacial sliding indicative of dynamic glacial response to climate conditions
Glacier sensitivity and vulnerability to climate change
In Greenland
     •   Strong seasonality and doubling of occurrence rate in last five years
                              6-15 quakes from 1993 – 2002
                              23 in 2004
                              32 in 2005
     •   occur every month but more frequent in late summer months


                                             Horizontal ice velocity (red curve) showing ice
                                             accelerations during the summer melt seasons
                                             and the abrupt transitions to deceleration around
                                             the times of melt cessation.
                                             The cumulative additional motion (horizontal
                                             residual, black) relative to a wintertime-average
                                             velocity of 31.33 cm/day is 6.0 m by the time of
                                             the maximum velocity in 1999.
                                             http://www.theoildrum.com/story/2005/12/9/31522/5910
       Frequencies: Greenland & Alaska




1993                     1999                     2003
            Southern Alaskan Earthquakes – 1 Decade
                                                         Source: NASA
In The News…


          Glacial earthquakes rock Greenland ice
                           sheet
                       (Mar 24, 2006)

       A rapid increase in "glacial earthquakes" – caused by
        sudden large movements of glaciers – over the past
          few years indicates that warmer temperatures will
       destroy the Greenland ice sheet faster than expected,
       a new study warns. Surface meltwater is not dribbling
       away, as if from a giant ice block melting slowly, but is
        seeping through cracks to the bottom of the glacier.
       Once there it forms a layer that "helps lift the glacier up
           from the rock" so it flows faster to the sea, says
         seismologist Goran Ekstrom at Harvard University,
                          who led the study.


                          Source: New Scientist
 Case Study : Antarctica’s Post-Glacial Rebound Earthquakes


Antarctica a unique case
    •majority of land mass still glaciated
    •limited observations available for crustal motion
    •No reports of large intraplate EQs in region



                                         Earthquake – March 25, 1998, Mw=8
                    •                     East-West strike does not coincide with
                                                  strikes of transform fault
                    •                       Northeast/Southwest orientation opposite of
                                                  orientation that occurs on transform fault



 Tsuboi et. al. uses deconvulision method
    (inversion of body waves) to provide 5
    subevents
 Model of historic crustal response shows
    horizontal crustal motion about epicenter in
    northeast direction, coincideing with deduced
    fault mechanisms
                                              References
Danesi, S. “Peculiar Seismicity in Antarctica: Swarms of glacial earthquakes with a recurrent magnitude under David
    Glacier, Victoria Land”. Geophysical Research Vol.8 2006

Dewitt, Sarah. “Glaciers Spur Alaskan Earthquakes” Journal of Global and Planetary Change. July 2004

“Earthquakes Canada” Natural Resources Canada 2007 (accessed March 15, 2007)
     <<http://earthquakescanada.nrcan.gc.ca>>

Ekstrom, Goran et. al. “Glacial Earthquakes”. Science Vol. 302, October 2003

Ekstrom, Goran et. al. “Seasonality and Increasing Frequency of Greenland Glacial Earthquakes” Science Vol. 311, March
     2006

Johnston, A.C. “Suppression of Earthquakes by Large Continental Icesheets”
    Nature Vol. 330, No. 3, December 1987

Hecht, Jeff “Glacial Earthquakes Rock Greenland Icesheet” Science Vol. 311, (1747-1756), March 2006

Staniford, Stuart. “Greenland, or Why you might care about Ice Physics”. The Oildrum. Jan.28, 2007 (accessed March 15,
     2007) <<http://www.theoildrum.com/story/2005/12/9/31522/5910>>

Steven, Earl “Icequakes” Earth Science News 2003 (accessed March 15, 2007)
    <<http://www.mala.bc.ca/~earles/ice-quakes-oct03.htm>>

Tsuboi, Seiji et. al. “The March 25, 1998 Antarctic Earthquake: Great Earthquake caused by postglacial rebound” Earth,
    Planets, Space, Vol. 52, (133-136), 2000

Winberry, Paul J. et. al. “Seismicity and Neotectonics of West Antarctica”. Geophysical Research Letters, Vol.30, No.18,
    2003

Wu, Patrick “Intraplate Earthquakes and Postglacial Rebound in Eastern Canada and Northern Europe” Dynamics of the Ice
    Age Earth: a Modern Perspective. TransTech Publications, Zurich, Switzerland, (603-628) 1998

				
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