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Glacial Earthquakes

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)

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Glacial Earthquakes

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  1. Glacial Earthquakes Huda Elasaad March 28, 2007

  2. 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

  3. Glacial Earthquakes- Two Sided Coin • 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

  4. Icequake Seisomogram Glacial (M=5.0) and Tectonic (M=4.2) earthquake in Alaska (Sept.1999) Seismograms aligned at predicted P-wave arrival time

  5. Documented Locations of Icequakes Current Seismic activity Previous record of seismic activity

  6. 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)

  7. 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

  8. 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)

  9. Icequake Mechanisms – Glacier Motivated 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 Post Glacial Rebound 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

  10. 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

  11. 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

  12. Frequencies: Greenland & Alaska 1993 1999 2003 Southern Alaskan Earthquakes – 1 Decade Source: NASA

  13. 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 In The News…

  14. 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

  15. 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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