Reviewing the tectonic setting.

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Challenges in observational seismology with
special reference to the 2004 Sumatra-Andaman
Islands earthquake

• Reviewing the tectonic setting.
• Reviewing what happened during the earthquake.
• Reviewing some of the effects of the earthquake.
• Comparing the earthquake to previous earthquakes

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COMPLEX PLATE BOUNDARY ZONE IN SOUTHEAST
ASIA Northward motion of India deforms the
region Eastward motion in China SE Asia Many
small plates (microplates) and blocks India
subducts beneath Burma microplate
EURASIAN PLATE
INDIAN PLATE
SUNDA PLATE
BURMA MICROPLATE
Molnar Tapponnier, 1977
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Stein Wysession 2003
Partitioning of strain between thrust motion at
the trench and strike-slip motion on the Sumatra
fault
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Layer anomaly maps
Left panels solutions representing upper
mantle, transition zone and lower mantle
structures Right panels results of resolution
tests
Widiyantoro van der Hilst (Science 1996)
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Vertical sections across the Sunda
arc
Widiyantoro van der Hilst (Science 1996)
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Cartoon of 3D seismic structure beneath the
Sunda arc (Widiyantoro van der Hilst, Science
1996)
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Newcomb and McCann, 1987
lack of recent large earthquakes
M gt 5.5 between 1960-2002
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The Earthquake

• Date 26th December 2004
• Time 0058 53s UTC 0758 53s Local time
• Locality 225km SSE of Banda Ache, Northern
  Sumatra
• 3.267 North, 95.821 East
• Depth 10km

http//www.geop.itu.edu.tr/taymaz/sumatra/
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Initial estimates of magnitude Mwp 8.0 - 8.5
(using 60 sec P waves) Reported by the
NEIC Ms 8.5 - 8.8 (using 20 sec surface
waves) Mw 8.9 - 9.0 (using 300 sec surface
waves) Focal mechanism
T
P
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Cause of the earthquake

• Caused by the Indian plate subducting beneath the
  Burma micro plate.
• Tip of Burma micro plate is dragged down with
  the Indian plate.
• Burma micro plate springs back to its
  equilibrium position, causing simultaneous uplift
  on its western tip and slight depression slightly
  further east.

Stein Wysession, 2003
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Diagram showing both Torodial and Sheroidal modes
of the Earth.
http//www.iris.iris.edu/sumatra/free_oscillations
_second.htm
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Normal Modes of the earth after the earthquake
indicate larger magnitude than originally
believed.
Revised magnitude 9.15-9.3
http//www.iris.iris.edu/sumatra/normalmode_second
.htm
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The duration of the rupture

• Long P arrival merges with PP
• PP more attenuated than P

Stein and Wysession 2003
Ni et al 2005 Nature
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Ammon et al 2005 Science
Station to the north

• rupture lasts 500 s
• variation in duration of P indicates ruptured
  northwards
• 3 distinctive bursts of energy

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Ishii et al. 2005 Nature
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Ishii et al., 2005 Nature
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Geodetic data
Subarya et al 2006 Nature
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Aftershocks Including swarm near Nicobar Islands
(gt150 Mag 5 earthquakes in 3 days)
Lay et al 2005 Science
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The effects of the Earthquake
http//www.iris.iris.edu/sumatra/GSNwaveforms_seco
nd.htm
http//www.iris.iris.edu/sumatra/circlemap_second.
htm
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• The Day was made shorter by 2.68 microseconds.

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The quake also made the earth wobble on its axis
and shift the North Pole.
http//www.nasm.si.edu/research/ceps/etp/earth/img
/EarthAS17.jpg
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The most devastating effect of the earthquake
came from the large tsunami that was generated.
http//www.shoa.cl/servicios/tsunami/imagen/gigant
e.jpg
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Slip vector 20 degrees closer to trench-normal
than the plate motion consistent with strain
partitioning
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Comparison to other earthquakes
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Dec 2004 Mw 9.3 Sumatra
March 2005 Mw 8.6 Sumatra
June 2004 Mw 8.4 Peru
Lay et al 2005 Science
Ground displacements for the 3 largest
earthquakes in the last 40 years
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Diagram of cumulative seismic moment
More seismic energy released in this one
earthquake than all the earthquakes recorded in
the last 10 years