The magnitude 9.2 Indian Ocean earthquake of December 26, 2004, produced
broad regions of uplift and subsidence. In order to define the lateral
extent and the downdip limit of rupture, scientists from Caltech,
Pasadena, Calif.; NASA's Jet Propulsion Laboratory, Pasadena, Calif.;
Scripps Institution of Oceanography, La Jolla, Calif.; the U.S. Geological
Survey, Pasadena, Calif.; and the Research Center for Geotechnology,
Indonesian Institute of Sciences, Bandung, Indonesia; first needed to
define the pivot line separating those regions. Interpretation of
satellite imagery and a tidal model were one of the key tools used to do
this.
These pre-Sumatra earthquake (a) and post-Sumatra earthquake (b) images of
North Sentinel Island in the Indian Ocean, acquired from the Advanced
Spaceborne Thermal Emission and Reflection Radiometer (ASTER) instrument
on NASA's Terra spacecraft, show emergence of the coral reef surrounding
the island following the earthquake. The tide was 30 plus or minus 14
centimeters lower in the pre-earthquake image (acquired November 21, 2000)
than in the post-earthquake image (acquired February 20, 2005), requiring
a minimum of 30 centimeters of uplift at this locality. Observations from
an Indian Coast Guard helicopter on the northwest coast of the island
suggest that the actual uplift is on the order of 1 to 2 meters at this
site.
In figures (c) and (d), pre-earthquake and post-earthquake ASTER images of
a small island off the northwest coast of Rutland Island, 38 kilometers
east of North Sentinel Island, show submergence of the coral reef
surrounding the island. The tide was higher in the pre-earthquake image
(acquired January 1, 2004) than in the post-earthquake image (acquired
February 4, 2005), requiring subsidence at this locality. The pivot line
must run between North Sentinel and Rutland islands. Note that the scale
for the North Sentinel Island images differs from that for the Rutland
Island images.
The tidal model used for this study was based on data from JPL's
Topex/Poseidon satellite. The model was used to determine the relative
sea surface height at each location at the time each image was acquired,
a critical component used to quantify the deformation.
The scientists' method of using satellite imagery to recognize changes in
elevation relative to sea surface height and of using a tidal model to
place quantitative bounds on coseismic uplift or subsidence is a novel
approach that can be adapted to other forms of remote sensing and can be
applied to other subduction zones in tropical regions.
ASTER is one of five Earth-observing instruments launched December 18,
1999, on NASA's Terra satellite. The instrument was built by Japan's
Ministry of Economy, Trade and Industry. A joint U.S./Japan science team
is responsible for validation and calibration of the instrument and the
data products.
The broad spectral coverage and high spectral resolution of ASTER provides
scientists in numerous disciplines with critical information for surface
mapping, and monitoring of dynamic conditions and temporal change. Example
applications are: monitoring glacial advances and retreats; monitoring
potentially active volcanoes; identifying crop stress; determining cloud
morphology and physical properties; wetlands evaluation; thermal pollution
monitoring; coral reef degradation; surface temperature mapping of soils
and geology; and measuring surface heat balance.
The U.S. science team is located at NASA's Jet Propulsion Laboratory,
Pasadena, Calif. The Terra mission is part of NASA's Science Mission
Directorate.