At 00:58:53 UTC (Coordinated Universal Time) on 26 December 2004, a magnitude 9.0 earthquake
occurred off the west coast of Sumatra, Indonesia. This was the fourth
largest earthquake in the world since 1900 and the largest in over 40
years. It was caused by the release of stresses in the Earth that are built
up as the Indian tectonic plate descends into the mantle beneath the Burma
plate. It is estimated that the sea floor was displaced several meters due
to the quake, resulting in large ocean waves, called "tsunamis" from the
Japanese for "harbor waves." The tsunami moved rapidly across the deep
ocean, with speeds estimated around 640 km/hr. When the waves reach shallow
water near land, they slow considerably, but their size increases
dramatically and they strike with catastrophic force. With human
casualities exceeding 150,000, this event is one of the deadliest natural
disasters in modern history, causing devastation along the shores of
Indonesia, Sri Lanka, India, Thailand, and other countries.
The initial tsunami waves reached the eastern Indian coast around 3:35 UTC,
based on tide gauge measurements made at the port city of Vishakapatnam.
The Multi-angle Imaging SpectroRadiometer (MISR) aboard NASA's Terra
satellite passed over the eastern Indian coast between 5:10 to 5:20 UTC,
when the tide gauge indicated the arrival of another series of waves.
Because MISR's nine cameras imaged the coast over a time span of about 7
minutes, and because the the waves are unusually large, MISR was able to
capture unique time-lapse imagery of the breaking waves. The still image
shows four frames from the instrument's backward-viewing cameras spanning a
period of about 2.5 minutes. This scene is located along the shores of
Andhra Pradesh, near the mouth of the Godavari River, and covers an area of
42 kilometers x 37 kilometers. The arrows show the progression of the
southwestern edges of the breakers. A series of frames spanning nearly 6
minutes has been made into a small animated GIF (below). An animated GIF
(5.8 MB) covering a somewhat larger area of 86 kilometers x 49 kilometers
is also available. A second animated GIF (3.8 MB) shows a region further
south, at the northern end of India's Coromandel Coast, and covers an area
of 43 kilometers x 58 kilometers. Cloud "motion" in these animations
results from apparent displacements due to parallax associated with their
height above the surface. The tsunami waves, on the other hand, are at sea
level and show actual motion. When the waves arrive in the shallower water
near the shore, they grow and, if they become large enough, they will break
in a manner similar to typical oceanic waves, but on a much larger scale.
The leading edge of the breaking waves is likely what is visible in the
imagery. Additionally, if the tsunami waves impact the coast at an angle,
they can produce what are known as "edge waves" which propagate parallel to
the coast. There is clear evidence of edge wave generation in these images.
Upon discovering the unique content of this imagery, MISR scientists
contacted Dr. Vasily Titov at the National Oceanic and Atmospheric
Administration's Pacific Marine Environmental Laboratory in Seattle, WA.
Dr. Titov is an expert in the propagation of tsunamis, and has generated a
model animation of the tsunami's progression from its origin near Sumatra
(see http://www.pmel.noaa.gov/tsunami/indo_1204.html). The MISR imagery
provides measurements of the location and timing of the breaking waves,
their angle relative to the shoreline, and their speed of propagation,
which is estimated from these data to be around 30 kilometers/hour. In
conjunction with bathymetric measurements of ocean depth, this information
can be used to refine and calibrate tsunami propagation models. According
to Dr. Titov, improving these models has two primary benefits. First, a
detailed understanding of wave interactions with coastal areas is necessary
for developing damage mitigation approaches. Second, a better predictive
capability of the models will make possible more accurate near-real-time
forecasts of tsunami arrival times and effects.
The Multi-angle Imaging SpectroRadiometer observes the daylit Earth
continuously and every 9 days views the entire globe between 82° north and
82° south latitude. These data products were generated from a portion of the
imagery acquired during Terra orbit 26720 and utilize data from within
blocks 77 and 78 within World Reference System-2 path 142.
MISR was built and is managed by NASA's Jet Propulsion Laboratory,
Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The Terra
satellite is managed by NASA's Goddard Space Flight Center, Greenbelt, MD.
JPL is a division of the California Institute of Technology.