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Frequency
spectrogram of T wave showing relative strength
of different frequencies through time |
When
the sea floor off the coast of Sumatra split on the
morning of December 26, 2004, it took days to measure
the full extent of the rupture. Recently, researchers
at Columbia University's Lamont-Doherty Earth Observatory
analyzed recordings of the underwater sound produced
by the magnitude 9.3 earthquake. Their unique approach
enabled them to track the rupture as it moved along
the Sumatra-Andaman Fault, raising the possibility
that scientists could one day use the method to track
underwater earthquakes in near real time and opening
new avenues in seismologic research.
Listen
to the December 26th earthquake (mp3) 
"We were able to constrain some details such
as the speed and duration of the rupture more accurately
than traditional seismic methods," said Maya Tolstoy,
a Doherty Research Scientist and lead author of the
study. "Moreover, we found the earthquake happened
in two distinct phases, with faster rupture to the
south and slower to the north, almost as if there were
two back-to-back events." The study appears in
the July/August edition of Seismological Research
Letters.
The earthquake occurred along
a stretch of the fault known as a subduction zone,
where the India Plate is slowly being pushed beneath
the Burma Plate. "The
fault basically unzipped from south to north," said
DelWayne Bohnenstiehl, a Doherty Associate Research
Scientist and co-author of the study. "By looking
at the direction the sound comes from, you can get
a pretty clean look at the way it broke."
When an earthquake occurs
underwater, part of its energy is released in the
form of sound, known as a tertiary or T wave, which
travels great distances through the ocean. T waves
travel significantly slower than primary and secondary
(P and S) waves, both of which are often recorded
in an overlapping pattern that can obscure important
parts of the seismic record. As a result, oceanic
sound energy sometimes provides a more direct look
at the entirety of a large underwater earthquake. "It's
like the hare and the tortoise" said Tolstoy, "The
tortoise is moving a lot more slowly, but it gets the
right answer in the end".
The T waves for the Sumatra earthquake were captured
by underwater microphones located at Diego Garcia,
more than 1,700 miles from the epicenter. These microphones
are part of arrays known as hydroacoustic stations
that are scattered throughout the world's oceans to
listen for the telltale sound of an atomic blast.
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Waveform
showing T wave amplitude |
"We can hear icebergs cracking and magnitude
four earthquakes underwater from across the ocean basin,
so it's not surprising that we heard this earthquake," said
Tolstoy.
What is surprising, however, is the fact that the
earthquake appeared to occur in two distinct phases.
The first phase encompassed the first three minutes
of the eight-minute earthquake, during which the rupture
proceeded north at about 1.7 miles per second (2.8
km/sec) from the epicenter. During the second phase,
the rupture slowed to 1.3 miles per second (2.1 km/sec)
and continued north for another five minutes until
it reached a plate boundary where the fault changes
from subduction to strike-slip, where the two plates
push past one another in opposite directions. This
suggests that had the subduction continued, this longest
ever recorded earthquake might have been even longer.
The method that the researchers
used also shows promise for helping officials quickly
determine where relief activities are needed. In
the case of the Indonesian earthquake, early seismic
data indicated that only the southernmost third of
the fault was involved. Later analysis revealed that
about 750 miles actually ruptured, a finding that
was supported by Tolstoy and Bohnenstiehl’s
study.
Because hydroacoustic stations like the one at Diego
Garcia operate around the clock, Tolstoy believes they
may hold the potential to provide a rapid and accurate
source of information on the duration and length of
an underwater earthquake, information that is critical
in determining where to send emergency relief in the
first hours of a disaster, as well as in determining
the risk of a tsunami.
Recently, the International
Monitoring System of the CTBTO began making their
data available on a trial basis to tsunami warning
organizations recognized by UNESCO. Tolstoy hopes
that eventually scientists will gain easier access
to these data as well, which would help them learn
more about the basic processes of the Earth. "There
is an opportunity here to make a contribution to international
disaster monitoring, as well as help us better understand
earthquakes and tsunamis and potentially mitigate these
events in the future." said Tolstoy. "It
makes sense to let others listen in.
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