Tsunami was expected after large quake
By Jennifer Lauren Lee
Jan. 6, 2005 9:00 p.m.
The deadly wall of water that devastated the coastal communities
of nearly a dozen countries in South Asia on Dec. 26 was the
ocean’s natural response to the massive underwater
earthquake, UCLA geophysicists say.
“As far as I know, it behaved itself as it should
have,” said John Vidale, professor of geophysics at UCLA.
“The only surprise was the earthquake. Nobody expects an
earthquake that big.”
Tsunamis can occur following any earthquake on an underwater
fault-line, provided the earthquake is big enough, said Emily
Brodsky, assistant professor of geophysics at UCLA.
“The earthquake moves the ground underneath the
ocean,” Brodsky said. “Imagine a big bucket of water.
If you punch the bottom of it, it’s going to create a big
wave.”
Though the earthquake released more than 20,000 times as much
energy as an atomic bomb, according to National Geographic, it was
the tsunami that caused the most damage, said Heidi Houston,
associate professor of geophysics at UCLA.
“I don’t think the shaking caused much
damage,” Vidale said. “I’m not sure they even
felt the earthquake.”
Vidale explained that the wave would have been barely noticeable
at the epicenter of the earthquake, which took place in the middle
of the Indian Ocean, roughly 200 kilometers away from Sumatra.
In the open ocean, the energy put into the water by the
earthquake is spread over a wide area. But as the ground becomes
shallower, the wave becomes larger and more powerful as its energy
becomes concentrated into a smaller volume of water.
“The weird thing about a tsunami is you don’t see a
very large swell out on the ocean,” Brodsky said. “It
builds up to a higher and higher front that crashes onto the
shore.”
The height of each wave of the tsunami also depends on how
shallow coastal waters are, Brodsky said, leading to 50-foot waves
in some areas, according to the National Geographic.
“The tsunami is as big as the earthquake,” Vidale
said, pointing out that this one “moved the sea floor a
couple meters up and down.”
Tsunamis are usually caused by what Brodsky calls
“shallow” underwater earthquakes.
Earthquakes occur when two of the plates that make up the
earth’s crust rub against one another as they move. But the
point at which the two plates rub together might occur nearly at
the surface, or hundreds of kilometers beneath the earth’s
crust. The closer the earthquake occurs to the surface, the more
likely it is to cause a tsunami. “This earthquake happened
right at the top,” Brodsky said.
Vidale adds that the two plates converging at the Sumatra
faultline are also meeting head-on in a “subduction
zone,” with one plate being pushed on top of the other
““ a situation which is conducive to producing tsunamis.
Because of its abundance of subduction zones, the Pacific Ocean
is thought to be at an even greater risk for tsunamis than the
Indian Ocean, Houston said. But although it is possible for a
tsunami to hit the coast of Los Angeles, Brodsky, Houston, and
Vidale say it is unlikely.
“The nearest fault is on land,” Vidale said,
“so it’s not likely to shake the water.”
In addition, Vidale said, the fault lines near Los Angeles are
of a different type from the fault lines which tend to produce
tsunamis. “The faults here are more sideways,” Vidale
said. “They don’t push the water up or pull it
down.”
Houston adds that Los Angeles is unlikely to have an earthquake
with a magnitude as high as 9.0 because the plates converging under
Los Angeles do not rub against one another over as broad or deep an
area as they did in Sumatra. “There’s no techtonic
structure that will produce a big enough fault plane,”
Houston said.
Despite the unlikelihood of a tsunami occurring locally, Houston
warns everyone to be aware of the signs of an impending tsunami.
“If you’re at the beach and the water withdraws or you
feel strong shaking, leave the beach and get onto higher ground
““ even in Los Angeles,” Houston said.
Although some of the waves did not reach shore until six hours
after the earthquake, Houston said, most victims were taken by
surprise.
“One of the reasons there was no warning system in place
was that there had not been that many tsunamis,” Houston
said. “They’ve had them, they just haven’t had
them sufficiently close to that region.”
Unlike the Indian Ocean, the Pacific Ocean has a tsunami warning
system in place, with seismometers to detect earthquakes, as well
as sensors to identify unusually large waves in the open ocean and
the receding tide characteristic of a coming tsunami, Vidale said.
A “call-down” system is also in place for contacting
all the countries in the area efficiently, Vidale said. “That
sort of structure is not in place in the Indian Ocean,”
Brodsky said. “It is not a particularly well-instrumented
part of the world.”
Brodsky hopes that the disaster will bring people together to
create a similar tsunami warning system in the Indian Ocean.
“The most important thing societally to come out of it is a
better interaction between scientists and society,” Brodsky
said. “It will improve communication between scientists and
policy makers, so we’re not just doing our science in a
box.”
