For Tsunami Forecasters, Speed Is Everything
By Henry Fountain
Jan. 23, 2018
Left, damage from the 9.2-magnitude earthquake that struck
Alaska in 1964. Right, after a 7.9-magnitude earthquake was recorded off the
Alaskan coast on Tuesday morning, phones lit up with warnings of a tsunami.
Left, damage from the 9.2-magnitude earthquake that struck
Alaska in 1964. Right, after a 7.9-magnitude earthquake was recorded off the
Alaskan coast on Tuesday morning, phones lit up with warnings of a tsunami. Credit Left, Bruce Roberts, via Getty Images; Right, Associated
Press
The strong earthquake that sent panicked Alaskans, fearing a
tsunami, to high ground early Tuesday is a reminder that, at least around the
Pacific Rim, the biggest danger from quakes often comes not from the land but
from the sea.
Given the need for speed — deadly waves can strike within
minutes in some cases — tsunami forecasters tend to shoot first and ask
questions later, sending out broad alerts that are often scaled back or
canceled within hours.
In the case of Tuesday’s quake, waves could have struck the
nearest land, Kodiak Island, within half an hour. The U.S. Tsunami Warning
Center, one of two operated by NOAA, sent out its first alerts within 3 minutes
of the event, a typical time frame, said Michael Angove, who manages the
tsunami program at the National Oceanic and Atmospheric Administration.
The quake Tuesday turned out to be largely harmless, a
function of its 7.9 magnitude, which made it about 100 times less powerful than
the world’s strongest earthquakes, and its location in the Gulf of Alaska about
170 miles from Kodiak Island.
But being centered offshore is no guarantee against
destruction. Some of history’s most catastrophic quakes have occurred under the
sea, with most of the deaths and damage caused by water rather than falling
buildings or crumbling infrastructure.
In the 2011 Tohoku earthquake in Japan, for instance, the
Fukushima nuclear reactors survived the magnitude-9.1 quake’s initial shaking.
It was the larger-than-anticipated tsunami that struck minutes later that
doomed the plant by knocking out emergency power generators.
In the 2004 Boxing Day earthquake near Aceh, Indonesia, also
of magnitude 9.3, almost all of the more than quarter-million deaths were from tsunami waves that inundated coastal zones
around the Indian Ocean.
And although the 9.2 magnitude 1964 Alaska earthquake — the third largest quake ever recorded —
was technically centered on land, on a fjord in Prince William Sound, about 90
percent of the 131 deaths were from waves. In one native coastal village, the
water arrived while the ground was still
shaking, killing one-third of the residents.
Those quakes occurred around the Pacific Rim, as do
thousands of others every year. The
margins of the Pacific are where the tectonic process known as subduction is
readily apparent. In subduction, one large section of the earth — a plate —
slides slowly under another, as the earth’s surface recycles itself over
geologic time.
Sometimes, parts of a plate get stuck until they break in an
earthquake. If this happens near or under water, then the movement of blocks of
rock displaces massive amounts of water, spawning a tsunami.
Some kinds of quakes are much more likely to cause tsunamis
than others; so-called megathrust earthquakes, in which one side of the fault
rides up over the other, are especially likely to generate waves. The Tohoku,
Aceh and 1964 Alaska quakes all involved megathrust faults.
Waves overwhelming a levee in Japan after the Tohuku
earthquake and tsunami in 2011.
Aflo/Mainichi Newspaper/European Pressphoto Agency
But tsunami forecasters seldom have the luxury of time to
wait for an analysis of the quake’s mechanism, Mr. Angove said.
“When we have an event that’s near our coast, it’s a very
challenging problem for us,” he said.
“There is a lot of sophisticated seismic waveform analysis. But we don’t have
time to do that level of analysis when you’re talking about eight to 10 minutes
of travel time.”
Instead, he said, the decision to send out the initial alerts is based on a rapid
analysis of two factors: magnitude of the quake and its location.
On Tuesday, the advisories continued in the hours after the
quake, with some cancellations announced for parts of the west coast of the
continental United States. Small tsunamis of about half a foot were reported in
several Alaskan communities on Kodiak Island and elsewhere before the last
alerts were canceled about 3 and a half hours after the earthquake.
The decisions to maintain or cancel alerts were based on
actual readings from buoys showing water levels in key locations, Mr. Angove
said, rather than from analysis of the quake motion.
As it turned out, within a few hours the United States Geological
Survey had determined that the quake was not a megathrust one. Rather, it had
more in common with the San Andreas fault, and its horizontal motion would have
been unlikely to displace much water.
Henry Fountain covers climate change, with a focus on the
innovations that will be needed to overcome it. He is the author of "The
Great Quake," a book on the 1964 Alaskan earthquake.
NYT, ScienceKorrections - Jerzy Chojnowski
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