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<DIV class=article-heading><FONT size=3><STRONG>Nature</STRONG></FONT></DIV>
<DIV class=doi><ABBR
title="Digital Object Identifier">doi</ABBR>:10.1038/nature.2012.10480</DIV>
<DIV class=article-heading> </DIV>
<DIV class=article-heading><FONT size=4>Satellite system will speed up tsunami
warnings</FONT></DIV></HGROUP>
<DIV>GPS networks could cut time needed for accurate alerts by a factor of
ten.</DIV>
<DIV><SPAN class=vcard><A class=fn
href="http://www.nature.com/news/satellite-system-will-speed-up-tsunami-warnings-1.10480?WT.ec_id=NEWS-20120424#auth-1">Richard
A. Lovett</A></SPAN></DIV>
<DIV class=pubdate-and-corrections><TIME datetime="2012-04-20" pubdate>20 April
2012</TIME></DIV></HEADER><SECTION>
<DIV class=section>
<P class="content no-heading cleared main_content">NASA and a group of
universities known as the READI network have begun testing an earthquake-warning
system based on satellite data from the Global Positioning System (GPS). The
method could have allowed Japanese officials to issue accurate warnings of the
deadly March 2011 earthquake and tsunami ten times faster than they did, say
scientists.</P>
<P class="content no-heading cleared main_content">The system is currently being
tested using the US Pacific Northwest Geodetic Array: hundreds of GPS receivers
placed along the North American coast between Northern California and British
Columbia in Canada. The sensors provide real-time measurements of ground
movement caused by nearby and offshore seismic faults.</P>
<DIV class="content no-heading cleared main_content" align=center>
<DIV style="WIDTH: 303px; HEIGHT: 264px" class="img img-right">
<DIV
style="PADDING-BOTTOM: 1px; PADDING-LEFT: 1px; PADDING-RIGHT: 1px; PADDING-TOP: 1px"
class=img-content align=left><IMG alt=""
src="http://www.nature.com/polopoly_fs/7.3981.1334925574!/image/1.10480.jpg_gen/derivatives/landscape_300/1.10480.jpg">
<P class=caption><FONT size=1>Seismic traces record the magnitude of earthquakes
— but satellite data can do it faster and more accurately. </FONT><FONT
size=1>F. FLORIN/AFP/Getty Images</FONT></P></DIV></DIV></DIV>
<P class="content no-heading cleared main_content">Tim Melbourne, a geodesist at
Central Washington University in Ellensburg and the array's lead scientist,
addressed a meeting of the Seismological Society of America in San Diego,
California, this week. "The Cascadia subduction zone is our main focus," he
said. "That is potentially a magnitude-9 fault."</P>
<P class="content no-heading cleared main_content">The GPS receivers were
installed in the late 1980s to study plate tectonics, and were upgraded in the
early 2000s to provide hyper-accurate data for land surveyors as a supplement to
conventional seismic arrays.</P>
<P class="content no-heading cleared main_content">They are placed so that
seismic waves travelling from dangerous faults towards populated areas first hit
the sensors, and trigger a warning message. This gives officials a few seconds'
notice of approaching shock waves and helps them to work out where the epicentre
of the quake is.</P>
<DIV class="related-stories-box box">"The data get to our lab in under a tenth
of a second," says Melbourne, "and we can process that into a position estimate
good to a couple of centimetres within a half-second."</DIV>
<H2 class="content no-heading cleared main_content"><FONT size=2>Earth-shaking
accuracy</FONT></H2>
<P class="content no-heading cleared main_content">Conventional seismometers
provide similar information. On 13 April, for example, a small earthquake of
magnitude 3.5 occurred near Aromas, California. "At my desktop in Berkeley, I
got 25 seconds' warning," said Richard Allen, director of the seismology
laboratory at the University of California, Berkeley.</P>
<P class="content no-heading cleared main_content">But seismometers have
limitations. "They do a beautiful job discriminating among magnitude 2, 3, 4, 6,
earthquakes," says Melbourne. "But they run into trouble where you have to
distinguish a magnitude 8 from a magnitude 9." This is partly because in big
quakes, the ground may shake for longer, but not significantly harder.</P>
<P class="content no-heading cleared main_content">GPS has no such problem,
because it directly measures the movement of the ground. "If the ground lurches
by many metres, it's unambiguous evidence for a very large earthquake," says
Susan Hough, a seismologist at the US Geological Survey in Pasadena,
California.</P>
<P class="content no-heading cleared main_content">Melbourne and Allen use the
Japan earthquake as an example. In March 2011, Japanese officials relying on
seismometer data were able to issue earthquake warnings within eight seconds of
detecting that something major was going on. "But they thought it was magnitude
7.1," says Allen. The estimate increased to 8.1 within 2 minutes, but it took
another 20 minutes to reach the final value of magnitude 9.</P>
<P class="content no-heading cleared main_content">Meanwhile, the tsunami caused
by the earthquake was on its way, striking the Japanese coast only 30 minutes
after the earthquake. Because they had underestimated the size of the
earthquake, officials also underestimated the size of the wave, and the Japanese
public were unprepared for the damage it would cause.</P>
<H2 class="content no-heading cleared main_content"><FONT size=2>Ten times
faster</FONT></H2>
<P class="content no-heading cleared main_content">Using his own calculations
with real-time GPS data from the Japanese earthquake, Melbourne got the right
magnitude within two minutes. He is surprised that the Japanese didn't do it
themselves in 2011. "Japan has the gold standard of everything [related to
earthquakes]," he says. "I thought the Japanese would have incorporated this a
decade ago."</P>
<P class="content no-heading cleared main_content">It is hard to tell whether
more lives would have been saved. "But I have a feeling that if they had known
within two minutes, it would have been very different," says Melbourne. Seth
Stein, a geophysicist from Northwestern University in Evanston, Illinois, adds
that while Japan's warning system worked well enough to save many lives,"a lot
of people who didn't evacuate or didn't evacuate far enough didn't think it
would be that big. So it's important to get a better take, early on, about how
big an earthquake is."</P>
<P class="content no-heading cleared main_content">But not everyone thinks that
GPS is the best warning system. The main complaint is that glitches could cause
false earthquake alarms. "For example, one tends to get spikes when satellites
come over the horizon, which can generate large, spurious signals," says
Hough.</P>
<P class="content no-heading cleared main_content">Yehuda Bock, a geodesist at
Scripps Institution of Oceanography in La Jolla, California, says that the best
solution is to add inexpensive seismometers to GPS stations, so that the
instruments can back each other up.</P>
<P class="content no-heading cleared main_content">But Melbourne thinks that GPS
signal processing is already good enough to be used on its own. "At some point
you have to say, OK, it all seems to be working," he says. "We're rapidly
developing it and pushing to make it better, but all the pieces are in place
now."</P>
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