What this really means is that GPS stations can be set up in target
areas and we really know most of them anyway, or at least to the
level of resolution needed. With that it will become possible to
determine what moved and better still immediately understand the
available tsunami energy.
That leads directly to a predictive output map produced in real time
minutes after the Earth moved. With that early warning equipment and
first responders already are targeted and in possession of
contingency orders. Better yet it will be possible to discount false
negatives in which a quake takes place but fails to produce
sufficient seabed energy.
Right now vulnerable areas need evacuation plans. Until now it has
been mostly pointless because you did not know until you saw the
wave unless it was a major event that woke everyone up. Recall
Indonesia in which the quake effectively went unremarked. Much the
same happened in Japan although an evacuation was underway in places
but unconvincing because of prior false alarms.
When the siren howls and you can turn on your TV to discover you are
in a must run zone, the system will be working.
GPS Solution
Provides Three-Minute Tsunami Alerts
May 17, 2013 —
Researchers have shown that, by using global positioning systems
(GPS) to measure ground deformation caused by a large underwater
earthquake, they can provide accurate warning of the resulting
tsunami in just a few minutes after the earthquake onset. For the
devastating Japan 2011 event, the team reveals that the analysis of
the GPS data and issue of a detailed tsunami alert would have taken
no more than three minutes.
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The results are
published on 17 May inNatural Hazards and Earth System Sciences, an
open access journal of the European Geosciences Union (EGU).
Most tsunamis,
including those in offshore Sumatra, Indonesia in 2004 and Japan in
2011, occur following underwater ground motion in subduction zones,
locations where a tectonic plate slips under another causing a large
earthquake. To a lesser extent, the resulting uplift of the sea floor
also affects coastal regions. There, researchers can measure the
small ground deformation along the coast with GPS and use this to
determine tsunami information.
"High-precision
real-time processing and inversion of these data enable
reconstruction of the earthquake source, described as slip at the
subduction interface. This can be used to calculate the uplift of the
sea floor, which in turn is used as initial condition for a tsunami
model to predict arrival times and maximum wave heights at the
coast," says lead-author Andreas Hoechner from the German
Research Centre for Geosciences (GFZ).
In the new Natural
Hazards and Earth System Sciences paper, the researchers use the
Japan 2011 tsunami, which hit the country's northeast coast in less
than half an hour and caused significant damage, as a case study.
They show that their method could have provided detailed tsunami
alert as soon as three minutes after the beginning of the earthquake
that generated it.
"Japan has a very
dense network of GPS stations, but these were not being used for
tsunami early warning as of 2011. Certainly this is going to change
soon," states Hoechner.
The scientists used
raw data from the Japanese GPS Earth Observation Network (GEONET)
recorded a day before to a day after the 2011 earthquake. To shorten
the time needed to provide a tsunami alert, they only used data from
50 GPS stations on the northeast coast of Japan, out of about 1200
GEONET stations available in the country.
At present, tsunami
warning is based on seismological methods. However, within the time
limit of 5 to 10 minutes, these traditional techniques tend to
underestimate the earthquake magnitude of large events. Furthermore,
they provide only limited information on the geometry of the tsunami
source (see note). Both factors can lead to underprediction of wave
heights and tsunami coastal impact. Hoechner and his team say their
method does not suffer from the same problems and can provide fast,
detailed and accurate tsunami alerts.
The next step is to
see how the GPS solution works in practice in Japan or other areas
prone to devastating tsunamis. As part of the GFZ-lead German
Indonesian Tsunami Early Warning System project, several GPS stations
were installed in Indonesia after the 2004 earthquake and tsunami
near Sumatra, and are already providing valuable information for the
warning system.
"The station
density is not yet high enough for an independent tsunami early
warning in Indonesia, since it is a requirement for this method that
the stations be placed densely close to the area of possible
earthquake sources, but more stations are being added," says
Hoechner.
Note
Traditional tsunami
early warning methods use hypocentre (the point directly beneath the
epicentre where the seismic fault begins to rupture) and magnitude
only, meaning the source of the earthquake and tsunami is regarded as
a point source. However, especially in the case of subduction
earthquakes, it can have a large extension: in Japan in 2011 the
connection between the tectonic plates broke on a length of about
400km and the Sumatra event in 2004 had a length of some 1500km. To
get a good tsunami prediction, it is important to consider this
extension and the spatial slip distribution.
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