In a race against time when every minute means the difference between life and death, earthquake researcher Satoshi Miura wants to be ready the next time a big quake hits, triggering a tsunami.
On March 11, the Japan Meteorological Agency (JMA) initially estimated the massive tremors emanating from the Great East Japan Earthquake at a magnitude of 7.9.
The agency would later upgrade the magnitude to an astounding 9.0, which meant the inaccurate early assessment led to huge underestimates of tsunami heights and dire consequences for those who were caught unaware.
That experience is driving seismologists such as Miura to seek new methods to better assess giant quake magnitudes both promptly and accurately.
"The magnitude can be calculated in about four minutes," said Miura, a professor at the Earthquake Research Institute of the University of Tokyo, of a new method he is developing through joint research with Tohoku University.
Miura has shown that the magnitude can be calculated on the basis of a rough estimate of the size of the underground seismic source fault by using data provided by the Geospatial Information Authority of Japan, which is monitoring crustal deformation at 1-second intervals using GPS.
Miura is currently trying to develop a system to issue tsunami warnings in five minutes.
Along with Miura, accurate earthquake magnitude assessment is also a popular topic with members of the Seismological Society of Japan.
At an October meeting in Shizuoka, presentations included "Instantaneous magnitude estimation" and "A simple method of magnitude estimation."
The JMA has also been calculating earthquake magnitudes and predicting tsunami heights for some time now, but scientists are revisiting these subjects because the March 11 disaster exposed limitations of the agency's current evaluation method.
Seismic waves are made up of a wide gamut of components, ranging from "rattling" waves of short periods to "billowing" waves of long periods.
The JMA is using a method that relies only on waves of short periods up to 6 seconds to calculate earthquake magnitudes in about 3 minutes. The method was introduced after tsunami warnings, issued five minutes after the magnitude-7.8 Hokkaido Nansei-oki earthquake struck near Okushiri island in 1993, turned out to be too late to save the lives of hundreds of people.
For earthquakes of magnitudes up to about 8, accurate measurement is usually not a problem.
When the magnitude exceeds 8, however, long-period waves grow larger, whereas short-period waves tend to remain much the same in amplitude. This means that the JMA method tends to underestimate magnitudes for such events.
The moment magnitude (Mw) is an alternative measurement of earthquake size that takes long-period waves into consideration and can be accurate for events with magnitudes exceeding 8. However, it takes more than 15 minutes to calculate Mw, because doing so requires about 10 minutes' worth of seismic records. It is therefore impractical to use it for issuing tsunami warnings, where speed is of the essence.
Instead of using Mw, the JMA has decided to first determine, in about three minutes, whether the magnitude is over or under 8, and to issue maximal tsunami warnings when they are likely to exceed 8. Seismic intensity distributions are expected to be useful in making that decision.
Shocks of at least "lower 6" on the JMA seismic intensity scale were observed over a stretch of about 250 kilometers when the magnitude-8 Tokachi-oki earthquake struck to the south of Hokkaido in 2003. Shocks of corresponding intensities were observed over more than 450 km during the magnitude-9 Great East Japan Earthquake. Drawing upon this fact, the Meteorological Research Institute is currently developing a technique to estimate the magnitude on the basis of the distribution of seismic intensity.
A search continues for methods to determine the magnitude quickly, even at the cost of accuracy.
Hiroo Kanamori, a professor emeritus at the California Institute of Technology, and coworkers have developed an alternative technique to calculate the moment magnitude by focusing on seismic waves called the "W phase."
The W phase refers to seismic waves of long periods (100-1,000 seconds) that arrive between the P waves, or the primary part of seismic records, and the S waves, or the secondary and principal part of seismic motion. Seismic waves that precede the S waves are relatively small, but they are large enough to be analyzed when the earthquake magnitude is 6 or larger.
Unlike in conventional methods, the analysis can be done quickly, because it uses only the initial part of seismic records.
The calculation takes only about 6 minutes for an earthquake that happens in a region with many deployed seismometers, and can be finished in about 20 minutes for a giant earthquake in a region that has few seismometers. The U.S. Geological Survey introduced the method in 2008.
Following the Great East Japan Earthquake, Kanamori and his coworkers analyzed the quake's W phase using data provided by the National Research Institute for Earth Science and Disaster Prevention. The data showed that the earthquake magnitude could have been estimated at 9.1 seven minutes after it struck if data had been acquired in real time. The JMA is also using this calculation technique on a trial basis.
The W phase helps to give an accurate estimate for the magnitude of a tsunami earthquake that the JMA magnitude may tend to underestimate. However, calculation may be unsuccessful if the earthquake is preceded immediately by another major earthquake striking somewhere else, experts said.
One lesson learned from the March 11 disaster is that there should be more than one method to cope with all possible scenarios.
Tatsuhiko Hara, a chief research scientist at the Building Research Institute, has developed a technique to calculate the magnitude in about 12 minutes on the basis of the amplitude and duration of P waves in a specific period range.
The Meteorological Research Institute is also comparing different calculation methods that use the P waves.
Scientists such as Hara are hoping the lessons learned from the recent disaster will pay off when the next mega-quake hits.
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