The Great East Japan Earthquake revealed the grim fact: The Japan Meteorological Agency is ill-equipped to quickly assign a magnitude to huge quakes striking near the Japanese archipelago.
The Great East Japan Earthquake revealed the grim fact: The Japan Meteorological Agency is ill-equipped to quickly assign a magnitude to huge quakes striking near the Japanese archipelago.
The agency took nearly three hours after the giant quake struck off northeastern Japan on March 11 to announce a magnitude close to 9.
That announcement came about two hours after the United States Geological Survey (USGS) estimated its magnitude at 8.9.
The agency's work to determine the size of the earthquake was delayed because the temblor was too powerful to be handled by its seismometers.
Still smarting from the delay, the agency is now considering the introduction of a different type of seismometer for swift estimates of the magnitudes of huge quakes.
The devastating earthquake occurred at 2:46 p.m. on March 11. Three minutes later, the agency announced that a magnitude-7.9 quake had taken place.
At 4 p.m., the agency revised the estimate of the magnitude up to 8.4. But the figure was again revised up to 8.8 at 5:30 pm.
Meanwhile, the USGS announced at 3:20 p.m. its fairly accurate estimate of 8.9.
Both institutions eventually revised their magnitude estimates to 9.0. The Japanese agency did so on March 13, while the USGS released a preliminary revision to 9.0, six hours after the seismic event. The U.S. estimate was confirmed on March 15.
Because of the logarithmic basis of the scale, each whole number increase in magnitude represents a tenfold increase in measured amplitude and corresponds to an increase of about 30 times in the amount of energy released.
The March calamity confirmed that the traditional method is not capable of accurately calculating the size of a huge quake with a magnitude exceeding 8.
The Meteorological Agency's estimate of the Great East Japan Earthquake based on the method was 8.4.
But the agency also has a system for automatically computing quake magnitudes by using special seismometers designed to detect long-period seismic waves.
But the March temblor was so powerful that all these seismometers, except the ones far away from the epicenter in Okinawa and the Ogasawara islands, swung past the maximum, thus preventing the system for automatic magnitude analyses from working properly.
The system can also calculate magnitudes in about 40 minutes by using records of seismic signals made by instruments overseas. But the cataclysmic disaster also swamped seismometers in South Korea.
The agency then looked for usable overseas data and used them to estimate the magnitude to be 8.8. But the process took a lot of time.
The agency is now considering the use of a type of powerful seismometer that can observe long-period seismic waves suitable for measuring massive earthquakes without being overwhelmed by the great amplitudes of the ground motions.
The 2004 Indian Ocean earthquake off the west coast of Sumatra highlighted the need to develop a more effective system to quickly estimate the magnitude of this kind of gigantic earthquake, which tends to trigger towering tsunami and cause serious damage.
At that time, the USGS first estimated the quake's magnitude to be 8.2 in about 30 minutes and then revised the number to 9 after 20 hours of analyses.
In addition to the conventional method, the organization also employed an algorithm using the "W phase" wave, very long-period energy traveling along the Earth's surface that can provide rapid and reliable estimates of the source parameters of large seismic events.
The W phase source inversion algorithm is capable of automatically calculating the magnitude of a huge earthquake at a distance by about 20 minutes and that of one occurring nearby in about six minutes.
The magnitude-8.9 estimate announced by the USGS some 34 minutes after the Great East Japan Earthquake was based on the traditional method.