Ten minutes after the March 11 earthquake, a Japan Meteorological Agency observatory accurately calculated the magnitude, but the figure was not used to revise early tsunami warnings, which underestimated the waves' height, The Asahi Shimbun has learned.
Ten minutes after the March 11 earthquake, a Japan Meteorological Agency observatory accurately calculated the magnitude, but the figure was not used to revise early tsunami warnings, which underestimated the waves' height, The Asahi Shimbun has learned.
The Matsushiro Seismological Observatory in Nagano city estimated the magnitude of the Great East Japan Earthquake at 9.0, using a system that calculates magnitudes of earthquakes overseas.
"The Matsushiro magnitude was not part of our operation routine," said a meteorological agency official about conditions on March 11. "So we could not have used it even if we had been informed of it."
Earthquake magnitude estimates are the basis for tsunami warnings.
About three minutes after the earthquake struck at 2:46 p.m. on March 11, the meteorological agency's main office evaluated its magnitude at 7.9. The agency issued "major tsunami warnings" with projected heights of 3 meters in Iwate and Fukushima prefectures, and 6 meters in Miyagi Prefecture. Thirty minutes later, the tsunami height projections were revised to 6 meters and more than 10 meters, respectively, on the basis of tsunami observations by Global Positioning System wave gauges.
Some regions never knew about the revised warnings because of disrupted communication lines.
On June 22, the meteorological agency decided to include Matsushiro magnitude estimates for earthquakes in Japan in its evaluations to avoid underestimations.
The "Matsushiro method" is prone to large margins of error, but produces estimates quickly.
In 2001, the Matsushiro Seismological Observatory developed a technique to quickly evaluate scales, called "Matsushiro magnitudes," of major overseas earthquakes by analyzing data found on the Internet.
When the 9.1-magnitude Sumatra-Andaman Earthquake struck in 2004, the Matsushiro system, which was under trial operation, estimated its magnitude at 8.8 about 15 minutes after it took place.
Since 2005, the observatory has been sending estimated magnitudes of large overseas earthquakes to the meteorological agency's main office in Tokyo.
The Matsushiro observatory also calculated magnitudes of earthquakes in Japan on a trial basis, although it was not tasked to do so. When the Great East Japan Earthquake hit, the observatory estimated its magnitude at 8.9 seven minutes after it took place, 9.0 three minutes later and 9.1 after 13 minutes. A difference of 1.0 in magnitude corresponds to about a thirtyfold difference in energy released by an earthquake.
The meteorological agency's routine system produces earthquake magnitude estimates in three minutes. But the system does not include seismometers capable of accurately evaluating giant earthquakes, leading to maximum magnitude estimates of about 8.5, and running the risk of underestimations.
It takes the agency at least 15 minutes to calculate magnitudes of giant earthquakes using data collected from more accurate types of seismometers. Information given by the Matsushiro observatory 10 minutes after an earthquake would not be in time for the first tsunami warnings, which are generally issued within three minutes of an earthquake. However, the data would be useful in revising the warnings.
The Matsushiro Seismological Observatory uses tunnels dug by the Imperial Japanese Army in 1944-45 for an underground military headquarters. Seismic observations began in the tunnels after the end of World War II.
The observatory is part of an international seismic observation network, and is also used to monitor nuclear tests.