A chain reaction that destroyed the Earth's crust in three different areas created the huge magnitude of 9.0 for the Great East Japan Earthquake, one of the biggest ever recorded.
At a news conference Sunday, Takashi Yokota, an official in charge of earthquake prediction information at the Japan Meteorological Agency, said: "Three huge earthquakes occurred one after the other. It is extremely rare for such a complicated shaking to occur."
Ordinarily, the Japanese Meteorological Agency will announce a tentative magnitude immediately after an earthquake has struck based on the seismic waves that have been observed. After Friday's quake, the initial magnitude announced immediately after the quake was 7.9. Later, as more data was collected by the agency, the magnitude was revised upward to 8.4 and then 8.8.
On Sunday, the magnitude was revised again to 9.0.
Based on statistics at the U.S. Geological Survey (USGS) and the Japanese Meteorological Agency, 9.0 is the fourth-largest magnitude ever recorded, matching one in Kamchatka in 1952.
The new analysis was based on seismic waves observed in various parts of the world, including Australia and Finland. The additional data altered original figures because seismographs close to an earthquake are unable to analyze seismic waves that are very large.
Different magnitude results can be produced depending on the data and calculation method used.
The USGS announced a magnitude of 8.9, but it later released another analytical result that put the magnitude at 9.1
An increase in the magnitude of 0.2 is equivalent to a doubling of the seismic energy. An increase by a full point is equivalent to a 32-fold increase in the energy.
Analysis of the data produced a picture of how seismic faults were destroyed.
After the initial quake at 2:46 p.m. Friday, there were two subsequent quakes that caused major destruction.
The Meteorological Agency initially calculated a magnitude of 8.8 based only on the initial quake, in which the destruction of the crust lasted for about 90 seconds.
A minute later, the crust in an area slightly south of the original quake also began collapsing. That crust destruction also lasted for about 90 seconds.
The third crust destruction began further south. In the end, three crust destructions took place and lasted a total of six minutes.
The second and third periods of destruction were similar in scale to the first. For that reason, the seismic energy for the overall series of earthquakes became gigantic.
Meteorological Agency officials believe the size of the tectonic plate that was destroyed measured 500 kilometers in a north-south direction and about 200 km in an east-west direction.
Observations of crustal movement by the Geospatial Information Authority found the largest movement on the surface at Shizugawa in Minami-Sanriku in Miyagi Prefecture, where the crust shifted 4.4 meters to the east-southeast and sank by 75 centimeters.
Katsuyuki Abe, a professor emeritus at the University of Tokyo who chairs the government's Earthquake Research Committee, said at a news conference following a Sunday committee meeting: "We were unaware that activity on that large a scale could occur in the Tohoku region. It made us realize more than ever that nothing comes easy when dealing with nature."
The Earthquake Research Committee had predicted eight areas of the Tohoku coast where earthquakes could occur at the plate boundary. While the committee had evaluated the possibility of an earthquake occurring through the coupling of a number of such areas, it never predicted that such a large number of areas as were involved in the latest earthquake would ever occur.
At Sunday's committee meeting, the evaluation was made that Friday's earthquake was the result of a coupling of four of the eight areas off the Tohoku coast, ranging from off the coast of Miyagi Prefecture to off the coast of Ibaraki Prefecture. Some committee members also pointed to the possibility that there was a coupling with three other areas, including off the Sanriku coast and off the coast of the Boso Peninsula.
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