Padangekspres.net-Friday's earthquake off the coast of the Tohoku region likely occurred along the boundary where the Pacific Plate is subducted under the inland plate.
It had a similar mechanism to the 2004 earthquake off the coast of Sumatra, Indonesia, that triggered the huge tsunami that killed tens of thousands of people.
At the same time, Friday's earthquake was much stronger than any predicted for the Tohoku region.
The focus of the first large quake was off the coast of Miyagi Prefecture, but aftershocks also occurred off the coast of Fukushima and Ibaraki prefectures further south.
Officials of the Japan Meteorological Agency believe that a fault of about 400 kilometers running from northern Iwate Prefecture to the boundary between Fukushima and Ibaraki prefectures was destroyed.
According to an analysis by Yuji Yagi, an associate professor of geodynamics at the University of Tsukuba, the earthquake was triggered by the destruction of a fault with a length of 600 kilometers and a breadth of 200 kilometers.
There was extensive destruction at three locations, and the Earth's surface may have shifted by as much as 8 meters in some areas.
The larger the scale of an earthquake the larger is the area of the fault that is destroyed.
The meteorological agency announced a magnitude of 8.8, while the U.S. Geological Survey in Colorado announced a magnitude of 8.9.
That means the earthquake likely exceeded the magnitude 8.6 to 8.7 earthquake that hit in 1707 through a serial earthquake in the Tokai, Tonankai and Nankai regions caused by the destruction of a fault boundary that extended from the Tokai region to off the coast of Shikoku.
Takashi Furumura, a professor at the Division of Disaster Mitigation Science at the University of Tokyo's Earthquake Research Institute, said, "It was the largest scale of earthquake that could have occurred in Japan."
He said it had an energy about 90 times greater than the one predicted for off the coast of Miyagi Prefecture.
The government's Headquarters for Earthquake Research Promotion had forecast that there was a 99 percent probability over the next 30 years of an earthquake with a magnitude of between 7.5 and 8 occurring off the coast of Miyagi Prefecture.
That prediction was based on earthquakes occurring off the southern part off the Sanriku coast and further toward land off the coast of Miyagi Prefecture.
Kunihiko Shimazaki, professor emeritus at the University of Tokyo who chairs the Coordinating Committee for Earthquake Prediction, said, "It might have been the one included in the long-term forecast for occurring off the southern part of the Sanriku coast near the Japan Trench."
Earthquakes occurred in that region in 1793 and 1896, or at nearly 100-year cycles.
Shimazaki said, "If that earthquake occurs concurrently with one off the coast of Miyagi Prefecture, there is the possibility of an earthquake occurring with a magnitude around 8."
The region has been hit recently with a series of earthquakes. An earthquake with a magnitude of 7.3 occurred on Wednesday to the northeast of Friday's focus.
Masakazu Otake, a professor emeritus at Tohoku University who is the past chair of the coordinating committee, said, "The earthquakes that occurred over the past few days may have been a precursor and the seismic energy might have erupted in a moment."
Hirofumi Yokoyama of the meteorological agency said, "It is a very rare example to have a major earthquake after one with a magnitude of 7."