The Great East Japan Earthquake was preceded by small, repeating quakes that migrated slowly to where the disaster eventually hit last March 11, according to researchers at the University of Tokyo citing a seismological phenomenon known as "slow slip."
The team from the university's Earthquake Research Institute said the activity occurred in two stages ahead of the magnitude-9.0 quake, the fifth most powerful on record.
The findings were published online Jan. 20 in the U.S.-based Science journal.
To better understand the processes involved, the team led by Aitaro Kato, an assistant professor of seismology at the ERI, analyzed records of seismic stations in Miyagi and Iwate prefectures from before the massive quake hit.
The team looked at 1,416 quakes, including tiny temblors, in the records of 14 seismometers operated by the Japan Meteorological Agency, Tohoku University and others.
The research revealed two sequences of small earthquakes whose locations migrated south in the month before the disaster.
The first sequence occurred from mid- to late February, whereas the second was observed between the initial magnitude-7.3 shock of March 9 and the March 11 main shock.
The February sequence migrated south at a speed of 2 to 5 kilometers per day, whereas the March sequence traveled about 10 km per day. Those migration velocities were roughly the same as those of slow slip observed in the past.
The southbound earthquake sequences were caused by slow slip, the researchers concluded.
Slow slip refers to a phenomenon where plates slide against each other along their boundary so slowly that humans sense no shock on the ground surface.
The migrating slow slip is likely to have caused stress to build up near the source of the Great East Japan Earthquake, the scientists said. It remains unknown, however, if all gigantic earthquakes are preceded by similar precursors.
"It is essential to continue monitoring to see whether similar phenomena accompany other earthquakes," Kato said.
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