TOTTORI – An active fault beneath the town of Hino, Tottori Prefecture, that is believed to have caused a magnitude 7 earthquake in the year 880 has been discovered by the Central Research Institute of the Electric Power Industry, institute officials said Saturday.
The institute said the 3-meter-deep fault may have also caused the magnitude 7.3 quake that rocked western areas of the prefecture on Oct. 6 last year.
The fault is located next to another fault and was found based on the locations of aftershocks of the October quake. The two lines also run in the same direction.
If the fault did cause both quakes then the likelihood of another powerful earthquake hitting the region is drastically reduced, as seismic energy stored in the fault would have been released by the October quake, the officials believe.
Until last year’s quake, neither western Tottori Prefecture nor eastern Shimane Prefecture had been affected by a major earthquake in living memory.
Current observations for earthquake predictions in eastern Shimane will be reviewed if the October quake lessened the danger of a strong quake in the near future, the officials said.
The Nov. 23, 880, quake, dubbed the Izumo quake, primarily affected Shimane Prefecture and caused extensive damage to buildings in the area, including Izumo Shrine. The epicenter of the quake was estimated to be west of Lake Shinji in eastern Shimane Prefecture.
A 30-cm vertical gap and a horizontal gap measuring more than a meter wide were confirmed along the newly identified active fault, the fault line running in a northwest-southeast direction.
In the October quake, the fault is believed to have shifted about 1.4 meters along the same line at a depth of about 1 km beneath the surface.
The institute, which in November began studying last year’s quake by using aerial photographs to analyze the ground, discovered a 20 km-long formation.
After excavating part of the central section of the formation, the institute uncovered the active fault.
Daiei Inoue, a senior member of the institute, said, “The faults in this region have been active in 1,000-year cycles and the newly discovered active fault is believed to have triggered the last October quake, although a more minute analysis is necessary.”
Yasuhiro Umeda, a professor of Kyoto University and a member of the Coordinating Committee for Earthquake Prediction, said it is an important discovery, adding that discussions on reviewing current seismic observations will be necessary.
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