The north east of Japan is subject to major earthquakes every 30 – 40 years. The last major quake was 32 years ago in 1978. But we have to go back 1142 years to the 869 Jōgan earthquake to find a similar magnitude and accompanying tsunami. This has led Japanese analysts to call the Great Sendai Quake of 2011 a one in a thousand year event.
“Three tsunami deposits have been identified within the Holocene sequence of the Sendai plain, all formed within the last 3,000 years, suggesting an 800 to 1,100 year recurrence interval for large tsunamigenic earthquakes. In 2001 it was reckoned that there was a high likelihood of a large tsunami hitting the Sendai plain as more than 1,100 years had then elapsed”.
The 1978 Miyagi earthquake occurred at 17:14 local time on 12 June. It had a magnitude of 7.7 and triggered a small tsunami. The earthquake caused 28 deaths and 1,325 injuries.
Earthquakes with similar magnitudes have occurred in this region periodically, about every 40 years. Such earthquakes include the ones that occurred in 1793, 1835, 1861, 1897, 1936, and 1978. The 2005 Miyagi earthquake is not considered to be the one that was expected to follow the 1978 Miyagi earthquake. More recent comparisons have confirmed the differences between the 1978 and 2005 events.
Over a thousand passengers were stranded in Sendai railway station after bullet train services were stopped. Ten families were evacuated from their homes for fear of landslides. About 35,000 homes lost power supplies but electricity was restored for most within a few hours.
A highway in northeastern Japan was temporarily closed. Phone networks were snarled. Regional utility Tohoku Electric Power Co Inc said an 825,000-kilowatt (kW) nuclear reactor, the Onagawa No.3 unit near Sendai automatically shut down due to the quake. Japan’s largest oil refiner, Nippon Oil Corp, said it had started shutting down its 145,000 barrels per day Sendai refinery, although its facilities appeared to be undamaged.
In Tokyo, buildings shook violently and lamps swayed, sending workers scurrying to doorways.
But the last time an earthquake with a magnitude similar to that 3 days ago on 11th March was on 13th July 869 AD where traces of a tsunami have been found about 4km inland.
The 869 Jōgan earthquake and tsunami struck the area around Sendai in the northern part of Honshu on the 13 July. The earthquake had an estimated magnitude of 8.6 on the surface wave magnitude scale. The tsunami caused widespread flooding of the Sendai plain, with sand deposits being found up to 4 km from the coast.
The tsunami caused extensive flooding of the Sendai plain, destroying the town of Tagajō. Archaeological investigations have identified the remains of 8th and 9th century buildings beneath the town, covered by sediments dated to the middle of the 10th century.
The estimated magnitude of the earthquake as 8.6 on the surface wave magnitude scale, has been taken from modelling of the tsunami. A source area of 200 km long by 85 km wide with a displacement of 2 m is consistent with the observed distribution and degree of flooding.
Three tsunami deposits have been identified within the Holocene sequence of the Sendai plain, all formed within the last 3,000 years, suggesting an 800 to 1,100 year recurrence interval for large tsunamigenic earthquakes. In 2001 it was reckoned that there was a high likelihood of a large tsunami hitting the Sendai plain as more than 1,100 years had then elapsed.
This time 1142 years after the 869 Jōgan earthquake and tsunami the focal zone was some 500 km long and 200 km wide with three simultaneous quakes within the zone.