March 27, 2011

Earthquake & Tsunami History in Sendai Area

A field survey from Japan's Port and Airport Research Institute put the height of the tsunami wave that struck Japan's Iwate Prefecture on March 11th at 23.5 meters or 77.4 feet high. 


The Fukushima power plants were required by regulators to withstand a certain height of tsunami. At the Daiichi plant the design basis was 5.7 metres and at Daini this was 5.2 metres. 


There has been a history of even larger tsunami waves hitting the Sendai area.  

Japan's domestic record for tsunami wave height was set at 38 meters or 125 feet during the 1896 Meiji Sanriku Earthquake and Tsunami, which killed 22,000 - 27,000 people. Surprisingly the massive tsunami wave was generated by a relatively small 7.2 magnitude earthquake. 

More recently, the 1933 Sanriku earthquake (magnitude 8.4) generated a tsunami wave of 28.7 meters or 94 feet, which killed 1,500 - 3,000 people. 

A much larger tsunami hit the Sendai area in 869AD, with tsunami waves reaching as far as 4 km or 2.5 milles inland.

The quake that triggered the tsunami was 8.6 magnitude, also much larger than the design standards for the nuclear reactors in the area. 

"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."


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