The recognition and dating of tsunami deposits is an important part of paleoseismology . The extent of a particular deposit may help to judge the magnitude of a known historical earthquake or to act as evidence of a prehistoric event . In the case of the 869 Sanriku earthquake , the identification of tsunami deposits over 4.5 km inland on the Sendai Plain , dated quite closely to a historical tsunami event , enabled the magnitude of this earthquake to be estimated and the likely rupture area offshore located . Two earlier deposits with similar character were identified and dated . The three deposits were used to suggest a return period for large tsunamigenic earthquakes along the Sendai coast of about 1,000 years , suggesting that a repeat of this event was overdue and that large scale inundation was likely . In 2007 , the likelihood of a great tsunamigenic earthquake striking this coast in the next 30 years was given as 99 % . Based partly on this information TEPCO revised estimates of likely tsunami heights at the Fukushima Daiichi Nuclear Power Plant to greater than 9 m , but took no immediate action . The tsunami triggered by the 2011 Tohoku earthquake had a wave height at Fukushima of about 15 m , well above the 5.7 m for which the plant 's defences had been designed . The inundation distance of the tsunami was almost identical to that reported for the three earlier events , as was the lateral extent
2011 Tohoku earthquake
2011 Tōhoku earthquake and tsunami
Tsunami deposit