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.