The current situation of tsunami geology under new policies for disaster countermeasures in Japan

This was the first known earthquake with a magnitude greater than 9.0 and a recorded maximum tsunami run-up (40.0 m, Mori et al., 2012) in the past 1,300 years of Japanese historical records.The earthquake and tsunami caused 15,885 deaths, with 2,623 missing (as of 10 April 2014; Metropolitan Police Department, 2014), and severe accidents at the Fukushima Daiichi Nuclear Power Plant.Although no clear differentiation of hazard and disaster exists in Japanese terminology, the Central Disaster Management Council (CDMC) (2011) defined the terms: "The 2011 off the Pacific coast of Tohoku Earthquake refers to the earthquake hazard of magnitude 9.0 that occurred on 11 March 2011 and was named by the Japan Meteorological Agency based on a standard naming convention.The Great East Japan Earthquake, named subsequently by the Cabinet, refers to the earthquake and tsunami disaster and the accompanying nuclear accidents".However, the "2011 Tohoku-Oki (or Tohokuoki) earthquake and tsunami" is more popularly used among scientists because important scientific journals such as Nature and Science have adopted the term for this event.The sizes of both the 2011 earthquake and tsunami were far greater than previously estimated.Therefore, disaster prevention countermeasures against such hazards were insufficient.The event was designated as an unexpected ("souteigai" in Japanese) hazard (e.g., Goff et al., in press).However, even before the 2011 event, historical and geological results of studies had suggested the potential occurrence of a low-frequency large earthquake and tsunami along the Pacific coast of Tohoku (e.g., Abe et al., 1990;Minoura, 1990;Watanabe, 2001).In fact, the 869 Jogan earthquake and tsunami, which is now regarded as a possible predecessor of the 2011 event (e.g., Goto et al., 2011; Namegaya and Satake, 2014), has been well studied since the late 1980s.Based on geological studies of the Jogan tsunami, the minimum inundation area was 3-4 km inland from the paleo-shoreline (Sawai et al., 2007).An earthquake of Mw>8.3-8.4 is necessary to explain the distribution of tsunami deposits (Satake et al., 2008a;Namegaya et al., 2010;Sugawara et al., 2011), and the magnitude is now revised as Mw>8.6 based on the lessons learned from the 2011 event (Namegaya and Satake, 2014).The tsunami recurrence interval was estimated based on geological evidence accumulated over 3,000 years: approx.800 years (Minoura and Nakaya, 1991), 600-1,300 years (Sawai et al., 2007), and now revised to be approx.500-800 years (Sawai et al., 2012).Therefore, in a geological sense, the occurrence of such a large tsunami at the Pacific coast of Tohoku was almost on time.However, geological information