TSUNAMIGENIC LANDSLIDES IN JAPAN ARC: TOPOGRAPHY, GEOLOGY AND THEIR MECHANISMS

Landslides generate large tsunamis. I discuss the potential tsunamigenic landslides in Japan arc. Famous on-land example is the Mayuyama landslide on southwest Japan. There is an active volcano Unzen nearby this landslide. The volcanic activity was being high from 1791 and the landslide occurred on 21 May 1792 by an earthquake. The landslide body has been flown into the sea to generate a tsunami of 22.5 m run-up height (Tsuji and Hino, 1993). The opposite coast of the landslide area was attacked suddenly by this tsunami and killed ~15,000 people. Rain fall also generates generally on-land landslides. Tsunami might be potentially generated if landslides occur on a coastal area. The formation mechanisms of the landslides should be related to pore water pressure associated with ground water level, but we do not know exactly the formation mechanism. We need further monitoring, survey and observation of potential tsunamigenic onland landslides. Underwater landslides could generate large tsunamis. We have a few lines of evidence for underwater tsunamigenic landslides so far. One example is 1896 Meiji Sanriku earthquake on the northeast Japan. This earthquake occurred on 15 June 1896 and generated a large tsunami of 38.2 m by run-up height. (JMA website). This earthquake was called tsunami earthquake, which was characterized by small surface wave magnitude (Ms = 6.8; Utsu, 1979) but large tsunami magnitude (Mt = 8.6; Abe, 1979). Kanamori and Kikuchi (1993) suggested that this tsunami was excited fundamentally by a large submarine landslide, which was triggered by the earthquake. The rupture area of the 2011 Tohoku-Oki earthquake on 11 March 2011 includes the hypocenter of the Meiji Sanriku earthquake. The large earthquake also generated a large tsunami of 37.8 m (Tsuji et al., 2012) or 39.7 m (TTJS website). There is a submarine landslide of 2-3 km wide and 10 km long at the tsunami source area. This landslide could excite the large tsunami (Kawamura et al., 2012, 2014). We believed that rupture by earthquake could generate seafloor deformation with tsunami, but tsunami generation might be closely related to underwater landslides being instability of a slope (Kawamura et al, 2014). We need carefully to study the tsunamigenic underwater landslides being invisible marine geohazards.