**1. Introduction**

A mega earthquake off the Pacific coasts of the Tohoku region, Japan, caused tsunamis, with an inundation height over 20 m, in 2011 (e.g., [1]). These tsunamis were generated by a rise, or a subsidence, in the seabed. For example, if a part of the seabed rises, owing to an underground fault movement, then the seawater over the deformation lifts, resulting in an increase in the

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

seawater's potential energy, then, in order to resolve this energy imbalance, waves, i.e., tsunamis are generated and travel in all available directions.

Tsunamis are, however, triggered not only by such a submarine earthquake, but also other phenomena as illustrated in **Figure 1**: landslide (e.g. [2]), sector collapse, glacier fall, submarine eruption, meteorite impact, and others. In the latter cases, where a tsunami source is not directly connected to an earthquake, the wave height of generated tsunamis could be under‐ estimated or be beyond estimation, by the general prediction for tsunamis based on only seismic data, as in cases with a tsunamigenic earthquake [3]. In this chapter, we concentrate our discussion on tsunami generation caused by landslide, or submarine eruption.

**Figure 1.** An illustration for examples of tsunami sources.

**Figure 2.** A huge boulder, which was proposed to have been carried by tsunamis, on Ishigaki Island, Japan. This photo was taken in 2013, and provided by Dr. T. Iribe. He is the person in this photo and is 1.71 m tall.

First, we will study several characteristics of tsunamis induced by landslides, or sector collapses. It is proposed that the 1771 Meiwa Earthquake Tsunami occurred owing to a submarine landslide, for although the earthquake was not strong according to the Japanese archive, huge boulders on Ishigaki Island, Japan, were determined to have been carried by the tsunamis [4]. Shown in **Figure 2** is a boulder supposed to have been carried by an older tsunami, which hit the same island. In 1792, the tsunamis due to a landslide, or a sector collapse, at Mt. Mayu, Japan, traveled over the Ariake Sea, resulting in a runup on the opposite shore, which killed more than 15,000 people [5]. **Figure 3(a)**, and **3(b)**, show the eroded slope of Mt. Mayu, and the view of its opposite side, respectively; the distance between them is about 20 km. Such tsunamis are not necessarily generated only by sands or rocks: an excursion ship could be hit by tsunamis, due to a partial collapse of glacier near a coast on Svalbard Islands, Norway [6].

seawater's potential energy, then, in order to resolve this energy imbalance, waves, i.e., tsunamis

Tsunamis are, however, triggered not only by such a submarine earthquake, but also other phenomena as illustrated in **Figure 1**: landslide (e.g. [2]), sector collapse, glacier fall, submarine eruption, meteorite impact, and others. In the latter cases, where a tsunami source is not directly connected to an earthquake, the wave height of generated tsunamis could be under‐ estimated or be beyond estimation, by the general prediction for tsunamis based on only seismic data, as in cases with a tsunamigenic earthquake [3]. In this chapter, we concentrate

**Figure 2.** A huge boulder, which was proposed to have been carried by tsunamis, on Ishigaki Island, Japan. This photo

was taken in 2013, and provided by Dr. T. Iribe. He is the person in this photo and is 1.71 m tall.

our discussion on tsunami generation caused by landslide, or submarine eruption.

are generated and travel in all available directions.

36 Tsunami

**Figure 1.** An illustration for examples of tsunami sources.

**Figure 3.** The eroded slope of Mt. Mayu in Nagasaki Prefecture (a), and the view of its opposite side in Kumamoto Prefecture (b). The distance between them is about 20 km. The author took these photos in 2012.

These tsunamis are generated through an interaction between water motion, and falling bodies, such that the tsunami generation process is rather complicated. An experimental study on tsunami generation due to a rigid object sliding down a slope, was reported by e.g. Wiegel [7], while tsunami generation due to particles moving down a slope, was studied by e.g. Walder et al. [8], and Shigihara et al. [9], using sands for falling bodies, Shigematsu and Kohno [10] glass beads, and Mohammed and Fritz [11] naturally rounded river gravel. In the laboratory experiments by Riu et al. [12], the falling bodies were glass balls with different diameters, glass beads, natural stones, acrylic rock‐shape blocks, ice balls, etc.

Tsunami generation due to a landslide, has been also investigated numerically, using various methods (e.g., [13–21]). Wu and Liu [22] had simulated tsunami generation from a rigid wedge sliding down a slope, using a modified volume of fluid (VOF) method, as well as a moving boundary algorithm, and compared their three‐dimensional numerical results, with the corresponding experimental data obtained by Raichlen and Synolakis [23]. In this chapter, we will study several fundamental characteristics of tsunami generation caused by a landslide, or a sector collapse, on the basis of two‐dimensional vertical results, obtained through a numerical simulation using a moving particle semi‐implicit (MPS) model [24], where the falling body is assumed to be a fluid, or a rigid body, which moves down a slope with a constant gradient.

Second, we examine tsunamis caused by a submarine eruption. When a submarine explosive eruption occurs, volcanic products are blown out, as in case for an explosion from a subastral active volcano. Conversely, if some volume of magma is released out of a chamber, owing to a submarine volcanic eruption, ground subsidence occurs, leading to a creation of a caldera. Although both volcanic products, and a caldera, should cause tsunamis as mentioned by Egorov [25], and Maeno et al. [26], respectively, we take up a different tsunami source, peculiar to a submarine eruption, for discussion in this chapter, i.e., a phreatomagmatic explosion [27]. In the process with a submarine phreatomagmatic explosion, seawater contacts high temper‐ ature magma in the seabed neighborhood, after which the seawater evaporates with an explosive increase in its volume, resulting in a water surface displacement that generates tsunamis. A new index for submarine volcanic explosion, concerning tsunami generation, is developed by assuming the relationship between a phreatomagmatic explosion, and the resultant initial tsunami waveform. We specifically assume the value of this index, to generate a numerical simulation for tsunamis caused by a submarine volcanic eruption in Kagoshima Bay, Japan, where a submarine explosion with tsunami generation has been observed [28].
