Earthquakes do not occur around the earth at random, but are mainly concentrated in limited belt-like zones. Figure 10.1 shows the epicentral distribution of large shallow earthquakes (M ≥ 7.5, focal depth ≤40 km) in about 100 years from 1903 to 2005. The five numbered large circles are the great earthquakes of Mw greater than 9.0. The first instrumental magnitude scale M was proposed by Richter (1935). However, M was proposed to apply to moderate-scale earthquakes and focus on relatively short-period seismic waves, so it is not appropriate as a quantitative scale for great earthquakes of M 8-9 class. Kanamori (1977) proposed the new magnitude scale MW, using seismic moment based on the fault length and amount of displacement. Magnitude MW and M are nearly identical for moderate earthquakes with a fault length 100 km or less, so the conventional Richter magnitude expresses an appropriate magnitude for such earthquakes. In this book, M is used to express the Richter magnitude for earthquakes of M 7.5-8 or smaller and the moment magnitude MW for earthquakes of greater magnitude.