2017 Volume 126 Issue 5 Pages 641-652
The development of observational, theoretical, and experimental studies on seismology in Japan from 1945 to 1965 is described. Damage caused by the Fukui earthquake in 1948 and its nature was surveyed from different perspectives. This survey later became the model for comprehensive surveys of major earthquakes. After the Fukui earthquake, observations on micro-earthquakes were carried out with sensitive electromagnetic seismographs. These studies confirmed the Gutenberg-Richter's formula and the Ishimoto-Iida's formula for small earthquakes. Explosion seismic observations have also been conducted by the Research Group for Explosion Seismology since 1950, and crustal structures in Japan have been precisely inferred from those observations. The Earth's interior was investigated by analyzing the attenuation of body waves in the 1950s, and surface waves and free oscillations of the Earth also began to be used to investigate the Earth's interior in the 1960s. Studies on earthquake mechanisms attracted great attention from seismologists worldwide in the 1950s. The elasticity theory of dislocations was applied to the earthquake mechanism and Takuo Maruyama's sophisticated mathematical theory was suggested by Hirokichi Honda's hypothesis of a pair of couples, i.e., that force system type II at the foci of earthquakes corresponds to earthquake faults. Furthermore, Keiiti Aki's studies in the 1960s on the earthquake mechanism using Love and Rayleigh waves strongly supported the double-couple source mechanism. During this period, not only studies analyzing waveforms of earthquakes but also statistical studies on the characteristics of earthquake occurrence were conducted actively. Some seismologists, such as Chuji Tsuboi, examined time, space, and magnitude distributions of earthquakes and discussed the energy of earthquakes. Experimental studies on the mechanical properties of rocks under high pressures and temperatures began in the second half of the 1950s. Magnitude-frequency relations of elastic shocks accompanying fractures were discussed comparing the statistical nature of earthquakes. Besides, the Earthquake Prediction Research Project was launched at the initiative of seismologists in the 1960s.