(3)The North Tango Earthquake (March 7, 1927, M 7.3)


The North Tango Earthquake occurred in a shallow location, and its source region extended on land from near the Tango Peninsula (Tango Hanto) in northwest Kyoto to the coast. There was strong seismic ground motion over a wide area, and seismic intensity 5 in JMA scale was observed in Kyoto and Nara, about 100 km from the focal region. There was ground motion that corresponded to a seismic intensity of 6 in JMA scale near the focal region, judging from the extent of the damage (Fig.7-20). The damage caused by this earthquake was the most severe at the root of the Tango Peninsula in northwest Kyoto and extended over a wide area that included the Kinki, Hokuriku, Chugoku, and Shikoku regions. There were a total of 2,925 fatalities (Fig.7-21).

There was slipping of the earth's surface with this earthquake at the Gomura fault and at the Yamada fault, which were perpendicular to each other (Fig.7-22). The western side of the Gomura fault rose a maximum of 80 cm relative to the eastern side, and slipped a maximum of 2.7 m to the south. The northern side of the Yamada fault rose a maximum of 70 cm relative to the southern side, and slipped a maximum of 80 cm to the east. Also, repeated measurements show that the crustal deformation on the Tango Peninsula side, whose boundary is the Gomura fault, slipped to the northwest, while the opposite side slipped to the southeast (Fig.7-23). These deformations suggest that the earthquake was generated by the force of compression in a roughly east-west direction. This is in rough conformity with the currently known crustal movements in the vicinity.

Survey results on active faults form the basis for the hypothesis that the interval of activity for the Gomura fault and the Yamada fault that caused the North Tango Earthquake is several thousand years.

Special observations were made for an aftershock investigation of North Tango Earthquake, and a three-dimensional distribution of the aftershocks was obtained for the first time. The largest aftershock (M 6.5) occurred on April 1, about one month after the main shock. While the number of aftershocks increased temporarily from the end of March to the beginning of April, they later diminished (Fig.7-24). In addition, the aftershocks also occurred at sea and the main shock generated a tsunami. This is the basis for the hypothesis that the source region of the main shock extended into the sea. Therefore, earthquakes that occur in shallow locations on land sometimes generate a tsunami when they occur near the coast.