Abstract
The 1854 (Ansei) Nankai earthquake is examined on the basis of historical materials, the wave height distribution of tsunami, the hypocenter distribution of recent microearthquakes in Shikoku and its surrounding areas and so on. The aftershock activity is higher in the central part than in the east part of Shikoku, while it is the reverse in the case of the 1946 (Showa) Nankai earthquake. From daily numbers of felt earthquakes in Kochi and Usa which were written in the Kiya Earthquake Diary and the Shinkakuji Earthquake Diary of historical materials, respectively, the difference of aftershock activities between those towns is clearly seen. Although the distance between those towns is only about 15km, felt earthquakes in Usa are two to three times as many as those in Kochi. It may be due to that the aftershocks are less active in Kochi than in Usa and their hypocenters are very shallow. Such phenomena suggest that the focal location of the main shock is located off the Tosa Bay. This inferrence is supported also by the height distribution of tsunami.
