1986 年 39 巻 4 号 p. 621-634
In the present study, we have accumulated most complete seismological data and reinvestigated the seismicity associated with the 1944 Tonankai earthquake. Because of poor observation and social difficulty during the World War II, incompleteness and insufficiency in observational data resulted in rather obscure view about the actual process and mechanism of the earthquake. Number of aftershocks and their location accuracy reported by existing studies seem to be dissatisfactory for detailed discussion.
We found some observational data that are still available but have not been used yet, then, we applied a modern hypocenter location method to the newly compiled data and succeeded in increasing the number of accurately located aftershocks more than twice. The followings are important findings of the study.
The mainshock initiated from the bottom of the fault plane, in the south-west corner of the focal region. This feature is in accordance with the results of some previous studies. Most of aftershocks concentrated in the vicinity of the Shima spur, which extends south-east from the Shima peninsula to the Nankai trough. Temporal broadening of the aftershock region toward south-east along the spur is recognized and this trend seems to continue over 40 years until now. Distribution of aftershocks around Shionomisaki, the southern top of the Kii peninsula, may suggest some overlapping of the focal regions of the Tonankai and the 1946 Nankaido earthquake. Aftershock activity or induced seismicity is also recognized in the middle part of Shizuoka prefecture where some seismologists assume the focal region of a future interplate earthquake, so called Tokai earthquake. Seismicity around the southern Izu peninsula may represent an induced activity along one of the active faults which strikes southeast from the peninsula. Other induced seismic activity in inland area of central Honshu indicates a large extent of the effect of the interplate earthquake on the tectonic environment of the mentioned area. This study will contribute to gain better understanding of the process and mechanism of the Tonankai earthquake.