The 1923 Kanto earthquake (M7.9), the most disastrous earthquake in Japan, is the first earthquake which left abundant seismometrical data of the main shock and aftershocks. It provides us with one of the best opportunities of studying aftershock activity and long term change in seismicity after an interplate earthquake. However, due to an uncertainty of data quality of that day and difficulty in integrating the available data, detailed examination of aftershocks has been remained untouched except several preliminary investigations carried out by the Central Meteorological Observatory and universities immediately after the earthquake in term of the current knowledge of seismology.
In this study, we made a complete integration of the data existing separately. The intensive data of half year after the 1923 event ensures statistical treatments such as making of S-P histograms and daily frequency of earthquakes observed at each station which may approximately indicate the extent of aftershock zone and time variation of the aftershock activity. By using S-P times mainly and P and S arrivals independently in some cases, we locate about 400 earthquakes. Although each epicenter may include location error of an order of 20km, relatively large extent of aftershock area makes the aftershock distribution show a good correspondence with the faulting models and the seismicity pattern of today.
A high concentration of aftershocks is recognized in the border of Yamanashi and Kanagawa Prefectures, the western boundary of faulting area where the main shock originated. Another concentration of aftershocks suggests that eastern boundary of the faulting zone exists in the middle of the Boso Peninsula. While northern boundary of aftershock region is defined rather clearly near the border of Kanagawa and Tokyo Prefectures, southern limit of aftershock zone is ambiguous because poor location accuracy.
Compared to the periphery of the aftershock area, two inactive areas of aftershock activity are recognized at the middle of faulting region which corresponds to low seismicity areas found with recent microearthquake studies and the nucleation zone of the main shock where large dislocation of the faulting is estimated by the dynamic source process studies. This result suggests that basic seismicity pattern of present day in the southern Kanto region including the 1923 focal region has been created from its aftershock activity. Seismicity pattern before 1923 might be almost similar to those of today except middle of the Boso Peninsula. Relatively high seismic activity is recognized there immediately after the 1923 earthquake and it has taken ten to twenty years the seismicity to recover the normal level.
Magnitude frequency relation of the aftershocks was compared with those of four interplate earthquakes of similar large size in Japan. It shows that the Kanto earthquake is accompanied with larger number of aftershock of M>6. Prominence of aftershock activity may be attributed to the complexity of tectonic setting of the focal region which is located near the collision boundary of the Philippine Sea plate and the Eurasian plate.
