GIS Analysis of the Slope Failures Caused by the 1923 Kanto Earthquake and Comparison With the Events of the 2004 Mid Niigata Prefecture Earthquake

Hazard Analysis of earthquake-induced slope failures is important in understanding what areas may be susceptible to slope failures in future earthquakes. Geographic Information System (GIS) is useful to understand the distribution and characteristic of slope failures, including elevation, slope angle, slope aspect, geology, and distance from epicenter. A spatial analysis for slope failures caused by the 2004 Mid Niigata Prefecture earthquake (the study area is about 820 square km) shows that slope failures occurred at 1711 locations and the total area was about 4 square km. The failures caused by the Mid Niigata event are compared with the slope failures triggered by the 1923 Kanto earthquake (M7.9). The Kanto earthquake triggered about 20,000 slope failures in Kanagawa Prefecture, and the total area reached about 58.5 square km (the study area is about 1200 square km). The major difference in distribution of failures between the Mid Niigata event and the Kanto event is distribution of slope angles. In the Mid Niigata event, 90% of the failures occur on slopes less than 30°, while more than 30% of the failures occur on slopes steeper than 30° in the Kanto event. For the Mid Niigata event, slope failure density (defined as slope failures per square km) values decrease from about 10 slope failures per square km at epicentral distance of 3 km to less than 1 slope failures per square km at greater than 10 km from epicenter. In Tanzawa-Oyama area, western region of Kanagawa Prefecture, relations with slope failure density values and distance from epicenter of the aftershock (M7.3) generated on January 15, 1924 of the Kanto event have the similar tendency with the data for the Mid Niigata event. The maximum of slope failure density values reaches about 50 slope failures per square km at epicentral distance 1 km, and the values become less than about 15 slope failures per square km at more than 15 km from epicenter. As for the main shock and another aftershock (after 5 minutes from the main shock, M7.3) of the Kanto earthquake, the similar tendency in the relations with slope failure density values and distance from epicenter is not observed. It means that the aftershock on January 15, 1924 may trigger many slope failures in Tanzawa-Oyama area. Comparing the failures of the Mid Niigata event with the failures of the Kanto event, it is obvious that the scale of slope failures is terrible in the 1923 Kanto earthquake.