Characteristics and triggers of earthquake-induced landslides of pyroclastic fall deposits: An example from Hachinohe during the 1968 M7.9 tokachi-Oki earthquake, Japan

Abstract

This work characterizes the geomorphological features and describes the factors controlling earthquake-induced landslides of pyroclastic fall deposits. Statistical analysis of the distribution of landslides caused by the 1968 M7.9 Tokachi-Oki earthquake and prior high-magnitude earthquakes was conducted using 0.5-m resolution Light Detection and Ranging data and aerial photographs. Most of the 1968 co-seismic landslides were shallower than 3.5 m and occurred on sliding surfaces with slope angles of less than 34° (average angle of 26.4°). The fitting relationship between the height and length of the 1968 co-seismic landslides was H = 0.226L with a modal apparent friction angle of 12-14°, indicating high mobility. The 1968 co-seismic landslide crowns were primarily located near ridge crests. The hillslope morphology and seismic Arias intensity direction greatly influenced the direction of the earthquake-induced landslides, which were predominantly oriented in a northwest-north-east direction. These earthquake-induced landslides occurred in a cluster where fine marginal portions of Towada-Hachinohe pyroclastic flow deposits, i.e., To-H (pfl), thinly (< 3 m) covered Towada-Hachinohe volcanic ash. Landslide surfaces were formed in the marginal To-H (pfl) layer. Light Detection and Ranging digital elevation model analysis suggests 84.7% of the 1968 co-seismic landslides were undercut by road or river incisions, which form convex slopes that reduce support for the downslope beds. This study supports a correlation between sensitive pyroclastic fall deposit materials and distribution, which can influence potential earthquake-induced landslides with putative seismic intensities of 5 or greater on the Japan Meteorological Agency scale.