Preliminary analysis of some characteristics of coseismic landslides induced by the Hokkaido Iburi-Tobu earthquake (September 5, 2018), Japan

Abstract

At 18:07 on September 5, 2018 (UTC), a Mw 6.6 earthquake struck the Iburi-Tobu area of Hokkaido, Japan. The Hokkaido Iburi-Tobu (HIT) earthquake is a reverse-slip earthquake with a focal depth of 33.4 km at 42.69°N and 142.07°E. According to the detailed landslide interpretations, there were approximately 7059 landslides induced by this earthquake concentrated in an area of 466 km2. The landslides induced by the HIT earthquake are mostly shallow landslides whose sources are mainly an air-fall pumice layer from the Tarumai volcano, and these landslides exhibit an obvious characteristic of a “low affected area, large landslide area”. The landslide area follows an exponential distribution, and many landslides are composed of multiple subsource areas. There are at least 8778 source areas in the whole landslide area. The landslides present high mobility, and the landslide deposits not only accumulate on the foot but also “flow” down along the gullies, similar to debris flows, despite the gullies being gentle (<6%), which may be related to recent heavy rainfall events. Most landslides occur on gentle hills with an elevation range of 100–250 m, and the landslide height is mainly concentrated between 15 m and 45 m. Major slide directions follow a nearly SSE-NNW direction, and all the landslides occur with PGA (peak ground acceleration) ≥0.5 g.