[H-DS10] Geohazards in humid, tectonically active countries and their precursors

[HDS10-P01] Landslide hazards induced by heavy rainfall in August 2022 in Aomori Prefecture, Japan

*Ching-Ying Tsou¹ (1.Faculty of Agriculture and Life Science, Hirosaki University)

Keywords:landslide hazard, rainfall, Aomori, geomorphological characteristic, geological characteristic

In August 2022, Aomori Prefecture in Japan experienced two major rainfall events from the dates of 2-3 and 8-13, bringing significant amounts of rainfall. These events resulted in widespread landslides, flooding, and infrastructure damage. Geological and geomorphological surveys of the landslides were conducted one to two months after the events. At Stogahama, three shallow landslides occurred in weathered basalt and traveled downslope, and developed into a debris flow. The landslide scars are located at a zero-order stream and foot slopes of a coherent landslide. According to the eyewitness accounts, the debris flow occurred at 4:30 AM on August 12th, 2 hours after the peak of rainfall intensity of 31 mm/hr and an accumulated rainfall of 479 mm since the start of the rainfall on 9 August. The estimated maximum velocity of the debris flow was 100 km/hr, calculated based on a runout of 840 m and a duration of 30 sec. At Tanosawa area of Fukaura, a shallow landslide, about 2 m deep, occurred in terrace deposits underlain by andesite pyroclastic rock at the upper reach of a first-order stream. The transformation of the landslide into a debris flow with a runout of about 140 m resulted in the destruction of houses and the blocking of a railway. An eyewitness reported that the landslide happened at 15:00 PM on August 9th, when heavy rainfall reached its peak intensity of 60 mm/hr. Furthermore, a large-scale landslide measuring 55 m in width and 600 m in length, occurred in Kurosaki area of Fukaura. It is assumed that the landslide occurred at 14:00 PM on August 9th, 2 hours after the rainfall reached its peak intensity of 47 mm/hr and accumulated rainfall of 170 mm had fallen since the start of the rainfall event. The landslide was reactivated from a coherent landslide that occurred on the rim of a 60-m-high marine terrace. The topographic map derived from Lidar DEM prior to the landslide revealed that 1-5 m high scarps and middle slope undercutting due to road construction were precursors to the landslide. The geological structure of the landslide is characterized by dip slopes of mudstone underlain by conglomerate. Additionally, a northwest-trending fault that separates the upper and lower strata seems located near the mid-slope of the landslide. The head of the landslide may have been formed along this fault, but further investigation is needed. It is also a future task to examine whether there was infiltration of water due to the fault and the resulting increase in water pressure within the slope.