Massive landslide triggered by 2008 Iwate–Miyagi inland earthquake in the Aratozawa Dam area, Tohoku, Japan

Abstract

On June 14 2008, an Iwate–Miyagi inland earthquake that had a magnitude of 7.2 hit the eastern foot of the Ohu Mountains in Tohoku district, Japan. The seismic peak ground acceleration was greater than 1,000 gal in the Aratozawa Dam area. The earthquake triggered a massive landslide at the upper reach of the dam. The landslide had the sediment volume of over 67 million cubic meters and is considered the largest catastrophic landslide in Japan during the last 100 years. This report presents a summary of our findings pertinent to the landslide’s activities based on our field investigations that started the day after the landslide. This report covers: (1) details of the land deformations caused by the landslide, (2) geological background pertinent to landslide development, and (3) estimation of the slip surface and the other physical properties of the landslide based on the analysis of the boring core specimens and landform features. The landslide is roughly divided into two sections, a lower and an upper half. The lower half moved almost simultaneously as one massive block of 700 m long, 800 m wide, and 70–80 m thick. The slip surface had developed on the very fine sand of the alternate layer of fine-grained sandstone and siltstone. The slickensided slip surface has a gradient of only 2°. This feature indicates that the type of the landslide movement is considered to be a block glide. The landslide body is nearly identical to the topography of the landslide area that was developed about 50,000 years ago. This shows the possibility that the landslide was reactivated. The upper half consists of two large ridges and the broad debris field and is 600 m long, 900 m wide, and 70–100 m thick. The maximum height of the main scarp is over 150 m.