The land-slide and mud-flow disasters caused by the Naganoken-seibu Earthquake and the Quaternary geology of the southern slope of Mt. Ontake

Abstract

The Naganoken-seibu Earthquake of M6.8 occurred at 8 : 48a.m. on the 14th of September, 1984. It centered at about 35.5°N, 37.5°E. The hypocenters of the first and aftershocks were rather shallow, and were distributed within a small area on the southern slope of Mt. Ontake, a solitary volcano, 3063 m high, of Quaternary age. Large and small land slides triggered by the earthquake and associated mud flows devastated the villages and forests, and killed tens of men and women. Several members of the present authors were just surveying the geology of the area, and they, together with other members, made an urgen survey of the land slides and mud flows. The Quaternary strata of the southern slope of Mt. Ontake are classified as shown in the following table. [table] Among the four Quaternary formations the Tarusawa Formation, composed of thick series of lava flows, beds of tuff-breccias, volcanic mud-flows and beds of pyroclastic falls, is the main constituent of the present mass of Ontake Volcano, while the overlying Ohtaki Formation is composed of lava flows, beds of agglomerates, beds of tuff-breccias, beds of sand and gravel and aeolian volcanic ash beds. Significant is the remarkable unconformity between the two formations. In other wards, the rocks of the Tarusawa Formation had been eroded deeply, so that many deep and steep valleys and slopes were existent before the deposition of the Ohtaki Formation. Many of the land-slides occurred in the rock sequence of the Ohtaki Formation under the following conditions. 1) Many of the land slides (or collapses) occurred where thick beds of pumices and scoria of the Ohtaki Formation, overlain by inclined beds of lavas and tuff-breccias, accumulated in the valleys or on the slopes that had been eroded out of the Middle Pleistocene Tarusawa Formation. The Ohtaki Formation in and on the steeply inclined valleys and slopes were unstable from the beginning. 2) Where the basal surfaces of the Upper Pleistocene Ohtaki Formation made valley forms the groundwater gathered into the valley bottoms, and it expedited weathering and argillization of pumices and scoria. It also made easy to slide the overlying load. 3) The precipitation of the preceding five days had attained about 150 mm, which seeped into the rocks of the Upper Pleistocene Ohtaki Formation. It raised probably the groundwater table, and also increased pore-water pressure near the base of the deposits.