Abstract
We observed time sequential changes of slopes prone to deep-seated landslides using aerial photographs and satellite images covering the last 60 years in the middle reach of the Ooigawa River Basin, in central Japan. We detected five deep-seated landslides that had shown significant expansion over the last 60 years（since 1947）, and examined their topographic features. In many cases, preceding small collapses were recognized on the foot of the bulging slope and otherwise, landslides expanded toward the upslope or laterally. The Houkinagi Landslide, the largest in this area, has shown a high expansion rate and significant mass creep over the past century, including a tension crack opening at a rate of around 10cm/year, found behind the main scarp. These phenomena found in deep-seated landslides indicate that precedent landslides at the foot of bulging slope will be useful premonitory features if accompanied with significant mass creep behind them.
