Kinematics of an alpine retrogressive rockslide in the Japanese Alps

The Aresawa rockslide in the Japanese Alps experienced a partial collapse (510 x 105?m3) in 2004, followed by an accelerated downward movement of the head area rather than stabilization. This paper discusses the kinematics of a retrogressive rockslide area and factors promoting slope instability subsequent to the 2004 collapse, on the basis of geodetic surveys and meteorological observations from 2006 to 2010. The unstable area covers at least 2.3 x 104?m2 and consists of five active blocks. The main blocks are separated from the stable rock slope by pre-existing sackung features that follow the strike of the bedding plane. Shear zones below these sackung features have developed into slip planes potentially causing the next collapse. In particular, the propagation of a slip plane causing movement at 70?cm?yr-1 in the head area is promoted by bidirectional movement. It consists of slow dip-slip movement in snow-accumulating periods and rapid side-slip movement in snow-melting and snow-free periods, both of which occur on the same slip plane. This bidirectional movement appears to result from the combination of a loss of lateral support by the 2004 event and the immature state of the slip plane. The primary trigger of the acceleration in the unstable area is the stress release by the 2004 collapse. In addition, the presence of a potential slip plane below the sackung features also promotes slope instability, suggesting that sackung features behind a rockslide area may predefine the new head scarp of a forthcoming collapse. Copyright (C) 2012 John Wiley & Sons, Ltd.