Evaluation of effective factors causing ground failures during 1990 Manjil-Iran earthquake

In this work, a study of the Gilan region(north part of Iran) is performed to determine what geological and environmental factors affected slope movements during an earthquake in the area. Engineering geology aspects concerning regional faulting and related minor structures, lithology, drainage and hydrology, geomophology, and effect of man were considered. Field works were conducted in the whole area and among tens of slope movements, six big slopes(because of volume or resulted damages) were selected. For these slopes a detailed site investigation were performed to determine the relative significance of the factors influenced on the slope failure. To find out about the effect of weathering rate on failed slopes X.R.D analysis were conducted on the bed rocks underlying the failure zones. This study revealed that, the area is an active area and geological structures specially faults had an important role in the ground movement. Most of the slope movements are located on or near the faults. The bed rocks underlying the area, in most of the landslides, are associated with certain kinds of the rocks such as weathered andesites, shale and marly limestones, that are indicative of the field instability. The other important factors which affected the slopes failure are existence of surface and ground water in failure areas. Key words: Earthquake, Earthquake damage, Landslide, Ground failure, Weathering rate