A subsidence trough (or graben) formed at the head of the "L" Street landslide in Anchorage during the earthquake. The graben extends from the lower right corner of the picture to the upper left and passes beneath several houses. The slide block, which is the virtually unbroken ground to the left of the graben, moved to the left. The subsidence trough sank 7 to 10 feet (2.1 to 3.0 m) in response to 11 feet (3.3 m) of horizontal movement of the slide block. The volume of the trough is theoretically equal to the volume of the void created at the head of the slide by movement of the slide block. Note also the collapsed Four Seasons apartment building at the top center of the picture and the undamaged three-story reinforced concrete frame building behind it, which are on the stable block beyond the graben.

Image Credit: U.S. Geological Survey