A subsidence trough (or graben) formed at the head of the "L" Street landslide in Anchorage during the 1964 Prince William Sound earthquake (magnitude 9.2). The slide block, which is virtually unbroken ground to the left of the graben, moved to the left. The subsidence trough sank 7 to 10 feet in response to 11 feet of horizontal movement of the slide block. A number of houses seen in this photograph were undercut or tilted by subsidence of the graben. Note also the collapsed Four Seasons apartment building (in red box) and the undamaged three story reinforced concrete frame building beside it, which are on the stable block beyond the graben. Photo courtesy of the USGS.