MAPPING LANDSLIDES TRIGGERED BY THE EARTHQUAKE

The Northridge earthquake provided an unprecedented amount of data for studying the distribution and effects of landslides triggered by an earthquake in an urban area. Detailed field investigations to document earthquake-triggered landslides, which were initiated the day of the earthquake, are continuing. In the first several days following the earthquake, we drove outward from the epicentral area in all directions to locate areas of concentrated landsliding and to find the farthest extent of landsliding, which is defined by small rock and soil falls from very susceptible slopes such as steep road and stream cuts.

High-altitude aerial photography (nominal scale 1:60,000) of the epicentral region was flown by the U.S. Air Force within hours of the earthquake, and we mapped fresh landslides visible on these photos to provide a detailed inventory of ground failures triggered by the earthquake. We mapped more than 11,000 individual landslides on 1:24,000-scale U.S.G.S. topographic base maps. The photos show all but the smallest slope failures and some that were hidden within deep shadows on the north sides of steep slopes.

Landslides as small as 1-2 m across are visible where the slopes were sunlit;where slopes were partly shaded, slides about 5-10 m across are the smallest that could be resolved; thus, the inventory is not complete. However, our field observations indicate that south-facing slopes in most of the landslide area generally are steeper and produced far more landslides than north-facing slopes. Therefore, landslides on north-facing slopes that are not visible on the photos because of shadows probably account for only a small proportion of the total landslides. From our field investigations we estimate that we missed no more than about 20 percent of the landslides that exceeded 5 m in maximum dimension and no more than 50 percent of those smaller than 5 m. In terms of area, however, we estimate that we have mapped more than 90 percent of the area covered by triggered landslides, because most of the landslides that are not visible on the photos are small.

We manually digitized the landslides mapped on the 1:24,000-scale base maps using the Arc/Info geographic information system (GIS). Landslides triggered by the earthquake are plotted as solid polygons on the digital inventory maps (plates 1 and 2). We estimate that location accuracy of landslides mapped from the airphotos to the paper base maps is generally within 10 m and no worse than 20 m. When the paper maps were registered on the digitizer, the computer calculated the root-mean-square (RMS) error in the base-map registration, which averaged 3.6 m and ranged from 0.2 to 10.4 m. Thus, landslide locations plotted on plates 1 and 2 are generally accurate within about 15 m and are no more than 30 m mislocated.