Abstract
Most landslides occur in winter, following storms that soak the ground. Research in an area of Contra Costa County, in the San Francisco Bay region, recently completed by two investigators from the Menlo Park Office of the U.S. Geological Survey showed that most of the damaging landslides were found to take place on old landslides, which were reactivated by grading, construction, an excess of water, or a combination of these.According to Tor H. Nilsen, senior author of a new report (USGS Bulletin 1388) on the Contra Costa County landslides, ‘most of the landslides that occurred during our study took place during or immediately after storms in which 7 inches or more rain fell, particularly if the land was already wet from earlier storms.’ How fast and when the rain falls, he emphasized, ‘are more important than the total amount of rainfall during the season. Long continuous rain can be more dangerous insofar as landsliding is concerned than sudden storms separated by dry periods.’
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