NEWS: Giant Landslide Seen From Space
According to the USGS website, the most serious immediate risk is from shallow landslides that become so wet that they can move rapidly over long distances. Such "debris flows," as they're called, have the potential to injure or kill people who can't get out of their path in time. They can move at speeds of up to 35 miles per hour.
The areas most susceptible to debris flows are steep hillsides that recently have burned in wildfires. But this winter the risk could extend further, because several years of drought have killed off vegetation that normally might keep hillsides from collapsing.
But predicting precisely where mudslides will occur in a particular season is tricky. While rainfall maps can give some indication of risks, they're not necessarily a reliable indicator, because the amount of moisture in the hillsides is affected by varying degrees of drying and evaporation that occur between storms. USGS researchers, who've studied mudslides in the San Francisco area, have been experimenting with sensor technology that would detect moisture levels.