Slip-Sliding Away

Article excerpt

I'm predicting a landslide in November, but not the political kind. I'm talking about the sudden shift of hundreds or thousands of tons of rock and soil, and that kind happens nearly every day. To catch up on the latest tolls in death and destruction from major landslide events, go to the U. S. Geological Survey's (USGS) main site on the subject (landslides.usgs.gov), and click on "Recent Landslide Events."

In some places, such as the San Francisco Bay area, weather and geology seem to conspire with gravity to bring down mountainsides on a regular basis. You can get the details from a section of the USGS Web site dedicated to the causes and effects of El Niño (walrus.wr.usgs.gov/elnino/). Scroll down the page to the "Landslides" section, click on "Potential San Francisco Bay Landslides," then click on "fly-by movies." Download a crosssectional view of a "slow-moving failure" and watch it undermine a typical California hillside home, or take a virtual night over Marin County or East Bay Hills to get some idea of how prevalent landslides have always been in the region. Beneath the fly-by features, you'll find movies of two actual slides from the 1996-97 rainy season.

Many things can set critically unstable rock in motion. On May 18, 1980, about a mile below Mount Saint Helens, a magnitude 5.1 temblor triggered the largest landslide worldwide in the past century (see pubs.usgs.gov/ publications/msh/climactic.html and click on "Debris avalanche"). The mountain shed 0.7 cubic miles of rock, uncorking the more infamous eruption.

Human activities sometimes set the stage for catastrophic landslides. Logging is a good example. Steep slopes denuded of trees and cut with new roads don't stay put for long. The Sierra Legal Defence Fund has issued a report on the problem in British Columbia, titled "Going Downhill Fast" (www. …