A research team led by the University of California, Berkeley, has detailed the downhill movement of San Francisco Bay Area landslides using powerful new space-born imaging techniques.
In what is perhaps the most detailed study yet of the active landslides in the East Bay hills, researchers found that the slides were moving downhill between 5 to 38 millimeters per year.
Their findings, reported in the June 25 issue of the journal Science, demonstrate the power of a satellite mapping technique that provides far more information than what was previously possible through labor-intensive field studies.
Using high-resolution interferometric synthetic aperture radar, the scientists analyzed data collected between 1992 and 2001 by two European Remote Sensing satellites. Not surprisingly, during the period of heavy rains brought on during the 1997-1998 El Niño years, when seasonal precipitation increased by 200 percent, the researchers found that sliding rates increased by as much as 30 percent.
“We believe the seasonal acceleration of these landslides may be strongly controlled by elevated water pressures in the ground subsurface,” said George Hilley, a postdoctoral researcher at UC Berkeley’s Department of Earth and Planetary Science and lead author of the study. “Once you go about understanding the physics of these slides and how they respond to changing conditions due to precipitation and groundwater flow, then you can actually start to develop strategies for mediating these types of structures.”
In this new technique, highly reflective objects such as buildings or rock outcrops are tracked through as many as 50 satellite images. By tracking only these reliable points, noise within the measurements can be greatly reduced, and in urban environments where there are many reflective surfaces, a high density of quality measurements can be attained.
The researchers identified 18,428 such points in an area of about 16 square kilometers, and compared their images with mapped landslides available from geotechnical engineering firms to confirm the data’s accuracy.
Researchers also say the refined satellite mapping technique could be used to monitor deformation of ground that is characteristically soft, such as in San Francisco’s Marina District or Treasure Island.