SCIENTISTS have often wondered why so many houses in California remain standing after an earthquake. But now they think they know why: stucco helps the buildings stick together. And this weather-resistant coating is very commonly used in the state.

Researchers at the University of California in San Diego built a simple, two-storey house-typical of 90 per cent of homes in Southern California-and placed it on a hydraulic shake table that simulates earthquakes. The artificial quakes were equivalent to the 1994 Northridge earthquake near Los Angeles, which registered 6.7 on the Richter scale and caused damage worth $20 billion.

"The first test houses didn't have stucco-they were just [wooden framed] skeletons with a plywood shell," says John Hall of the California Institute of Technology, manager of the house-shaking project. In these tests, the top of the house swayed back and forth 12.5 centimetres further than the bottom. But when it was given an outer skin of stucco-a plaster made of dehydrated lime, powdered marble and glue, mounted on a wire mesh-the top swayed only 2.5 centimetres, says Hall. In the house, appliances toppled, and water heaters and shelves fell off the walls. But the structure suffered nothing worse than a few cracks.

The degree of protection depends on the gap behind the stucco. "If it's placed far enough away from the wood frame, it allows the wire mesh enough room to absorb most of the tension when the house shakes," says Hall. "It's like having a protective box of concrete around your house." The researchers next plan to find out how old stucco can be repaired to make older buildings less vulnerable during quakes.

Author: Diane Martindale

New Scientist issue: 5th August 2000

PLEASE MENTION NEW SCIENTIST AS THE SOURCE OF THIS STORY AND, IF PUBLISHING ONLINE, PLEASE CARRY A HYPERLINK TO: http://www.newscientist.com