In case of temblor, run downhill. (Seismology).

Scientists have long observed that the ground motions from an earthquake are stronger on a hill than they are on nearby plains. Now, they're developing computer models that reflect that phenomenon.

Most previous simulations of a temblor's ground motions on uneven terrain have emulated geologic features made of a single material or used simplified representations of earthquake waves, says Jacobo Bielak of Carnegie Mellon University in Pittsburgh. Those models haven't accurately reproduced the amplification of earthquake waves that seismologists actually measure. Bielak and Chiaki Yoshimura of the Taisei Research Center in Yokohama, Japan, have developed a more complicated representation.

The researchers created two sets of computer models. One set included a shallow hill made of homogeneous material, but the other incorporated a thin soil over-layer.

When earthquakes occurred deep beneath the hill, ground motions on the bare hills were about 60 percent greater than those on the nearby plains. On the soil-topped hill, however, the ground motions were more than 80 percent greater than those in flat areas.

The largest amplifications of ground motions occurred for shallow earthquakes centered far from the hill, says Bielak. In the models, the high-frequency vibrations that travel along Earth's surface--the ones that tend to cause most earthquake damage--were magnified as much as 10 times by the hills that incorporated the layer of soil. The incoming waves tended to get trapped in that thin layer and reinforce themselves as they bounced between the layer's upper and lower boundaries.

These results show that structural engineers may need to reconsider the earthquake vulnerability of buildings on hills or other geological features, says Bielak. --S.P.