Science / Medicine : Quake ‘Stress Map’ Being Developed

Scientists in 20 countries have joined together to create a “world stress map” that will show patterns of geological stress around the entire planet.

That, in turn, should help experts identify areas that are more likely to generate major earthquakes than had been thought.

“The first order of business is to establish global patterns,” said geologist Mary Lou Zoback of the U.S. Geological Survey’s Menlo Park office, who is coordinating the project.

Establishing those patterns will help define the enormous forces that drive the giant slabs of the Earth’s crust--called tectonic plates--around the globe. The plates crash into each other, as in the Pacific Northwest and Alaska, producing earthquakes and volcanoes, or they rub past each other, as along the coast of California, creating faults like the giant San Andreas.

Once those forces are clearly established, scientists will have a better chance of understanding localized stresses that do not fit the overall pattern, Zoback believes.

“We can look at earthquakes that have occurred, and we think we know what the stress field is, so we can find out if the slip (along the fault) is consistent with these forces,” she said. “If it isn’t, then there has to be some localized force or stress.”

In many areas of the world, localized stresses, caused by such things as the weight of a mountain range on the Earth’s crust or the removal of oil or water from the soil, are believed to have caused devastating earthquakes.

If the primary forces are clearly understood, it will be easier to identify the secondary forces and thus better establish the seismic potential for any area.

“We are really searching for localized stress anomalies, which a lot of people hope exist so they can say this area is going to have a big earthquake, but we know that this area over here is safe,” Zoback said in an interview.

She and other scientists who recently attended the International Geological Congress here doubt, however, that the answers will ever be simple.

“Most researchers believe earthquakes could occur anywhere,” she said.

Zoback and her husband, Mark, a geology professor at Stanford University, have done pioneering work that revealed stress patterns along the San Andreas. Before their research, it had been generally thought that the primary geological stress patterns ran in a north-south direction, corresponding to the direction of the movement of the North American Plate, which is inching south, and the Pacific Plate, which is moving north.

However, they found that the primary stress patterns are at right angles to the fault, so California is being crunched together along a northwest-southeast axis.

That is because the stress caused by the two plates pushing against each other is far greater than the stress caused by one plate sticking as it tries to slip past the other. Thus the primary stress pattern is across the fault rather than in the same direction as the plate movement.

That force has pushed huge slabs of the earth up thousands of feet, creating mountain ranges such as the San Bernardinos and the San Gabriels, which run parallel to the fault.

Zoback and the other 30 scientists working on the world stress map hope to identify regional stress patterns that could help explain why earthquakes occur in some areas and not in others, particularly in regions that are some distance from the plate boundaries.

She said she hopes to publish the map early next year.