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10/01/91

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Volcanic action can prevent earthquakes -- sometimes

STANFORD -- There are well-documented associations between earthquakes and volcanoes, but two Stanford geophysicists have found evidence for an unexpected twist to that relationship.

Instead of increasing the pressure that causes earthquakes, molten rock sometimes may prevent large quakes by filling spaces in the Earth's stretching crust that otherwise would form faults.

Their explanation -- based on a study of underground flow of basaltic magma, a molten rock from deep in the earth that sometimes creates volcanoes - may change how volcanic and earthquake hazards are assessed.

In a Sept. 20 article in Science magazine, geophysics graduate student Tom Parsons and professor emeritus George Thompson report on three regions around the globe where earthquake faults would normally be expected when tectonic plates stretch apart. Instead, in parts of those regions, basaltic magma has forced its way into the rock in the form of dikes - vertical layers of molten rock - with enough pressure to open up new cracks in the rock and then fill them. When the magma cools, it equalizes tectonic stresses and prevents faulting.

They propose a scenario new to geology: that intrusions by magma may suppress the creation of both faults and the mountainous terrain faults cause. The normal quake cycle sees stress building slowly until it is suddenly released when faults slip. In this scenario, though, the stress is prevented by rapid magma injection, causing only tremors that are barely detectable at the surface.

Magma is heated in the mantle of the earth below the crust and is forced by pressure up through the rock like mercury in a barometer tube, creating a vertical dike of basalt between blocks of granite or other rock.

One striking example is in the Basin and Range region of the western United States. Here long, narrow ridges and valleys are formed by repeated earthquake faulting, as the granite blocks of the earth's crust tilt and break apart. Major quakes are common, including the 7.3 magnitude Borah Peak earthquake of 1983.

Only 60 miles from Borah Peak, in southern Idaho, the oblong flat plain of the Eastern Snake River Basin has only micro-earthquakes. The absence of mountains suggests that this has been true for at least four million years.

The basin is dotted by hundreds of volcanic cinder cones. Dozens of tilted block faults run perpendicular to the 70 by 200 mile basin, stopping abruptly at the southern edge of the plain and starting up again on the northern side.

A simple explanation of the topography would be that a puddle of cooled magma has covered up the underlying faults. But Parsons and Thompson contend that there are no underlying faults: Repeated intrusions of magma have absorbed the stresses instead as the crust in the region pulled apart.

This earthquake-stopping action only applies to basaltic magma, the slow-flowing type familiar from Hawaii's Kilauea volcano. Silicic magmas, such as those at Mt. St. Helens, are more viscous and more explosive.

The action is also less likely to occur where continental plates are grinding together, as along the California coast. It requires a region of tectonic spreading like the Great Basin, the 3,000-mile-long sea-floor rift called the East Pacific Rise or the Great Rift Valley of East Africa.

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