Seismic Waves Shed Light on Magma Body

SOCORRO -- Geophysicists at New Mexico Tech have been using refinements in data analysis and an expanded seismic-monitoring network to redraw the boundaries and origins of a thin, yet extensive, horizontal layer of molten rock that lies about 11 miles below the surface of the middle Rio Grande valley, between Socorro and Bernardo.

In a soon-to-be-published article in Bulletin of the Seismological Society of America, New Mexico Tech geophysics professor Allan Sanford, Hans Hartse of Los Alamos National Laboratory, and Tech graduate students Robert Balch and Kuo Wan Lin report that they have re-estimated the lateral extent of the Socorro magma body by monitoring reflected seismic waves from micro-earthquakes which frequently occur in the same area.

As seismic waves from over 1,000 small earthquakes reflected off of the molten rock, back toward the surface, the researchers began using the new information to draw up a more accurate map of the magma body's vast expanse.

The newly plotted map of what the geophysicists describe as "one of the world's most well-known, distinct bodies of molten rock" virtually doubles estimates made in 1979 of the Socorro magma body's size, extending its range farther to the south, southeast, and northwest than had been previously mapped.

While the new estimate places the magma body as encompassing nearly 3,400-square-kilometers, the researchers emphasize that there is nothing to show that it has physically doubled in size since 1979.

"Rather, the increase in estimated size is the result of improvements in data analysis and an expanded seismic network," Tech graduate student Balch emphasized.

The study also confirms that the upper surface of the magma body is very flat, consistent with the magma ponding at the boundary between the denser rocks of the lower crust and the lighter rocks of the upper crust.

The probable extent of the magma body may be about the same size as the Socorro Seismic Anomaly (SSA), a similarly situated region of unusually high earthquake activity which occupies a somewhat larger area of 5,000-square-kilometers and includes most of the magma body's outline.

Although occupying only about two percent of the area of New Mexico, the SSA has been responsible for over 36 percent of the larger earthquakes in the state over the last three decades.

While the geophysicists were reassessing the Socorro magma body, New Mexico Tech professors John Schlue and Richard Aster were looking even further underneath.

"A longstanding mystery associated with the Socorro magma body has revolved around where the magma rises up from deeper within the Earth to inflate the body," Aster notes. "To try to learn about the plumbing, we performed an experiment to look for thicker regions."

In a paper published in the November issue of the Journal of Geophysical Research, Aster and Schlue, in association with Robert Meyer of the University of Wisconsin, examined local changes in seismic waves from large South Pacific earthquakes which passed through the body of the Earth and transited the magma body.

They found that these signals were affected in ways that were consistent with the presence of a magma body "root," located deep beneath the western portion of the Sevilleta National Wildlife Refuge.

The New Mexico Tech geophysicists speculate that this thickened "root" area may indicate a conduit for transporting molten rock up to the Socorro magma body from the Earth's mantle, some 60 miles below the surface or more.

"We know that the Socorro region is being inflated by this type of magmatic intrusion," Aster says, "and that this inflation drives the region's persistent earthquake activity, such as the large and damaging earthquakes of 1906 and the more recent small earthquakes which were felt during 1995. . . . This is the first indication as to where the magma supply is likely to be centered.

"But don't expect a volcanic eruption soon," Aster adds.

Although, speaking strictly in a geologic time frame, the Socorro area may be overdue for a volcanic eruption, New Mexico Tech geoscientists further downplay the possibility of an imminent eruption fueled by the Socorro magma body.

They point out that even though, on a geologic-time scale, volcanoes have erupted frequently in the intermountain United States, on human-time scales, they are rare occurrences.

"The Socorro magma body has probably been here for at least several tens of thousands of years," Aster notes, "and it could be many thousands or even millions of years more before the next volcanic eruption occurs in the region."

-NMT-

(George Zamora)