Pasadena’s Foothills Are on the Move

The foothill areas of the San Gabriel Mountains near Pasadena seem to be moving toward the San Andreas Fault at a rate of about 3 centimeters a year, according to two scientists who are monitoring such movement along the fault.

Such research could help scientists to further understand the powerful forces that shape one of the most active seismic regions in the world.

The two Jet Propulsion Laboratory scientists, who use radio telescopes focused on a star trillions of miles away to measure infinitesimally small movements on the Earth, found that the area is moving almost due north. Such a direction, while consistent with the movement of the giant tectonic plate that comprises the North American continent, puts it on a collision course with the San Andreas.

But “the significance of that movement may not be very great” because it was measured over a relatively brief time span and could change with further movement along the fault, said Gregory A. Lyzenga, a JPL geophysicist who published the findings in today’s issue of Science magazine.

Lyzenga and a JPL colleague, Matthew P. Golombek, a geologist, used radio telescopes at three sites in California as part of a program sponsored by the National Aeronautics and Space Administration. One of the sites is on a mesa above JPL, near Pasadena; the other two are on the other side of the San Andreas: The National Aeronautics and Space Administration’s Owens Valley Radio Observatory and the Mojave Base Station in the northeastern Mojave Desert.

The two scientists are pioneers in the use of “very long base line interferometry” to measure movement along the fault. The technique involves the use of a distant star, far beyond the Milky Way Galaxy, which emits microwaves that can be intercepted by radio telescopes on Earth. By correlating the signals received at each of the three sites, the scientists can measure movement of any of the sites. It was the JPL site that moved.

If those signals are taken over a 10-year period, the results should reveal movement with an accuracy of .07 of an inch, Lyzenga said.

Lyzenga and Golombek based their findings on a four-year period, from 1980 through 1984, but the project is continuing and they believe the results so far are reliable enough to draw some conclusions.

They suggest, for example, that such movement may be controlled more by deep Earth forces associated with the tectonic plates than with movement along the San Andreas Fault. The giant plates that make up the Earth’s crust migrate on the Earth’s molten core, and the boundary between the Pacific Plate and the North American Plate runs north and south along the coastline of the Western United States.

Their research indicates that is the same direction that the JPL site is moving, although the San Andreas Fault runs in a northwesterly direction.

Differences in Rate

They said the JPL site is moving at about three centimeters a year, about half the speed that the Pacific Plate is slipping past the North American Plate in California, Lyzenga said.

The difference in the rate of movement “has remained an enigmatic problem,” the scientists wrote.

The most logical explanation, they said, is that the fault is moving more slowly because it is locked in an area known as the “big bend” east of San Luis Obispo, and the difference in the rate of movement reflects growing strain within the San Andreas.

The direction of the movement of the JPL site also indicates that “we are seeing motions driven by more deep-seated forces (the tectonic plates) than the near-surface forces (the San Andreas Fault),” Lyzenga said.

Implications of Direction

Lyzenga is also intrigued by the fact that the JPL site is moving directly toward the big bend area of the San Andreas rather than paralleling the direction of the fault.

“That implies that we are building up compressive strain, consistent with the presence of the San Gabriel Mountains,” which were believed to have been formed by uplifts caused by the compression of the fault zone, he added.

Lyzenga is the first to admit, however, that the findings are inconclusive. The difference in the rate of movement, and even the direction that the JPL site is moving, could be altered dramatically by a massive quake on the San Andreas.

“But it’s fun to get these numbers and try to figure out what they mean,” he said.