August 27, 2008
Scientists See New Detail in Measures of Geological Movement
By Lisa Krieger
It takes an earthquake for humans to feel the shift of geological plates beneath our feet. But a new network of hypersensitive tools installed around the Bay Area detects even the most subtle shrugs, providing important insights about the slow stretching, splitting and drifting of our region.
Part of the National Science Foundation-funded EarthScope, the most ambitious earth science project ever attempted, the Plate Boundary Observatory Project measures the movement and deformation of the Earth below the western United States with a level of detail and data accessibility never seen in geophysics.
Mounted on four legs for stability, these so-called "monuments" are tiny laboratories that talk to GPS satellites to transmit a steady stream of data, documenting millimeters of motion -- whether up, down or sideways -- relative to the stable central region of the United States.
When mapped over time, each monument is watched as it migrates. Geologists hope that these maps will yield a clearer understanding of the forces that are building our terrain and that will someday trigger a catastrophic earthquake.
"GPS is great at observing motions that a person cannot feel," said geophysicist Jessica Murray-Morales of the U.S. Geological Survey in Menlo Park.
One monument, for instance, reveals that in the past year Russian Ridge in the south Bay Area moved 1.2 inches to the north, 0.8 inches west and grew 0.3 inches in elevation.
During installation, the steel legs must be drilled deep into bedrock, so as not to measure shallow landslides. With solar panels and batteries, the monuments are self-sufficient. Most are out of public view.
The monuments measure a different motion than the seismic waves released by earthquakes. For instance, they are now monitoring the slow creep of earth that followed the explosive burst of energy from the 5.9 magnitude earthquake near Alum Rock Park on October 31, 2007.
The Bay Area monuments hold particular interest to geologists because of the tension along the complex series of faults such as the San Andreas, the Hayward/Rodgers Creek and the Calaveras, where the North American tectonic plate and the Pacific plate converge. This wide fault zone is the site of much grinding, straining and stress and the likely locale of the Bay Area's next big earthquake.
Because Bay Area monuments are located along different faults, some are traveling more quickly than others. They are part of a network of 1,000 GPS instruments, installed from the edge of the Pacific Coast to the Rocky Mountains and from Alaska to Mexico.
"It is similar to the GPS in a car, or that a hiker would use. It gives a position," said Andre Basset, an engineer with UNAVACO Inc. of Boulder, Colo., which installs the stations. "But it does so much, much more accurately."
The array was started in 2004 along the West Coast and will occupy about 2,000 locations across the continental United States in the next decade.
Some of the stations include "strainmeters," clustered along active faults and in volcanic areas. About three meters long and 10 centimeters in diameter, they are cemented into rock below the surface to measure strain in three dimensions.
When a rock squeezes or relaxes, the strainmeters sense changes in their diameter to a part in a billion.
In other parts of the nation, the monuments reveal different problems. A collapsing station might mean that too much groundwater is being pumped out. On a melting glacier, they can measure the speed of retreat.
Although GPS is a proven workhorse, what's new is its use on such a massive scale. Advances in moving and crunching big data sets make it possible, said Basset.
"The stations pump out coordinates all day long, every 15 seconds," said Basset.
Geologists watch the monuments move day by day, even hour by hour. A typical instrument near a major fault would move about three centimeters a year. In a big earthquake, it could move meters.
In the Bay Area, "a lot of the stations are chugging northwards at a regular rate," said Basset. "Even if there is no earthquake, they're still moving."
Originally published by Lisa Krieger , MediaNews staff.
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