Research Stories

by Diane Boudreau

In the northwestern United States, in the rain shadow of the Cascade Mountains, stretches a unique area of high-desert terrain. Surrounded by picturesque snow-capped mountains and inhabited by deer and antelope, this tranquil area conceals an extremely violent past.

Beneath the surface, a tectonic plate is being shoved under North America like unwanted peas under a napkin. As water is released from this slab, rocks melt and bubble up to the surface. No place on Earth has seen more volcanic activity than this area, which stretches across almost 75,000 square miles and includes parts of Oregon, Idaho, California, and Nevada. Geologists refer to it as the High Lava Plains.

"This subduction zone caused the Cascades–Mount St. Helens, Mount Adams, and others," says Matthew Fouch, an ASU seismologist. "When you move east that entire area is just coated in lava. It's a geologist's nightmare. It's not like the Grand Canyon where you can see the layers. You can't see beneath the surface."

In October 2007, Fouch traveled to Oregon to help install 100 seismometers. The readings from these instruments are helping Fouch and other scientists to understand why the area is so volatile. They also use data from other seismometers installed by Earthscope, a national Earth science program dedicated to studying the structure and evolution of the North American continent.

"It's like the Earth science version of the Hubble Space Telescope," says Fouch.

As part of Earthscope, scientists have installed about 400 seismometers across the western United States. The instruments will remain there for one or two years, then be moved in footprints across the country. These seismometers record earthquakes from all over the world.

Fouch doesn't just study the data from the seismometer readings. He gets out in the field, digging holes and running cables.

Matthew Fouch

The scientists dig a hole about 3 feet deep, pour concrete in the bottom, then insert a drum to keep the machinery dry. The solar-powered seismometer sits in the bottom. It is connected to a global positioning satellite (GPS)–not to keep track of its location, but to measure time.

"We want to be accurate to within about five-millionths of a second," says Fouch.

A seismometer is a sensor that detects ground motion. Seismometers operate on the principle of inertia–stationary objects remain stationary unless a force is applied to them. In this case, the stationary object is a weight at the end of a pendulum. When the ground shakes, the instrument's frame moves, but the pendulum weight remains stationary. It appears as if the pendulum is moving, but in fact, it stays still while everything around it moves.

By attaching a recording device to a seismometer, scientists can see how much the Earth moves. The combination of a seismometer and a recording device is called a seismograph. Early seismographs used a pen and a rotating drum to record motions. Today, computers record the electrical voltage generated by the relative motion between the weight and the frame. The seismographs in use today can detect movements as small as 1/10,000,000 centimeters–almost at the atomic size.

"The technology has been revolutionized in the last 25 years," says Fouch. "Broadband seismometers can record really high and low frequencies. We also have digital cameras and flash drives now. We used to have to lug around huge hard drives."

Fouch says the High Lava Plain is one of the more understudied areas of geology in the United States. This research will help scientists understand how the plate tectonics of the western U.S. has developed over time.

"This is part of the exploration theme of SESE," says Fouch, referring to ASU's School of Earth and Space Exploration. "We're going places no one has ever gone before to collect that data."

Read more about deep-Earth research in "The puzzle deep beneath our feet."

For more information, contact Matthew Fouch, School of Earth and Space Exploration, College of Liberal Arts and Sciences, 480.965.9292, Fouch@asu.edu