GRANTS PASS, Ore. — A well drilled 300 feet deep to study fluctuations in groundwater has proven extremely useful for monitoring major earthquakes around the world.

The well is especially sensitive because it's drilled into an underground rock formation of granite with fractures that all run the same direction, channeling more water into and out of the well, said Evelyn Roeloffs of the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Wash.

Roeloffs, who has studied the well for more than a decade, said the earthquake that unleashed the devastating Asian tsunami last month roiled the water for more than five hours in a monitoring well 8,429 miles away in Southern Oregon.

The well just north of Grants Pass near the town of Merlin has recorded every quake of magnitude 7.4 or greater throughout the world since 1989, as well as most smaller quakes in California, Oregon, Washington and Alaska.

It's not unusual for wells of all types to register fluctuations in water levels during quakes, but this well is more sensitive than most, geologists say.

In the case of the Dec. 26 tsunami quake, the well's water rose and fell every 20 seconds. The maximum change in water level during one of the cycles was almost 18 inches.

Earthquakes generate seismic waves that contract and expand the Earth, alternately squeezing out water stored in rocks and then allowing the water to flow back into fractures.

Water levels in wells also rise and fall with the Earth's tides — the daily expansion and contraction of the planet's crust caused by the gravitational pull of the sun and moon. Most wells respond to the tides and nearby earthquakes.

Employees of the state Water Resources Department noticed something strange on the paper chart that recorded the well's water levels with a pen after it began recording water levels in the late 1980s.

"What would show up on the paper chart was just a vertical line," said Ivan Gall, the state hydrologist who now oversees data collection at the well.

In the beginning, state water officials just clipped news reports of earthquakes to the well's data records and stuck them in a file, says Doug Woodcock, who works for the state water department in Salem.

But after the Landers quake in California's Mojave Desert in 1992, when water levels fluctuated more than 30 inches in the well, the officials contacted Geological Survey scientists. Scientists went back over the records and matched quakes to water levels, including all big quakes since 1989.

Understanding the changes in the well water could help scientists understand the role of underground fluids in causing earthquakes, Roeloffs says.

"What we'd really like to know is: Do natural fluid pressure changes trigger natural earthquakes?" she said.