January 14, 2005|By Frank D. Roylance | Frank D. Roylance,SUN STAFF
The Sumatran earthquake that triggered the devastating South Asian tsunami on Dec. 26 was so powerful its seismic waves coursed through the Earth's crust and sent water levels surging in a monitoring well in southwestern Virginia, 10,000 miles away.
State and federal geologists said water in the 450-foot limestone well in Christiansburg began oscillating, surging upward at least 2 feet, and down at least 3 over a half-hour period as the seismic waves passed. It was five hours before the water returned to its former levels and calmed down.
Although it's designed to monitor groundwater levels in the area, geologists say the Christiansburg well has long been noted for its sensitivity to distant earthquakes.
"The first one I remember was the 1985 Mexico City earthquake; that really looked nice," said David L. Nelms, district groundwater specialist in the Richmond office of the U.S. Geological Survey. That magnitude 8.1 quake moved water in the Christiansburg well 3 to 5 feet, and to a record low.
"These big earthquakes send out these teleseismic waves that can go - well, we can see how far they can go, and fast," Nelms said. "That is the thing that amazes me most."
Water-pressure data from the Christiansburg well is recorded digitally and relayed via satellite to the Internet every 15 minutes. It's part of a growing network of "real-time" monitoring wells in Virginia operated cooperatively by the USGS and the state Department of Environmental Quality.
Every earthquake unleashes a mix of seismic waves, or vibrations, all traveling at different speeds, into the surrounding rock. High-frequency waves can penetrate through the Earth's core to the other side of the planet. Low-frequency vibrations move through the Earth's crust 10 times faster than an airliner, scientists say, and circle the planet before fading away.
As they pass, they can send water in pools and lakes sloshing, squeeze the fractured rock like a bellows and cause water levels in wells to rise and fall. A 2002 quake in Alaska caused well water to grow cloudy in Maryland and Pennsylvania, and affected well-water levels from Arizona to Maine.
Eugene Powell, environmental manager at the Virginia DEQ, said the Christiansburg well surged by 2.4 feet on Nov. 14, 2001. That was an hour or two after a magnitude 8 earthquake in western China. The well hiccupped again, by 3 feet, on Jan. 20, 2003. That was several hours after a 7.8 quake in the Solomon Islands in the west Pacific.
There's no proof these were cause-and-effect events, Powell said, but it seems at least a possibility.
Sometimes the water levels in groundwater wells will change permanently as seismic shifts alter the rock fractures that contain it.
Hours after a 1998 quake in Pymatuning, Pa., residents began reporting their wells had gone dry. After three weeks, about 120 household wells in northwest Pennsylvania had dried up. Water levels dropped as much as 100 feet on one ridge near Jamestown, while levels in valley wells increased as much as 62 feet.
The Christiansburg well returned to normal just hours after the seismic wave from the Sumatran quake passed by last month.
But "normal" at this remarkably sensitive well means water levels that rise and fall rhythmically, by a few inches, with the daily rotation of the Earth and the monthly changes in the relative positions of the sun and the moon, Nelms said.
It's called "Earth tide" - caused by the same regular tug of gravity from the sun and moon that causes the ocean tides, except that it's pulling on the bedrock itself. "That squeezes the aquifers," he said, causing the water in them to rise and fall.
Asked why the Christiansburg well is so exquisitely sensitive to these things, he said, "It's a mystery."
For Christiansburg well data, go to http://va.water.usgs.gov/. Click on "ground water" and then the Montgomery County well link.
