A minor earthquake that shook Maine on Oct. 2 caused the water level in a U.S. Geological Survey (USGS) monitoring well to drop more than 2-1/2 feet. Nearly 17 hours later, the groundwater level was still dropping, according to scientists at the USGS Maine Water Science Center in Augusta. Hydrologists call the change in the well &quotdramatic,&quot and note that well-water users could notice changes in their drinking water. USGS hydrologists will continue to monitor this well several times per day.

The preliminary magnitude 3.9 earthquake was the third such event in Maine in the past few weeks. This event was centered about 4 miles south-southeast of Bar Harbor, or 45 miles southeast of Bangor, Maine. A magnitude 2.5 earthquake on Sept. 28 and a magnitude 3.4 on Sept. 22 were centered in the same location.

&quotIt isn’t unusual for earthquakes to cause minor changes in water levels in wells, but this is the most memorable in Maine in the last decade,&quot says USGS hydrologist Gregory Stewart. &quotUsers of well water could notice cloudy water and possibly a change in availability of water.&quot Stewart notes that the region’s fractured bedrock means it is difficult to predict whether or not other wells will be affected. &quotWater-level responses can occur over time periods of a few minutes to several months,&quot he says.

The well, located in Acadia National Park in Bar Harbor, is a drilled bedrock well that is 98 feet deep. On a normal day, water-level changes are generally 3 to 4 inches at this well. One other well near the epicenter also showed a drop in water level from this event. Other local earthquakes in the past two weeks were not detected at water level monitoring wells in the state.

The largest earthquake centered in Maine occurred March 21, 1904. Its magnitude was 5.1. This event toppled chimneys and was widely felt throughout New England. Historic earthquakes centered outside Maine have been large enough to cause damage in the state. The largest historic earthquake to shake the region was a magnitude 7.0 earthquake in 1663, centered in Quebec along the St. Lawrence River. It toppled chimneys in eastern Massachusetts. In 1755, a magnitude 6.0 earthquake off Cape Ann, Mass., shook down chimneys and several brick buildings in eastern Mass.

More information on the earthquake history of specific U.S. states is found at http://earthquake.usgs.gov/regional/states.php .