May 31, 2006
Wednesday

The great Alaska earthquake of March 1964 jarred Earth's plumbing system far beyond Alaska. More than 700 groundwater wells in the continental United States showed water-level changes, including a 12-foot rise in a South Dakota well. A well in Australia fluctuated more than two feet after the 1964 earthquake. The Denali Fault earthquake of 2002 caused a well in Wisconsin to rise more than two feet.

There's a mysterious connection between water wells and earthquakes, and scientists seem to notice it after every large earthquake, and even after some smaller ones. After reading how the giant Sumatra earthquake of 2004 triggered activity within volcanic Mount Wrangell, an Alaska graduate student took a look at recent large earthquakes in Alaska and Sumatra to see how wells in the state reacted to the big shakes.

Samik Sil, a graduate student from Calcutta, India, recently looked at a few dozen Alaska wells for his master's thesis at the University of Alaska Fairbanks. Hydrologists have monitored most of the same wells for years; they shared their data with Sil so he could check for water-level differences caused by earthquakes.

The wells, two on Ester Dome near Fairbanks, 18 near North Pole, and one near Anchorage, all showed some response to at least one of three large earthquakes, a magnitude 6.7 earthquake in the Alaska Range in late October 2002, the 7.9 Denali Fault earthquake of November 2002, and the giant 9.2 Sumatra earthquake that caused the devastating tsunami in December 2004.

The water levels in 10 wells rose a few millimeters in response to the Denali Fault earthquake, which had an epicenter within a few hundred miles of the wells. A bigger surprise was the wells' reaction to the giant Sumatra earthquake, which happened about 7,000 miles away. Sil found wells in North Pole responded to seismic signals from the earthquake by rising about 1.4 millimeters.

The wells reacted to the giant Sumatra earthquake in a way similar to volcanic Mount Wrangell, which heated up with small internal earthquakes as a response to surface waves reaching Alaska from the Sumatra earthquake. The waves compressed the volcanic plumbing system within Mount Wrangell, forcing a reaction.

Sil said there are several ways an earthquake can affect water levels in wells. Earthquakes can compress loose materials, reduce pore space and increase pore pressure to change the water level; a seismic wave can raise or lower water levels as it passes; and earthquake shaking can cause fractures in the ground that break the seal around wells. Permafrost confines many wells in Interior Alaska, Sil said, and these are vulnerable to fracture formation during a large earthquake.

That earthquakes can affect water wells halfway across the world shows how elastic the Earth's crust is and how freely seismic waves flow through the Earth, Sil said. Earthquakes near or far can affect the quality of well water (sometimes muddying it for awhile), or quantity. A 1998 magnitude 5.2 earthquake in northwestern Pennsylvania caused about 120 house wells to go dry within three months after the earthquake.

Today In Earthquake History:

May 31, 1917 - Shumagin Islands, Alaska - 7.9 - One of the Largest Earthquakes in the United States. (source USGS)

On the Web:

Earthquakes in the United States, Magnitude 7.0 and Greater
http://earthquake.usgs.gov/regional/states/large_usa_7.php

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