Eric Calais, Purdue University (IMAGE) Purdue University Caption Eric Calais, a Purdue professor of geophysics, shows GPS equipment used to track movements of the Earth's surface as small as one millimeter. These tiny movements show the build up of stress that could lead to an earthquake and are used in evaluating the potential threat to an area. Credit Purdue News Service file photo/David Umberger Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.