John Galetzka, SCIGN Network Coordinator, holds a GPS antenna on top of the SCIGN site at Occidental College in Los Angeles. (The antenna actually sits inside the GPS "globe" in the background.)

Pacoima Dam and Occidental College host two of the 55 stations in the Southern California Integrated GPS Network ( SCIGN ). These GPS (global positioning system) receivers continuously send data up to satellites that measure hard-to-discern, constant creeping in the earth's crust. The Global Positioning System ( GPS ) is the latest technology to be employed in the study of earthquakes. This array of satellites in orbit around the earth makes it possible to measure locations on earth to within a centimeter or less. Why are these careful measurements of the earth's shapeÑcalled geodesyÑuseful to seismologists? A major earthquake is often preceded by a number of measurable changes near its epicenter. Among these so-called precursor phenomena are uplifting and tilting of the ground, caused by increasing pressure on the rock below. Both phenomena usually happen too gradually to be measured by seismometers. But precise GPS measurements can detect these subtle changes in the earth's surface, information that may make earthquake prediction possible. SCIGN is a collaboration between NASA's Jet Propulsion Laboratory, USGS (United States Geological Survey), SOPAC (Scripps Orbit and Permanent Array Center) and SCEC (Southern California Earthquake Center).

Shrouded in early morning fog/smog, the 113-meter-tall Pacoima Dam was constructed in the 1920s.

A GPS receiver is bolted to the face of the dam and sends continuous information up to satellites orbiting the earth.

On the Road with the Faultline Project
©1999 Exploratorium