Uplift (red) and subsidence (blue) based on GPS data (top) confirm predicted motion (bottom).

An array of GPS instruments near the San Andreas Fault System in Southern California detects constant motion of Earth’s crust — sometimes large, sudden motion during an earthquake, and often subtle, creeping motion. The GPS array records vertical and horizontal motion of Earth’s surface. The challenge was to discern the broad, regional tectonic motion from the shorter-scale, local motion.

The team used a comprehensive statistical technique to extract from the GPS data a pattern of large-scale, smoothly varying vertical motions of the local crust. The pattern resulting from their data analysis was similar in magnitude and direction to motions predicted by previously published earthquake cycle model results.