Harry Fielding Reid, an American geophysicist, revolutionized seismology by moving beyond simple observation to mechanical explanation. By examining the displacement of fences and roads along the San Andreas Fault, he concluded that earthquakes are not caused by the movement of the fault itself, but by the release of accumulated elastic strain. His theory displaced older ideas that earthquakes were purely volcanic or chemical. Today, “Elastic Rebound” remains the core principle taught in structural geology, explaining the cycle of stress accumulation and release that defines the behavior of active fault lines across the globe.