Milne's instruments were eventually deployed throughout the British empire, and the first crude world maps of earthquake source regions, or epicenters , were subsequently obtained. The answer became evident from detailed studies of two notable earthquakes during this period that showed particular large and simple surface ruptures. On such faults, strain energy builds up gradually until some critical strength is overcome and slippage begins. Most of the energy released during fault slip goes into local heating and alteration of the fault zone, but a few percent goes into seismic waves that radiate out from the source region and can circle the earth many times. This elastic rebound process, in which earthquake faults are gradually loaded, slip suddenly, and then are gradually loaded again, was first formally proposed by the American seismologist Harry Reid following his study of the 1906 San Francisco earthquake.
Elastic rebound is an accurate model for most earthquakes and has withstood the test of time.
However, the elastic rebound model begs the question of the ultimate cause of earthquakes. A comprehensive answer to this question would have to wait another half century and would ultimately involve the overturning of many dearly held global geologic concepts. In 1912, Alfred Wegener postulated his theory of continental drift , in which the Earth was not statically cooling, with the continents and ocean basins remaining nearly fixed, but where continents somehow moved through the ocean basins, colliding and splitting, building mountain ranges, and generating earthquakes. Wegener's visionary hypothesis, although supported by fossil and geological evidence, particularly in the southern hemisphere, was largely dismissed by the scientific establishment for decades because he was unable to convincingly demonstrate how continents could somehow plow through the rocks of the ocean floor. These previously unimagined Benioff zones were soon recognized as enormous faults.