Scientists now know that they can predict the locations of future earthquakes by mapping the locations of fault lines. However, just a century ago this simple truth was not known.
Harry Reid’s discovery that earthquakes happen along existing fault lines provided the first understanding of the source and process of earthquakes. This discovery laid the foundation for the discovery of Earth’s crustal plates and plate tectonics in the late 1950s.
Reid’s discovery was called a major breakthrough in earth science and provided the first basic understanding of Earth’s internal processes and of how rocks behave under stress.
By 1750, scientists knew that there were fault lines (like long cracks) snaking through Earth’s upper crust where two dissimilar kinds of rocks came together. By 1900, scientists knew that these fault lines were associated with earthquakes.
The mistake scientists made, however, was to agree that earthquakes caused the fault lines. It was as though the crust had been a smooth block of rock that had been cracked by an earthquake, with one side sliding past the other to create the rock mismatch. Earthquakes happened, and fault lines were the telltale residues of past earthquakes.
Harry Fielding Reid was born in Baltimore in 1859. When he received his early schooling in Switzerland, these ideas were what were taught in geology classes. They were what Reid learned. However, earthquakes and fault lines were of little interest to Reid. Living in Switzerland focused his primary interest on mountains and glaciers.
Reid returned to Baltimore to attend college at Johns Hopkins University in 1865 (at the age of 16). He stayed long enough to receive a doctorate in geology in 1885. Beginning in 1889, Reid took positions as a university professor with a research emphasis on glaciers.
Reid traveled extensively through Alaska and the Swiss Alps mapping and studying glaciers, their movement, their formation, and their effects on the landscape. He wrote articles and papers on glacial structure and movement.
In April 1906 the great San Francisco earthquake struck and most of the city either toppled or was burned. In late 1906 the state of California formed the California State Earthquake Investigation Commission to study the San Francisco earthquake and to determine the risk to the state of possible future earthquakes. Reid was asked to serve as a member of this nine-member commission.
This commission study turned Reid’s interest toward earthquakes and fault lines. He mapped and studied the San Andreas fault line and roamed the central California coastal region mapping other fault lines. Always he searched for an answer to the question: What caused earthquakes?
Reid carefully studied the rocks along California fault lines and concluded that they suffered from long-term physical stress, not just from the jolt of a sudden earthquake. Reid saw that great stresses must have existed in the rocks along the San Andreas fault line for centuries, even for millennia, before the earthquake happened.
That meant that the fault lines had to have existed first and stress along them caused the earthquake. Stress built up and built up in the rocks until they snapped. That “snap” was an earthquake.
Reid developed the image of the rock layers along fault lines acting like rubber bands. Stresses deep in the earth along these fault lines pulled the rocks in different directions, causing these rocks to stretch, like elastic. Once the stress reached the breaking point, the rocks elastically snapped back, causing an earthquake.
Fault lines caused earthquakes, not the other way around. That meant that studying fault lines was a way to predict earthquakes, not merely study their aftermath. Reid had discovered the significance of the earth’s spider web maze of fault lines.
The destructive San Francisco earthquake of 1906 horizontally shifted land surfaces on either side of the San Andreas fault up to 21 ft (6.4 m).