Oil pumping probably triggered six moderate earthquakes in the Los Angeles area between 1938 and 1944.
As the Second World War loomed, oil companies drastically ramped up drilling in southern California. Rigs sucked up so much oil that in some cities, such as Long Beach, the ground dropped. The resulting pressure and stress changes in the underlying rock probably triggered the quakes, say Susan Hough at the US Geological Survey in Pasadena, California, and Roger Bilham at the University of Colorado, Boulder.
The scientists calculated the rock stresses created by oil production and compared the results with historical records of earthquake damage. The comparison showed that quake locations overlapped with areas where rock stress would have increased because of drilling. If some of these quakes were triggered by drilling, then the number of natural earthquakes in the Los Angeles area may be smaller than previously thought.
In around 1960, oil companies began injecting water into their wells, and the number of triggered quakes declined.