Oklahoma has the unfortunate distinction of becoming the capital of human-induced earthquakes in the US. That's a recent development that comes thanks to deep injection wells used to dispose of contaminated water from oil and gas production. But the oil and gas industry has a long history with minor seismic activity—one that, it appears, may go back to Los Angeles in the early 1900s.

Los Angeles’ early growth had more to do with petroleum than celluloid. Oil was struck in 1892, and production took off over the next few decades. A recent study reviewed the subsequent history of oil production and earthquakes there but found no signs that earthquakes were triggered by human activity, at least dating back to 1935. The quality of earthquake data drops off rapidly prior to that, but US Geological Survey researchers Susan Hough and Morgan Page decided to see what they could find.

Starting at 1900, they compiled all the personal accounts of shaking they could find. There were a few early seismometers operating, partly the work of seismologist Charles Richter (of the no-longer-used Richter Scale of earthquake magnitude), though earthquake monitoring didn’t get serious until the 1920s. The limited monitoring left the researchers with only crude constraints on the location and magnitude of most earthquakes, but it’s something.

That something was then compared with industry records of oil activities in the Los Angeles area to look for suggestive correlations.

The researchers identified a handful of significant quakes—an estimated magnitude 4.9 near Inglewood in 1920, a 4.5 in Whittier in 1929, and a 5.1-5.4 quake in Santa Monica in 1930, for example. And then in 1933, a well-known magnitude 6.4 earthquake damaged buildings in Long Beach; aftershocks went on for more than a year.

Most of these earthquakes occurred close to active oil fields, and many had technical characteristics that made a connection to human activity plausible. Out of 18 earthquakes, the researchers argue that there is a “possible or likely association” with oil production for 13 of them, including the largest ones.

Although there were enough people living in the area by 1900 to notice earthquakes (plus both the Los Angeles Herald and the Los Angeles Times were in print), very few were documented between 1900 and 1915. But between 1915 and 1932, there were more than 300 of them. Oil production really took off in the early 1920s.

Over this time period, oil wells were also drilled progressively deeper, as operators figured out where the best reservoirs were. Critically, they had not yet begun the practice of injecting water back down the wells to replace the volume of fluid removed and help push the remaining oil up. The land surface over a Long Beach oil field eventually sank about 20 feet as the oil was sucked out, for example. The researchers think that this removal of mass without replacement could have triggered earthquakes by relieving some of the pressure on faults below.

The magnitude 6.4 Long Beach earthquake is a little more complicated. A larger and deeper earthquake, it looked more like a typical earthquake for the area. But since it occurred so close to an oil field that had just begun tapping deeper reservoirs, the researchers still think that oil production could have helped trigger it.

Even if there is no clear evidence for a general pattern of human-induced earthquakes after 1935, there may at least have been a handful in the very early days of LA oil production. Beyond historical curiosity, this kind of information is useful for figuring the area’s natural seismic risk. Earthquakes triggered by discontinued human activity—a few apples in the basket of oranges—could distort our understanding of future risks.

Bulletin of the Seismological Society of America, 2016. DOI: 10.1785/0120160157 (About DOIs).