Oklahoma has the unfortunate honor of becoming the capital of man-made earthquakes in the US. This is a recent development thanks to deep injection wells used to drain contaminated water from oil and gas production. But the oil and gas industry has a long history of minor seismic activity — one that, it seems, dates back to Los Angeles in the early 1900s.
The early growth of Los Angeles had more to do with petroleum than celluloid. Oil was found in 1892 and production boomed over the next few decades. A recent study reviewed the subsequent history of oil production and earthquakes there, but found no evidence that earthquakes were caused by human activity, dating back at least to 1935. The quality of earthquake data is declining rapidly before that, but U.S. Geological Survey researchers Susan Hough and Morgan Page decided to see what they could find.
From 1900 onwards, they collected all personal accounts of shaking they could find. A few early seismometers were in operation, partly the work of seismologist Charles Richter (of the disused Richter scale of earthquake magnitude), although earthquake monitoring did not become serious until the 1920s. Limited monitoring left the researchers with only gross constraints on the location and magnitude of most earthquakes, but it’s something.
That something was then compared to industrial data on oil activities in the Los Angeles area to look for suggestive correlations.
The researchers identified a handful of significant earthquakes — an estimated magnitude 4.9 near Inglewood in 1920, a 4.5 in Whittier in 1929, and a magnitude 5.1-5.4 earthquake in Santa Monica in 1930, for example. And then, in 1933, a well-known 6.4-magnitude earthquake damaged buildings in Long Beach; aftershocks lasted more than a year.
Most of these earthquakes occurred near active oil fields, and many had technical characteristics that suggested a link to human activity. Of the 18 quakes, the researchers say 13 of them have a “possible or probable relationship” to oil production, including the largest.
Although by 1900 enough people lived in the area to notice earthquakes (plus both Herald of Los Angeles 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.
Oil wells were also being drilled deeper and deeper during this period as operators figured out where the best reservoirs were. Crucially, they had not yet started injecting water back into the wells to replace the volume of liquid removed and push up the remaining oil. The land surface above a Long Beach oil field eventually sank about 20 feet as the oil was sucked out, for example. The researchers think this removal of mass without replacement could have triggered earthquakes by relieving some of the pressure on fractures below.
The 6.4-magnitude Long Beach earthquake is a bit more complicated. A larger and deeper earthquake, it looked more like a typical earthquake for the area. But since it happened so close to an oil field that had just started drilling into deeper reservoirs, the researchers still think oil production may have caused it.
Even if there is no clear evidence for a general pattern of man-made earthquakes after 1935, there may have been at least a handful in the very early days of oil production in LA. Historical curiosity aside, this type of information is useful for determining the area’s natural seismic risk. Earthquakes caused by halted human activity – a few apples in a basket of oranges – can distort our understanding of future risks.
Bulletin of the Seismological Association of America2016. DOI: 10.1785/0120160157 (About DOIs).