Some Early 20th Century Earthquakes in Los Angeles Area Might Have Been Man-Made

A new study from the USGS suggests that some early 20th century earthquakes in southern California might have been induced (man-made) by past practices that were used by the oil and gas industry.

In the new study, scientists evaluated the likely cause of several significant earthquakes within the Los Angeles Basin between 1900 and 1933, together with consideration of available oil industry records over this period.

They found that several damaging earthquakes, including a 1929 event near Whittier, California (estimated magnitude 5) and the 1933 Long Beach earthquake (magnitude 6.4) might have been induced by oil and/or gas production during the early decades of the Los Angeles-area oil boom. During the early decades of the oil boom, withdrawal of oil was not balanced by injection of fluids, in some cases leading to dramatic ground subsidence, and potentially perturbing the sub-surface stress field on nearby faults.

In the middle of the 20th century, so-called water-flooding techniques were increasingly employed to compensate for oil withdrawal and to increase production of fields that were becoming depleted. Because industry practices have changed, the results of this study do not necessarily imply a high likelihood of induced earthquakes at the present time.

“It has been widely assumed that induced earthquakes do not contribute significantly to hazard in regions west of the Rocky Mountains, but our research suggests that damaging induced earthquakes might have occurred in the past, ” said Susan Hough, USGS seismologist. “Our study further suggests that the rate of natural tectonic earthquakes in the Los Angeles basin for this time period might have been lower than previously estimated.”

Earthquake rates have increased sharply in recent years in some parts of the United States, including in Oklahoma, where earthquakes were formerly infrequent. Extensive past research has supported the conclusion that much of this increase is due to disposal of wastewater into deep geologic formations.

The study, “Potentially induced earthquakes during the early 20th century in the Los Angeles Basin,” was published Nov. 1 in the Bulletin of the Seismological Society of America.