Oil and gas production may have contributed to four of the five most powerful Los Angeles Basin earthquakes of the early 20th-century oil boom, a new study showed Monday.

Scientists said the Inglewood earthquakes in 1920, Whittier quakes in 1929, Santa Monica in 1930 and Long Beach in 1933 may have all been caused by oil activities.

Oil or gas work started in these areas shortly before these earthquakes struck, according to Susan Hough and Morgan Page of the US Geological Survey, who wrote the study in the Bulletin of the Seismological Society of America.

The researchers did not show direct causation.

"What they showed is that the conditions are such that the earthquakes could well have been triggered by oil pumping activity," explained David Jackson, professor emeritus of seismology at the University of California, Los Angeles, who was not involved in the study.

The most powerful of the quakes -- in Long Beach, near Los Angeles -- registered at 6.4 on the Richter scale. It killed 120 people and caused $50 million in damage in 1933.

The findings that human activity was at play could lead scientists to revise estimates of seismic risks in the Los Angeles Basin and improve understanding of the effects of oil and gas as trigger mechanisms for earthquake elsewhere.

"Maybe the Los Angeles Basin is geologically more stable than is currently estimated," Hough said.

Previous studies had concluded that there was no indication of other earthquakes caused by human activity in this area of California after 1935 when output slowed.

For their research, scientists relied on a set of geological research, oil industry data, government agencies and newspaper articles from the time.

"This study brings in the idea that oil and gas production activities can generate large-magnitude earthquakes," said Richard Allen, director of the Seismological Laboratory at the University of California, Berkeley, who also was not involved in the study.

"We need to start recognizing that there's a growing body of evidence that oil and gas production activities can generate large-magnitude, damaging earthquakes. And that's something we should all take very seriously."

The scientists urged against comparing these California earthquakes, which were potentially related to oil and gas drilling, to present-day temblors in Oklahoma and Texas apparently caused by fracking, a process involving injecting massive amounts of waste water into very deep wells.

There were nearly a thousand earthquakes of at least magnitude three in Oklahoma last year, compared to an average of two per year between 1978 and 2008 in that state.

In September, Oklahoma recorded the strongest earthquake in its annals with a magnitude of 5.8.