The U.S. Geological Survey is raising serious doubts about a study that calculates a 99.9 percent chance of a large earthquake in the Los Angeles area in the next three years.
The USGS took the rare step of issuing a statement raising questions about the study, recently published in the journal Earth and Space Science and co-authored by a research scientist at NASA’s Jet Propulsion Laboratory in Los Angeles County.
“I have serious doubts that the conclusions of the paper are supported by the analysis that’s presented there,” Robert Graves, a USGS seismologist and the Southern California coordinator for earthquake hazards, said in an interview last week. The study lists Graves as a consultant who helped with calculations for the study, but Graves says he has deep concerns about the study.
The USGS said in a statement that the forecast “has not yet been examined by the long-established committees that evaluate earthquake forecasts and predictions made by scientists.”
The study found a 99.9 percent probability that a magnitude-5 or larger earthquake would strike the greater Los Angeles area between April 1, 2015, and April 1, 2018.
The study’s authors had been looking at the likelihood of another temblor within a 60-mile radius of last year’s La Habra earthquake, which knocked people to the floor, crumbled brick walls and displaced people from homes and apartments.
Graves said the 99.9 percent figure is suspect.
“It does not seem like a reasonable number,” he said. “I don’t know the method that was used to derive that. But basically, that’s saying that’s going to happen. And that level of certainty, to my knowledge, is just not attainable. We can never be that certain.”
The probability used by the USGS for such an earthquake in that area over the next three years is 85 percent, Graves said.
The study’s lead author, JPL principal research scientist Andrea Donnellan, said in an interview that the 99.9 percent figure was not a central conclusion of the paper and should not be viewed as an official forecast.
“As scientists, we were not putting out an official forecast. We were putting out something in a paper to test,” Donnellan said.
Donnellan said she considers the 99.9 percent number a test of a model, or algorithm, on the probability of future earthquakes.
“We never said in this paper we were predicting an earthquake. And we said that’s the probability of an event,” Donnellan said. “There is still a 0.1 percent chance it won’t happen. So we need to test it. And that’s what we are doing as scientists.”
She added that the USGS also leans toward a big earthquake in the next three years.
“If an earthquake happens in three years, we’re both right,” Donnellan said.
Donnellan said the estimate was actually a small point in the study. Its central conclusion was that the deeper layers of the Earth did not move in the La Habra earthquake in the way that the shallower areas did. That suggests that the deeper layers still have seismic strain, and an earthquake must eventually release that strain.
Graves said how the scientists came to that conclusion, too, is unclear. “The specifics in terms of the modeling, it’s a little difficult to tell the details; it’s not fully described in the paper,” Graves said.
Although Donnellan characterized the 99.9 percent number as not a central conclusion of the report, it has dominated public discussion about the study.