A new study that looked at earthquakes in Southern California over the last decade found that 72 percent came after smaller, precursor quakes — and this could represent a breakthrough in the science of earthquake prediction.
The study, published in late July and led by Los Alamos National Laboratory seismologist Daniel Trugman, looked at earthquakes of magnitude 4.0 or greater in the Southern California region between 2008 and 2017. The data confirms what theoretical models and lab experiments on earthquake nucleation have long suggested: Foreshocks almost always precede big quakes by a matter of days or weeks.
Earlier studies suggested that less than half of big quakes were preceded by foreshocks, but the new study was able to use data from "the tiniest magnitude events that were basically invisible before," as study coauthor Zachary Ross tells the LA Times. The range of time between foreshocks and mainshocks seen in the study was between three and 35 days.
A total of 46 earthquakes happened in the study region during that nine-year period, and of those, 33 were found to have been foreshadowed by smaller quakes. A magnitude 5.1 earthquake in March 2014, called the La Habra earthquake, was actually preceded by a long sequence of foreshocks, some in the zero to 1-magnitude range, beginning 17 days before the main event.
"It’s never just silent until the final failure," says Ross, regarding lab simulations in which seismologists have often found foreshock activity prior to bigger fault ruptures.
Still, knowing which earthquakes actually are foreshocks is a trickier matter — only about 5 percent of earthquakes are followed by larger events.
The study results are likely to lead to improvements in earthquake early-warning systems — and these can use all the help they can get. An early-warning system that's been years in the making in LA failed to alert people to the two large earthquakes in the Mojave Desert on July 4 and 5 — though that was apparently because the app is only set to warn of quakes that occur in Los Angeles County.
A piece in the New York Times last month discussed how those two large quakes actually triggered 16,000 smaller ones on the same fault system over two weeks — a so-called swarm that scientists say they did not predict because their understanding of the interconnected fault system was incomplete.