Every 174 seconds the ground in Southern California trembles as Earth’s tectonic plates shudder past one another. That amounts to 495 earthquakes every day, according to a new analysis of seismic data that isolated the very weakest measurable tremors in a region famous for its seismic jitters.

The new study, published this week in Science, adds nearly two million earthquakes to the catalogue of total seismic events in Southern California over the past decade. Although the newly known quakes were imperceptible to humans, the findings help fill gaps in the earthquake record, shedding light on the geophysical processes that lead to the dreaded “Big Ones.”

Scientists have long understood that earthquakes increase in number as they decrease in strength. But cataloguing those tiny tremblers is tricky. Their signals are often hidden within the general noise that seismometers pick up—including the shaking caused by construction, passing trains and even air traffic. That means the majority of earthquakes have long been hidden from science.

But 15 years ago, scientists noticed a pattern that enabled them to find these cloaked quakes: If two earthquakes originate in the same place, the shapes of their seismic waves are almost identical from the vantage point of a single seismometer. If one waveform zigs or zags, the other will follow suit—even if their magnitudes are vastly different. And that is key: If one knows the precise waveform of a quake, it is easier to find it within the noise. This means the signals of big quakes can be used to identify other, much tinier tremblings that travel along the same path.

This method is computationally intensive, however, so in the past scientists had only ever attempted to use it on relatively small data sets spanning days or weeks. But this did not deter Zachary Ross, a scientist at the California Institute of Technology, and his colleagues. The team had a loftier goal in mind: they wanted to scour the Southern California Seismic Network’s archive—which includes data from 500 stations that have constantly monitored the ground over the last 10 years—in search of previously overlooked earthquakes. The extensive search took 90 days’ worth of computing time. By the end the team identified nearly two million additional earthquakes that had occurred between 2008 and 2017—a tenfold increase over the original catalogue. “That alone might not be that exciting, but [the study authors] demonstrate how those additional detections of really small earthquakes do tell us more about how faults work,” says Roland Burgmann, a geophysicist at the University of California, Berkeley, who was not involved in the new research. “That makes it a beautiful study.”

For one thing, Ross and his colleagues discovered a number of earthquakes that were clustered together in time and space. “Everything seems to be related to some degree,” Ross says. Take the El Mayor–Cucapah earthquake that occurred on Easter Sunday in 2010 as an example: The shock originated in northern Mexico but triggered a series of smaller shudders within Southern California—some up to 275 kilometers from the epicenter of the initial earthquake. It is well known that large temblors can set off smaller ones, called aftershocks, and indeed scientists had already spotted such aftershocks associated with the 2010 event—but not at such great distances. So, the new study extends the known reach of earthquakes, Burgmann says, as well as shedding light on how that triggering works. Near an earthquake epicenter, aftershocks are brought about by the added stress resulting from the rearrangement of the Earth’s crust. But farther away from the epicenter, aftershocks are touched off by the shaking itself. The study results thus suggest that the latter might play a more significant role than previously expected—a crucial finding that will help scientists better forecast the likelihood of aftershocks on various faults in the future. If those faults contain the potential to unleash catastrophic events, the aftershocks will only help prime the fault for a Big One. As such, a better forecast could help save lives.  

The team also revealed a number of minuscule foreshocks—an equally important finding, given that scientists do not yet understand the details of how earthquakes get going. One idea scientists have floated is that a small section of a fault starts to slip slowly before picking up speed—and that this creates a series of shocks that gradually increase in strength over time and culminate in the main earthquake. Another idea is that a tiny rupture in one fault creates a cascade of shocks that travel along that fault (or a number of interconnected faults) like a set of dominoes as one event triggers the next, which triggers the next, until the main shock event is suddenly unleashed. 

Without better data on foreshocks it has been hard to determine which hypothesis is correct, so the new results might be a game-changer. For example, in the Brawley earthquake swarm that occurred in California in 2012, 10 earthquakes with a magnitude greater than 4.0 struck the desert over the course of three days. Although they appeared at the time to initiate quite suddenly, Ross and his colleagues were able to show that the onset was more gradual, with several minor earthquakes preceding the major ones. That hints the swarm might have been caused by the first scenario (i.e., a slow creep), says co-author Daniel Trugman at the Los Alamos National Lab.

Although Trugman notes that the hints the study gleaned on how foreshocks work are far from definitive, he is confident the new data set will allow scientists to better understand what precisely kicks an earthquake into high gear. And that may one day help scientists forecast earthquakes. “If we could really predict when the next big earthquake will occur, we’re in business—that’s the Holy Grail in seismic hazard analysis,” Trugman says. “I definitely wouldn't say we're there yet, but it's this type of work that's going to hopefully push us forward.”

Either way, Greg Beroza, a Stanford University seismologist who was not involved in the study, suspects that the most interesting findings are yet to come. He is excited to pour through the new catalogue in search of as-yet unknown phenomena—and there should be many. “It’s just like if a new telescope comes along and its magnification is 10 times greater,” Burgmann says. “Suddenly we see all of these stars and planets that previously we didn't see.”