For the past week, the ground near San Ramon, about 25 miles east of San Francisco, has been shaking. And shaking and shaking and shaking. Over 200 small quakes have now rattled the notoriously earthquake-prone region.

So time to panic, right? Or wait—is it time to breathe a sigh of relief, with all these small earthquakes dissipating the pent-up energy? Science, what is the answer?

“Any seismologist who tells you he or she knows is not correct,” says Peggy Hellweg, a seismologist at the Berkeley Seismological Laboratory. So….elaborate? “Every small swarm could be a foreshock. We think it’s a very small chance given the history of the fault,” adds Ole Kaven, a geophysicist at the United States Geological Survey. “But there’s no telling when and where exactly that type of event could happen.”

Welcome to the world of earthquake non-prediction. But as Kaven notes, the history of the Calaveras fault, which runs under San Ramon, does tell us something—and it’s actually pretty interesting.

USGS

Several earthquake swarms have struck the Calaveras since the 1970s, and none of them have triggered a big earthquake. If history is any indication, says Kaven, the swarm is likely to last another two weeks with the total number of quakes going into the hundreds. The longest recorded swarm in the area lasted 42 days.

The particular geology of the Calaveras fault makes it prone to earthquake swarms. The fault is creeping along at a more or less constant rate, and it has many different secondary faults branching perpendicularly from the main Calaveras fault, like twigs coming off a branch. When one of those secondary faults gets critically stressed, it could set off a swarm. The quakes in swarms are small, usually 4.0 or below, because those secondary faults are small; not much energy is stored in them in the first place.

That also throws some water on the “small earthquakes prevent the Big One” idea. True, those small quakes are dissipating energy, but it’s a tiny, tiny amount. “The fault patch slips a couple tens of centimeters—not even a drop in the bucket,” says Kaven. Even a 6.0 earthquake like the one in Napa last year is puny compared to the Big One. On our current earthquake scale, a 8.0 releases 1,000 times as much energy as a 6.0, which is, yes, logarithmic and completely counterintuitive.

While very unlikely, it’s possible a swarm could set off another fault. The Calaveras fault is connected to the Hayward fault, which is due for a big earthquake soon. Seismologists only recently realized the extent of the connection between the two faults; in fact, underground faults are very poorly mapped.

It’s a bit of a chicken and egg problem. “Earthquakes and our seismographs are our way of looking underground,” says Hellweg. In other words, seismologists map faults by looking at where earthquakes are happening. A fault that is not very active can fly under the radar for a long time. A 2002 earthquake swarm on the Calaveras fault turned out to be on a previously unknown secondary fault.

For now, says Kaven, the seismic activity is too much of “blob” to see exactly what’s going on at Calaveras. Once this earthquake swarm is over, seismologists might just have a better picture of the fault system under San Ramon.