New Web of Sensors Aims to Pinpoint San Ramon Earthquake Source

Mona Epstein thought the violent shaking that woke her was just a nightmare. But when she checked her phone, she saw that an earthquake had struck the San Ramon Valley.

“Earthquakes have infiltrated my dreams,” Epstein said. “It’s really nerve-wracking.”

Epstein lives in San Ramon, a typically quiet Bay Area suburb that has been jolted by 162 earthquakes of a magnitude 2.0 or higher since November. Three dozen small tremors shook her home in early February, popping open cabinet doors.

“It just won’t stop,” Epstein said. “It’s like the new normal, I’ve become accustomed to it.”

Seismologists call the sequence of small earthquakes a “swarm” and say it is normal for this part of the Bay Area, which lies atop a spiderweb of active faults.

They don’t know exactly how or why they’re happening, or whether they are occurring on a major fault — which may hint at the risk of a larger earthquake coming — or on one of the many smaller cracks nearby.

That’s why in mid-March, a group of United States Geological Survey seismologists and volunteers buried a network of 78 blue and grey toaster-sized seismometers across San Ramon and Danville.

“I don’t have any preconceived notions, I just want to find out what’s going on,” said Rufus Catchings, a research geophysicist with USGS, who is leading the work. “The data will hopefully tell us a lot more detail, like which faults are involved and whether earthquakes are happening on major faults.”

Catchings said a swarm on a smaller fault is “unlikely to generate a very large earthquake. But if it is connected, say, to the Calaveras or to one of the thrust faults, it could be very significant, and we need to know that.”

San Ramon sits in a part of Contra Costa County that is prone to swarms and has experienced them a handful of times since the 1970s due to a complex system of faults in the region, including the Calaveras, Concord-Green Valley, Pleasanton, Mt. Diablo Thrust, Greenville and Sherburne Hills Thrust.

Epstein has lived in San Ramon for 12 years and experienced multiple swarms. The latest quakes have left her with a mountain of unanswered questions.

“I read as much as I could to understand whether or not this means that [a big one] is coming,” Epstein said. “I don’t think there is any solid answer. That’s why they’re putting all those little sensors everywhere.”

A web of sensors

The grid of sensors will track movement underground for the next six months, and afterward, seismologists will dig them up and analyze their readings.

“We know for sure that we’re going to catch some really small earthquakes because they’re just going on all the time, and if the swarm does pick up again, then we’ll definitely catch that,” said Annemarie Baltay, a research geophysicist with USGS.

The battery-operated seismometers gather data around 200 times per second, Catchings said.

“We know that eventually there’s gonna be a big earthquake here,” Catchings said. “You can look at these mountains and see how they popped up through the tectonic forces.”

The researchers theorize that the swarm occurred along smaller sub-faults or due to liquid moving around these marginal cracks. But it’s hard to know exactly what’s happening five to 10 miles underground, Baltay said.

“It’s very likely that the fault goes down, turns, dips and moves around,” Baltay said. “This will help us sort of understand that fault structure at depth.”

Baltay said the findings could also show how different soils affect how waves travel through the earth. What researchers learn might require adjustments to the USGS’s National Seismic Hazard Model, which informs local building codes and insurance risk models.

Mark Armstrong, mayor of San Ramon, lives in a two-story home smack dab in the epicenter of the recent swarm of quakes. He said the shaking he feels is a reminder that he lives in earthquake country.

“A couple of earthquakes were literally in my backyard or across the street,” Armstrong said, recalling rumblings that shook pictures off his bookshelves. The USGS scientists installed a seismometer in his backyard. “There’s a little bit of excitement when you get one of the big ones.”

Armstrong said residents have one burning question: “When is it going to stop?”

Armstrong said the city is planning an emergency exercise with the American Red Cross, in collaboration with the City of Danville and Contra Costa County. The run-through will likely include activating an emergency operations center and shelters.

USGS seismologists also buried a sensor in Gina Veazey’s front yard. The Danville resident can tell an earthquake is going to shake her home when her dog, Stinky, goes “absolutely crazy” and “jumps off the bed.”

“Then all of a sudden we feel the tremor, so she’s a good indicator that something’s coming,” Veazey said.

Every time Veazey hears the groan of an earthquake, and then the rock and sway that follow, it’s a reminder that “Mother Nature is telling us we just got to roll with the punches that she throws. That’s all we can do.”