Two major earthquake-generating faults in California’s Bay Area may be connected. That means that the next temblor to shake the Bay’s 7 million or so residents could be much stronger — and much more destructive — than seismologists anticipated, new research shows.

A longer fault can produce a bigger magnitude quake

The study, published today in the journal Science Advances, shows that earthquake forecasts for the Bay Area could be underestimating the magnitude of a future quake. The US Geological Survey (USGS) estimates that there’s a 72 percent chance a dangerous quake might shake the area in the next 30 years; the earthquake could be at least as powerful as Southern California’s magnitude 6.7 Northridge quake in 1994 that killed 60 people and left 20,000 homeless. Knowing how strong a future quake could be is critical for disaster preparation.

The two faults described in today’s study, which are both part of the San Andreas fault system, are well known to scientists. One is the Hayward fault, which stretches from the southern tip of the San Francisco Bay to the northern inland bulge that forms the San Pablo Bay. Another is the Rodgers Creek fault, which starts on the northern edge of San Pablo Bay and continues deep into wine country.

Whether — and how — these two faults connect underneath San Pablo Bay has been a mystery for years. But knowing if in fact they do is important, since quakes travel along a fault’s length: that means a longer fault can produce a bigger magnitude quake. (However, most earthquakes don’t travel the full length of a fault because bends, steps, and other geological discontinuities can stop the quake in its tracks).

Hayward fault and Rodgers Creek faults connect at San Pablo Bay, the little round body of water to the northeast of San Francisco Bay.

A team of scientists led by Janet Watt with the USGS used a kind of high-resolution sonar to visualize shallow depths in the Earth’s surface beneath San Pablo Bay. The researchers looked for the telltale signs of a fault — areas where lines of sediment suddenly jump, or bend. They also looked at magnetic anomalies in the area, which can reveal changes in mineral composition typical of faults. By combining the underground images with magnetic analysis, the researchers concluded that the Hayward fault bends about 10 degrees to the right to connect smoothly with the Rodgers Creek fault underneath San Pablo Bay. Watt first presented this research last winter at the American Geophysical Union conference.

"This opens up the possibility of an earthquake that could rupture through this connection," Roland Burgmann, a seismologist at the University of California, Berkeley who was not involved in the study, tells The Verge in an email.

Last year, Burgmann and other seismologists discovered that the Hayward fault also extends south to connect with the Calaveras fault, which stretches about 70 miles from near Mount Diablo to the Salinas Valley. Adding the Rodgers Creek fault to the north now means that there’s a wider range of earthquake sizes and lengths that could occur along this fault system, Burgmann says.

How these two faults connect underneath San Pablo Bay has been a mystery for years

Right now, predicting exactly when and where a quake will strike is impossible. But sensors deployed near fault lines can pick up the first waves of a quake and send an alert before those waves reach population centers. During the 2014 South Napa earthquake, the USGS’s early warning system called ShakeAlert gave test users in Berkeley a five-second warning before the shaking started. That could give people enough time to turn off gas lines and stop trains, if they were ready to.

In August, the USGS distributed $3.7 million to six universities working on the next generation of ShakeAlert — but it’s going to take a sizable investment to develop it into a functioning public warning system. In June, the State of California budgeted $10 million for the early warning system, but that falls short of the $38.3 million the USGS estimates it will need to finish developing and deploying ShakeAlert across the West Coast, and $16.1 million every year to maintain it.

In the meantime, better understanding the fault system that could cause the next big earthquake is the only way to plan for a possible worst case scenario — a 7.4 magnitude earthquake. That’s five times stronger than the Loma Prieta quake that shook San Francisco in 1989, killing 63 people and causing $6 billion in damage. "It is a scenario we shouldn't rule out," Burgmann says. "That is important when it comes to being prepared for the biggest possible earthquake we believe we can have in the East Bay area."