When Will the Next Big One Hit? California's Bay Area Gets Hit With Two Earthquakes in One Day

Updated | If you are living in the Bay Area and thought you felt some shaking last night, it wasn't just you; two earthquakes hit outside San Jose. The quakes were relatively small—one was a 3.9-magnitude quake, the other was a 3.1, according to the United States Geological Survey (USGS). Can minor earthquakes like this provide any clues about future large ones? 

Earthquakes happen because the surface of the Earth is not one continuous sheet of rock. There are cracks, called fault lines, that split the Earth up into pieces. These pieces move around—and sometimes, their edges bump up and catch on each other. After a while, though, the pieces move so much that things become unstuck—and that causes an earthquake. 

It makes sense, then, that areas near these fault lines will have more earthquakes than others. Some fault lines have even become famous, like the San Andreas Fault, responsible for the deadly 1906 earthquake that struck San Francisco. Tuesday's earthquakes weren't on that fault line directly; they were on a branch known as the Calaveras Fault, CBS News reported.

San Andreas Fault Research geophysicist with the United States Geological Survey (USGS) Brad Aagaard shows members of the media a series of images that illustrate how shock waves from a catastrophic 7.8 magnitude earthquake on the San Andreas Fault would likely fan out across Southern California, at the three-day Los Angeles International Earthquake Conference on November 12, 2008, in Los Angeles. David McNew/Getty Images

However, as much as we might want or need to know when the next big one will hit, that’s a question that’s difficult to answer. We can sometimes tell when an area is particularly overdue by looking at previous earthquake patterns. As Kathryn Schulz described in her 2015 New Yorker article on the Cascadia Subduction Zone, seismologists know that region is overdue for a massive earthquake because that fault line has had massive earthquakes before—generally spaced out about every 243 years. Since the last big one on that fault line happened more than 243 years ago, the next large earthquake at the Cascadia fault line running under Washington state should happen—sometime.

Transitioning from when a big earthquake might happen to when it will happen is a leap that’s impossible to make. Seismologists do try to make predictions about where earthquakes are likely to do the most damage, or how the number of earthquakes might change over time. For example, some scientists at the University of Colorado in Boulder believe that small changes in the Earth's core might make earthquakes in the next five years more severe than usual. (That connection, however, isn't a surefire thing and the findings aren't yet published.)

"At this time, earthquake prediction is very far in the future. We can only predict areas where earthquakes are more likely to happen," said Camille Brillon, a seismologist at Natural Resources Canada. "At this point, it's unpredictable."

Some of that unpredictability is because we don't have a long history of scientifically tracking earthquakes. Yes, we can make some best estimates based on historical data, but that historical data is incomplete. Instruments to detect earthquakes have only been around for about a century, and early instruments weren't the most sensitive.

It’s a safe bet to assume that an earthquake—likely a very small one—will happen somewhere in the world today. Since the second California quake, there have been at least two others in North America, according to the USGS’s earthquake map. One hit in Colorado, and the other was on the Baja California peninsula in Mexico.

Neither was major.

This article has been updated to include comments from Camille Brillon.