There was some cause for concern over the weekend when the United States Geological Survey (USGS) announced there was a 1 in 100 chance of a large earthquake on the San Andreas Fault, but thankfully the danger seems to be averting. So what happened?

The USGS had tracked a so-called earthquake swarm in the Salton Sea, near Bombay Beach in California, beginning on Monday, September 26. These swarms, small tremors in a fault line, can be an indicator that a large earthquake is about to occur, prompting the warning. Without swarms, the risk of an earthquake in a given week is 1 in 6,000, according to the LA Times.

The USGS said 96 earthquakes above magnitude 2 were reported by September 30, at a depth of 4 to 9 kilometers (2.5 to 5.6 miles), with the largest being magnitude 4.3. This initially led the USGS to say there was as high as a 1 in 100 chance of a magnitude 7 or greater earthquake occurring by October 4. As of Friday, September 30, though, this likelihood was decreasing.

“Preliminary calculations indicate that… there is 0.006% to 0.2% chance (less than 1 in 10,000 to 1 in 500) of a magnitude 7 or greater earthquake being triggered on the Southern San Andreas Fault within the next seven days through October 7,” the updated warning states. “These revised probabilities are lower than those made earlier this week, due to decreasing swarm activity.”

Swarms are caused when two plates slip alongside each other. In this case, California is slipping sideways with respect to the main North American continent, which can create a small line of mini-earthquakes along the fault.

Speaking to IFLScience, David Rothery, Professor of Planetary Geosciences at the Open University, said that earthquakes normally occur without warning, with no swarm activity, but they can be an indicator of a larger upcoming quake.

“The big earthquake in Italy, L'Aquila [in 2009] was heralded by a series of small earthquakes,” he said. “So that is a case of activity before a major earthquake. It’s a very different tectonic situation in California, but it’s an example of why people are likely to be concerned when there is a swarm of earthquakes.

“California in particular is well monitored by seismologists, who do a pretty good job. If they say there’s a 1 in 500 chance in the next seven days, I tend to think they’re probably right, and there’s no need for anybody to panic.

“But you don’t always get a swarm. Earthquakes tend to strike out of the blue.”

Of course, people in California are well prepared for a quake. But while this danger of an earthquake seems to be passing, it’s a reminder that, at some point in the future, there’s probably going to be a “really big one”.

Image in text: Earthquakes in the Brawley seismic zone as of the evening of September 30. USGS