San Andreas Fault Earthquake Risk Increased after Ridgecrest, Scientists Say

The 2019 Ridgecrest earthquakes may have increased the risk of a large quake along the San Andreas Fault, scientists have said. The earthquakes, which hit California in July last year, included one of the biggest events to hit the area for over 40 years. The magnitude 6.4 and 7.1 earthquakes were followed by over 1,000 aftershocks, with scars from the events so prominent they could be seen from space.

In a study published in the Bulletin of the Seismological Society of America, researchers have now analyzed the rupture process of the two largest earthquakes. Kang Wang, from the University of California, Berkeley, and colleagues used seismic data coupled with information on how events had changed the geometric shape and orientation of the land, to understand where the rupture started and how it evolved. Their findings showed the magnitude 6.4 earthquake played an important role in triggering the 7.1 event to the southwest.

The team found these events at Ridgecrest created a large amount of stress on the nearby Garlock fault segment. This is a 160 mile long fault that runs to the northeast of the San Andreas Fault. Ridgecrest links the Garlock and San Andreas faults.

The San Andreas Fault is part of the boundary between the North American and Pacific tectonic plates. It is around 750 miles long and is considered to be one of the most dangerous faults in the U.S.. This is because three major cities—Los Angeles, San Francisco and San Diego—sit along it.

Tectonic plates are constantly moving, so over time faults build up stress. Eventually, when enough stress has built up, it is released, sometimes as large earthquakes. There has not been a large earthquake at the San Andreas Fault for over a century, raising concerns one could be due. The U.S. Geological Survey currently calculates the odds of an earthquake of magnitude 6.7 in the Los Angeles area over the next 30 years at 60 percent. For the San Francisco Bay area, the risk is 72 percent.

In an article posted to the Temblor, Inc. website, a catastrophe modeling company, researchers say that the additional stress placed on Garlock could trigger an earthquake along San Andreas to the north of Los Angeles.

CEO Ross S. Stein and Shinji Toda, from Tohoku University, Japan, were not involved in the study by Wang et al.

In their own research, the pair looked at previous seismic activity to look at how fault friction changes over time and space. They used machine learning to create earthquake forecasts to try to work out future risk. In a statement, they said there is a 2.3 percent chance of a magnitude 7.7 earthquake along the Garlock fault over the next year.

They also said that if Garlock ruptured within 30 miles of the San Andreas Fault, it could raise the risk of an earthquake at the latter by a factor of 150.

"That translates into a 50/50 chance of a San Andreas Mojave section rupture (with a range, 25 to 67 percent), either immediately following a Garlock quake, or after some delay," they wrote. "We thus estimate the net chance of a large San Andreas earthquake in the next 12 months to be 1.15 percent, or one chance in 87."

This, they say, is still low, so "no one should panic." However, they say their findings should be taken into account by policy-makers when looking at how to prepare for a disaster.

san andreas
Stock image representing the San Andreas Fault. Some scientists have said 2019's earthquakes may have increased the earthquake risk along the fault. iStock