According to a new study by the U.S. Geological Survey (USGS), some branches within the San Andreas fault system are locked, meaning that major earthquakes could be coming soon. This could affect millions living in the San Francisco Bay Area, as some of the fault lines are located underneath the heart of the area’s infrastructure. The research was led by James Lienkaemper and the results were published in the Bulletin of the Seismological Society of America.
The study examined aseismic creep, which is a slow steady movement in the Earth’s crust that relieves strain on fault lines as tectonic plates move relative to one another. However, if creep does not occur, the strain from the friction begins to build. The fault is then deemed to be “locked” and stores up that potential energy miles beneath the Earth’s surface. When the pressure becomes too great, it is released in the form of an earthquake. The magnitude of the earthquake is dependent upon how much potential energy is stored in the fault.
"The extent of fault creep, and therefore locking, controls the size and timing of large earthquakes on the Northern San Andreas Fault system," Lienkaemper explained in a press release. "The extent of creep on some fault sections is not yet well determined, making our first priority to study the urban sections of the San Andreas, which is directly beneath millions of Bay Area residents.”
The San Andreas fault system has five main branches that total about 1,250 miles in length in Northern California. The Pacific Plate to the west of the fault is pulling away from the North American Plate to the east at a rate of about 2.5 inches per year. The researchers found that four of the branches—Hayward, Northern Calaveras, Rodgers Creek, and Green Valley—are considered locked and in danger of creating an earthquake. Three of those are also nearing or overdue their historical averages for producing major earthquakes.
"The San Andreas Fault and its two other large branches, the Hayward and Northern Calaveras, have been quiet for decades. This study offers a good reminder to prepare today for the next major earthquake," said Lienkaemper.
The Hayward branch is about 40% locked and has a 30% chance of producing a magnitude 6.8 (M 6.8) earthquake within the next 30 years. The average length of time between major earthquakes along this fault is 160 years, and it has been 146 years since the last big one struck. However, that is just an average based on historical and geological records.
The other branches are also poised to unleash devastating earthquakes. The Northern Calaveras fault could create a M 6.8 earthquake, just like Hayward. Rodgers Creek (90% locked) and Green Valley faults each have enough locked energy to cause M 7.1 earthquakes. These three are at or near their historical recurrence averages, though it is difficult to predict exactly when they will strike.