A new study in the journal Geology presents an analysis of rocks drilled from a section of the San Andreas fault here in California showing that the slow creep of the faultline may be more dangerous than previously thought.

The San Andreas faultline is nearly 1300 kilometers long and runs the length of the state of California, from Baja in the south to the Mendocino Triple Junction in the north. It’s the dividing line between the Pacific Plate and the North American Plate, which are rubbing alongside each other as one moves north and the other moves south. It has been the site of several major earthquakes in the past couple of centuries, including the 1906 San Francisco earthquake, the 1989 Loma Prieta quake, and the 1994 Northridge quake. All three of these earthquakes resulted in death and destruction and were the result of a sudden, fast, and extreme slip between the two plates. Basically, they stick for a long time and then suddenly slide and cause a massive quake.

But if you know your California geography, you might have noticed something about the locations of those three quakes – two were in northern California and one was in southern California. So what happens in the middle of the state and the middle of the faultline? Not much. It’s not that quakes don’t happen. It’s that they happen pretty regularly but on a smaller scale. The central section of the San Andreas moves at a nice, steady 26-millimeters per year pace. It’s a process known as aseismic creep, and it has generally been thought to be a safe process than the massive slippages of the northern and southern sections.

In fact, the quakes that do occur in this central section tend to happen at the southern end of it and are only around magnitude 6, which is pretty mild by large earthquake standards. For contrast, the 1906 quake was a 7.9 magnitude, and the most deadly quakes around the ring of fire tend to be even greater than that, with several in excess of magnitude 9, including the Tohoku quake in Japan in 2011.

But just because we think this central section of the San Andreas is safer doesn’t necessarily make that statement true, and scientists always want to be certain of their results. So a team of researchers drilled out samples about three kilometers below the surface to see just how often the rocks had been altered due to the frictional heat of the slipping fault. And they found signs of more than 100 quakes that occurred in that deep section of rock, with the fault slipping about 1.5 meters during one event. That comes out to a 6.9 magnitude quake, which is the same as the 1989 Loma Prieta event and slightly larger than the 1994 Northridge quake. Of course, the method being used to calculate the magnitude is still evolving, so that quake may have been more energetic than the current calculations suggest.

What this research comes down to is to tell us that the central section of the San Andreas, which most scientists have considered relatively quiet and therefore safer than the other sections, could actually be just as dangerous and harbor the same potential for a massive quake. Not very reassuring to that section of my fair state.

But remember, California is very experienced with earthquakes, and the laws and building codes have improved in the past 40 years with lots of retrofitting going on where necessary. To be safe, if you live someplace prone to natural disasters such as earthquakes, it’s always best to be prepared, so make sure you check with local disaster agencies and prepare an emergency kit ahead of time.

This has been a PSA from the Daily Space.

More Information

Columbia Climate School press release

“History of earthquakes along the creeping section of the San Andreas fault, California, USA,” Genevieve L. Coffey et al., 2022 February 25, Geology