In earthquake prediction, geologists work in probability distributions rather than absolute terms. Consensus is growing, however, of the likelihood of a large and devastating earthquake to strike California. Why are scientists beginning to worry? The San Andreas Fault hasn’t produced a big earthquake in that region of the fault since the M7.9 earthquake hit California in 1857, 159 years ago.

The theory behind why we expect the next significant earthquake to be a big one becomes apparent in a few back of the envelope calculations. The average rate of plate movement along the San Andreas Fault has remained fairly consistent at approximately 2 inches per year for the last several million years. Given the aforementioned 159-year gap in a major earthquake along part of the San Andreas Fault, that would mean average plate movement has accumulated 26 feet of movement since then.

The accumulated strain built up from 159 years and 26 feet of plate movement provides the ammunition needed for a major earthquake on the order of M8.0. At some point the rock holding that strain will break when the strain overcomes the tensile strength of the surrounding rock. At this point, the two plates will quickly snap to a state of equilibrium or close to it, thrusting the plates from their current position.


Why Can’t We Predict Earthquakes?

As you likely know, the 1906 San Francisco earthquake (M7. was one of the most powerful in the recent century and led to mass destruction of the San Francisco bay area from earthquake related damages as well as fire. Unfortunately, there are no reliable ways to predict an imminent earthquake, much like we do for practically every other natural disaster.

We can measure increased toxic gasses and increased heat flow to predict volcanic eruptions, measure wind and weather patterns via satellites, and measure tsunami potential from earthquakes. Unfortunately, there are no telltale signs of an imminent earthquake. There are early warning messaging apps that will notify you of any impending danger. The only certainty is that the plates continue to move whether there is an earthquake or not to relieve the stress. The longer the time between earthquakes, generally leads to more powerful and devastating earthquake.

You can find a multitude of articles that claim with almost certainty that we will have a major earthquake in California “tomorrow.” However, the truth is we just don’t know when the next earthquake will strike. Scientists recognize it is becoming increasingly probable of a significant earthquake, but those conclusions are largely grounded in the aforementioned back of the envelope calculation and limited concrete predictive analyses.

A scary truth is that one single earthquake in this region is unlikely to happen in isolation. Stress has built up along much of the San Andreas Fault and the San Jacinto Fault. It is likely that a major release of stress through an earthquake would trigger the release of stress along the entire fault complex. With earthquakes, it is often unlikely to completely remove the stress within a fault zone, but rather push it down the line to another point within the fault.

In 2008 the U.S. Geological Survey published a report on the predicted damage from a M7.8 earthquake along the San Andreas Fault. The report predicted more than 1,800 deaths, 50,000 injuries and a total of over $200 billion in damage.

The truth is we just don’t know when the next big earthquake along the San Andreas Fault will be. It could be tomorrow or 10 years from now. However, we can say with a good deal of confidence that the area is primed for a large earthquake with devastating potential. Now is the time to upgrade infrastructure, early warning systems, and education on earthquake preparedness before it’s too late.