If a major earthquake hits in Southern California, does that mean an earthquake will soon happen in Northern California, and vice-versa? How worried should I be when a major earthquake occurs hundreds of miles away from me? Read on to learn more about fault lines and what the experts say.

Moving Fault Lines

We often start thinking about earthquakes just after a major earthquake occurs somewhere else. That’s a great moment to prepare because a major earthquake hundreds of miles away could trigger an earthquake near you.

Really? Yes…

When a major earthquake happens on one fault line, it affects other faults, even ones that are hundreds of miles away. The earth’s tectonic plates are like a bath filled with rubber ducks. If one duck moves, all the others move – at least a little. Earth’s floating tectonic plates are similar, but a lot more complex. The edges of the earth’s tectonic plates are not smooth like a rubber duck, so the movement of the plates is not as easily transferred. Crags and irregularities in the bedrock create friction that prevents the plates from sliding smoothly.

Movement at one fault line causes stresses to be distributed to a different part of the fault line and nearby fault lines, either adding stress or reducing stress. The earthquake reduces stress on the fault line in the ruptured region and at nearby parallel fault lines. The energy of the earthquake dissipates built-up frictional energy, which means that, after the initial aftershocks, another large earthquake is not likely in the immediate vicinity. But beyond the ruptured region, where there wasn’t a slip, the fault stress increases, leading to a greater probability of an earthquake.

1906 San Francisco Great Earthquake

Scientists understood this post-earthquake energy release and distribution from the 1906 San Francisco earthquake. The 60-second long, M7.8 earthquake was so large that it reduced all earthquake stresses on the SF Bay Area’s many fault lines and led to a “quiet” period. See the timeline below, showing the earthquake lull between 1906 and 1989.

Earthquakes Triggered Thousands of Miles Away

Did the 2019 Ridgecrest earthquake trigger minor earthquakes near Seattle, Washington and in the SF Bay Area? Or, would the M4.3 and M.4.6 earthquakes have occurred at that time anyhow? Major earthquakes not only affect areas that are hundreds of miles away, but also thousands of miles away. If a fault is already at the brink of its stress capability, it can be tipped over the edge by a major earthquake thousands of miles away. This is called “dynamic triggering.” Experts at the United States Geological Survey (USGS) state there’s “evidence that some major earthquakes manage to trigger seismicity over much greater distances (thousands of miles), but these triggered quakes are small and very short lived.” The M7.9 Denali Fault, Alaska earthquake in 2002 is a well-known example of dynamic triggering. The earthquake triggered 19 other earthquakes from Alaska to San Diego, California, roughly 2,300 miles away. See the red crosses below for for the Denali Fault dynamic triggering.

The Next Major Earthquake

When we feel the slight shaking of a nearby M4.0 earthquake, we start wondering if it’s a foreshock to something bigger. But, larger earthquakes farther away could also be a clue to future shaking. This is especially true in California where many fault lines are waking up from their lull and approaching their tipping points. Check out Are We Due for a Big One, for a fault line basics refresher.