Which parts of Sacramento County have the highest earthquake risk? Here’s what experts say

The United States Geological Survey alerted Sacramento County residents to an earthquake detected in the area Wednesday morning through a ShakeAlert.

A 4.2-magnitude earthquake rattled the Sacramento-San Joaquin River Delta region of Northern California, but initial reports say it caused no damage.

Here’s how often earthquakes happen in Sacramento County and California, and where you might feel one:

How often do earthquakes occur in California?

According to California Geological Survey’s website, earthquakes are detected in California every day by seismographs that record the minute vibrations of the earth.

About 150 earthquakes occur every year that are big enough to be felt but do not cause any damage.

Major earthquakes occur in California about every 10 years, the website states.

How frequently do earthquakes affect Sacramento County?

Though there are no major active faults under the capital region, that does not mean an earthquake cannot occur.

In a message to The Sacramento Bee, seismologist Dr. Julian Lozos said Sacramento County as well as the entire Sacramento-San Joaquin Delta is one of the least faulted parts of the state. This is due to most of the land being formed from sedimentary processes compared to continental collisions.

But when quakes happen, the soft and wet Delta sediment only amplifies the shaking.

“Think of a box full of sand versus a box with a solid piece of granite, and what would happen if you shook those at the same intensity,” Lozos, an associate professor at Cal State Northridge, told The Bee. “The sand is going to go all over the place.

“There’s enough buildup of energy that the faults do occasionally reach their breaking points.”

There is a 76% chance that at least one earthquake of magnitude 7.0 or greater will strike Northern California within the 30-year period that started in 2014, according to California Earthquake Authority.

Where are Sacramento’s nearest faults?

While there are no known “active faults” within Sacramento County, there are at least two faults in the area.

Mormon Island Fault Zone runs along the eastern border of Sacramento County near Folsom’s Mormon Island Dam, and shaking from Willows Fault has also been detected near Sacramento County, but its presence within the county has not been confirmed, according to California’s Seismic Safety Commission.

Lozos said western portions of Sacramento County have a higher likelihood of strong earthquakes because they are near San Andreas and its associated faults.

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