(FOX40.COM) — A earthquake with a magnitude of 4.1 hit near Isleton, a small city in Sacramento County along the river, according to the United States Geological Survey. 

The city is 40 miles southwest of Sacramento, located on the easternmost edge of the Delta Region near the Sacramento River.

While earthquakes are common throughout California, they usually don’t occur at the fault line near the epicenter of Isleton.

Officials with the United States Geological Survey said the earthquake happened along the Midland Fault Line, which lies between Sacramento and Stockton.  

“Earthquakes are not too frequent there,” Robert de Groot with the USGS told FOX40.com. “It’s a known source of earthquakes in the area, so it was a matter of time that we have earthquakes on these faults, but they’re not too frequent.” 

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What are the fault lines near the Sacramento area? 

The Midland Fault isn’t the only fault line near Sacramento County, as some are more active than others.

Of the larger and more active fault lines, the Hayward Fault is closest to the Sacramento region, running along the East Bay in a north-south direction close to the water. 

The San Joaquin Fault Zone lies south of Sacramento and is only active on its edges, even farther away from Sacramento, according to the California Earthquake Authority. 

“There are thousands of faults in California in places where earthquakes could occur so it just matter if there’s motion on those faults,” de Groot said. “Basically there’s some slip that happens underground and often times many miles underground, then what happens are seismic waves release energy from that flip and it’s process we know well and it’s a process that happens very frequently.”

While earthquakes are uncommon in the Midland Fault, they are common throughout the state, as there could be at least 50 a day, according to de Groot. 

“Now it’s only certain earthquakes like ones that people can feel that actually notice, but most earthquakes are small and go unnoticed,” de Groot said.

How earthquakes are measured

To determine the size of earthquakes, a magnitude scale is used to measure the amount of energy released by an earthquake. 

“Every time you go up a unit in magnitude, the amount of energy is 30 times larger,” de Groot said. “For example, a magnitude 5.0 is 30 times larger than a magnitude 4.0.”

According to the California Department of Conservation, the largest magnitude earthquake in California was a 7.9 magnitude at Fort Tejon on Jan. 9, 1857. Two people died in that earthquake and the quake left a 220-mile surface scar.