When It Comes To Earthquake Intensity, Fault Line Size Is A Big Deal

Large swaths of the San Joaquin and Sacramento valleys are not at high risk for earthquakes, because there are no fault lines that have been active in the last 11,700 years, according to the California Department of Conservation. 

But the Sierra Nevada Mountains and the areas west of the Coastal Mountains are active, though the size of the fault lines in the Sierra are very different from the more famous ones to the west. 

The San Andreas and Hayward fault lines are long. Hayward is 74 miles and San Andreas is 800 miles — or half the length of California. The active faults in the Sierra are often only a couple of miles in length.

Richard Armstrong, an assistant professor of earthquake engineering with Sacramento State, says fault line length and width often contribute to more impactful earthquakes. 

"The faults on the coastal side are moving more,” he said. “They store energy. As they store more and more energy, at some point, they release energy."

The Hayward Quake of 1868 measured 7.0 and leveled the town, and  the San Francisco Quake of 1906 was on the San Andreas Fault and measure 7.8.

But Armstrong says recently active fault lines in the Sierra near Mammoth and Truckee aren't nearly as long, and aren't as likely to cause as much destruction. But the amount of rainfall and the type of soil can help a quake create damage.

"If it's saturated during shaking, the material can what's called ‘liquify,’" he said, adding that it “kind of looks like quicksand.” 

“If you go to the beach and shake your feet on the sand, it kind of liquifies,” Armstrong said, “That can be damaging to structures because they can settle, they can rotate.”

Armstrong says saturated, sandy levee soil in the Delta is of concern during an earthquake, though other levees around Sacramento are constructed in a manner to avoid liquifying. He couldn’t think of an instance in recent memory where a tremblor had caused a levee to fail.

There are hundreds of earthquake fault lines in the Sierra, but most have been inactive for thousands or millions of years.

There are seven small faults that have been active as recently as 1966 between Lake Tahoe and Stampede Reservoir, which is about 30 miles north of the lake. Another small fault caused a 5.9 earthquake south of Lake Oroville in 1975.

None of the multiple faults running under Lake Tahoe have been active in at least 11,700 years, according to the California Department of Conservation. On the agency’s map faults are color coded by the length of time they have been inactive, in some cases, more than 1.6 million years.

The study of fault lines in California is still a relatively new science and not all have been identified.

"They use paleoseismology, and that's where they dig a trench across a fault and from that what they determine is the age of some of the deposits," Armstrong said.

He says it will take time to for geologists and seismologists to develop a clearer picture of the actual intensity and fault-line location of the recent Southern California quakes, which took place in a cluster of shorter, active fault lines.