New USGS map shows increased likelihood of major earthquakes in California

The U.S. Geological Survey's new National Seismic Hazard Map suggests California may be at greater risk for larger quakes than previously thought.

However, the state may also see fewer smaller quakes than earlier models had forecast.

Related: Mapping the Santa Monica Fault presents unique challenges

The Seismic Hazard Map characterizes seismic danger across the country.

According to the map, 42 states face some risk of earthquakes, while 16 states, including California, are considered at "high risk."

USGS researcher Mark Petersen says the goal of the map is to show "where future earthquakes will occur, how often they will occur and how hard the ground will shake."

Related: What does it take to map an earthquake fault?

Petersen was part of the team that created the new map using the latest scientific and engineering models. The last version of the map was released was in 2008.

Back then, researchers thought most faults were isolated fractures in the Earth’s crust. Over time, that view has changed, USGS researcher Ned Fields said. 

"The big conceptual breakthrough for the new model is really the recognition that faults are not isolated," Fields said.

Instead, he said there is an interconnected network of faults, "kind of like a tree with different branches." 

Movement on one branch can spread to others, creating a bigger quake.

However, these larger quakes release a lot of seismic energy. That means there will be less energy left to create smaller quakes.

USGS’ Mark Petersen adds that larger quakes are also less frequent than smaller ones.

"Instead of a lot of small earthquakes, you are having some huge earthquakes."

In fact, the risk of a magnitude 8 or larger quake has gone up 40% from the 2008 map, while the chances of a magnitude 6.7 or less have decreased by 25%.