The chance of a moderate-size earthquake striking the Los Angeles area soon is almost guaranteed, if a new study is correct.
The Greater Los Angeles area has a 99.9 percent chance of having an earthquake of magnitude 5.0 or greater in the next two and a half years, thanks to several hidden faults that have built up considerable strain, according to a study published Sept. 30 in the journal Earth and Space Science.
But exactly where this next medium-size temblor could strike is less clear, because any one of the many faults that thread through the area could rupture.
"Identifying specific fault structures most likely to be responsible for future earthquakes for this system of many active faults is often very difficult," Andrea Donnellan, a geologist in the Science Division of NASA's Jet Propulsion Laboratory in Pasadena, California, said in a statement. [The 10 Biggest Earthquakes in History]
A whole lot of shaking
The findings are based on the researchers' analysis of the deformation of the ground surface in the L.A. region that occurred as a result of the magnitude-5.1 earthquake that struck La Habra, California, in 2014. That temblor caused no injuries, but its shallow depth and epicenter in a dense, urban region resulted in $12 million in damages.
After the quake, geologists analyzed how the crust had deformed. To do so, they used several tools, including GPS data from the Plate Boundary Observatory and radar measurements taken from a plane using an instrument called Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR). UAVSAR detects teeny shifts in the Earth's surface as the plane flies above.
The measurements revealed that the La Habra temblor caused more surface deformation than should be expected from a magnitude-5.1 earthquake. In fact, almost one-fifth of the deformation occurred without causing any ground shaking. That suggests that even relatively small-magnitude earthquakes can destroy water mains or cause other structural problems far away from the heart of the earthquake, the researchers said.
Network of faults
The team also used computer models to better understand exactly how the ground deformation occurred. They deduced that shallow movement along several buried faults in the earth in both Los Angeles and the surrounding counties had caused the ground movement.
In particular, the researchers focused on the faults running through the region's San Gabriel Valley (which lies east of L.A.) and Chino Hills (a suburb of the city located in San Bernardino County). These faults are threaded into a network, and not all of them have been mapped.
The new measurements suggest that some of those faults are still locked together, producing strain that could trigger the next earthquake. A magnitude 6.1 to 6.3 earthquake would relieve that accumulated strain, the study found.
Because the earthquake faults in this region are part of a system, "they can move together in an earthquake, and produce measurable surface deformation, even during moderate-magnitude earthquakes," Donnellan said. "This fault system accommodates the ongoing shortening of Earth's crust in the northern Los Angeles region," she said.
However, not every model says a decent-size earthquake is a sure thing for the Los Angeles area in the immediate future. The U.S. Geological Survey, which uses different models than the one the researchers used, puts the chance of an earthquake of magnitude 5 or greater in this area in three years at 85 percent, according to the agency's Facebook page.
But whether there's an 85 or 99.9 percent chance of a big quake, it's probably not a bad idea for Angelenos to have their earthquake kits and disaster plans ready.