Study finds California faces new tsunami risk

As the new big-budget disaster movie "San Andreas" put the major Californian fault in the spotlight, a new study has called public attention to new risks of earthquakes and tsunamis offshore Southern California.

The latest study on the little known, fault-riddled, undersea landscape off Southern California and northern Baja California has revealed more worrisome details about a tectonic train wreck in the Earth's crust with the potential for magnitude 7.9 to 8.0 earthquakes.

The study, accepted for publication in the Journal of Geophysical Research: Earth Surface, supports the likelihood that these vertical fault zones have displaced the seafloor in the past, which means they could send out tsunami-generating pulses towards the nearby coastal mega-city of Los Angeles and neighboring San Diego.

A real-life offshore earthquake and tsunami wouldn't exactly follow Hollywood's script for a washout of Los Angeles or San Diego. Nevertheless, the hazard deserves more attention than it's received, said Geologist Mark Legg, the study's lead author.

Legg said in the study that surveys of the region show a "complicated logjam" of faults caused by the smash-up of the Pacific tectonic plate and the North American plate.

The blocks are wedged together all the way from the San Andreas Fault on the east, to the edge of the continental shelf on the west, from 150 to 200 km offshore, he said in the study.

These chunks of crust get squeezed and rotated as the Pacific plate slides northwest, away from California, relative to the North American plate. The mostly underwater part of this region is called the California Continental Borderland, and includes the Channel Islands, he said.

By combining older seafloor data and digital seismic data from earthquakes along with 4,500 km (2,796 miles) of new seafloor depth measurements, or bathymetry, collected in 2010, Legg and his colleagues found that along the Santa Cruz-Catalina Ridge Fault are ridges, valleys and other clear signs that the fragmented, blocky crust has been lifted upward, while also slipping sideways like the plates along the San Andreas Fault do.

Further out to sea, the Ferrelo Fault zone showed thrust faulting, which is an upwards movement of one side of the fault. The vertical movement means that blocks of crust are being compressed as well as sliding horizontally relative to each other -- what Legg describes in the study as "transpression."

As Southern California's pile-up continues, the plate movements that build up seismic stress on the San Andreas are also putting stress on the long Santa Cruz-Catalina Ridge and Ferrelo Faults. And there is no reason to believe that those faults and others in the Borderlands can't rupture in the same manner as the San Andreas, said Legg in the study.

However, "there is no need to panic and worry but you should be prepared now that we know there is a local tsunami potential," Legg advised Southern Californian residents. Endi