Science Spotlight: Offshore Faults

by Aylin Woodward

The Santa Cruz community is fortunate to live in a coastal paradise—but the waters of Monterey Bay may obscure potential dangers. Far below the surface—10 miles beneath the seafloor, in fact—are seismically active faults that could be the site of offshore earthquakes in the bay.

Gary Greene, an emeritus geologist at Moss Landing Marine Laboratories, says there are two seismically active faults offshore: the San Gregorio Fault and the Monterey Bay Fault Zone. These faults rarely rupture: Paleoseismic studies suggest that a magnitude-7 earthquake probably occurred on the San Gregorio Fault before 1775, but that the Fault Zone shows no evidence of activity that recent.

Still, seismologists cannot overlook the potential hazards. Mapping from Sam Johnson of the U.S. Geological Survey shows that the San Gregorio Fault could produce a quake as large as magnitude 7.4—10 times more powerful than the magnitude 6.9 Loma Prieta earthquake of 1989. The much shorter Fault Zone would top out at magnitude 6.4.

Any offshore temblor bigger than magnitude 6 could threaten coastal residents, Greene says. Ground motions could damage buildings near the coast—especially if they haven’t been retrofitted since Loma Prieta.

A resulting tsunami in Monterey Bay isn’t likely. But if an earthquake set off a landslide in the submarine Monterey Canyon that bisects the bay, the crumbling walls could trigger a “local tsunami,” Greene notes. Scientists at the Monterey Bay Aquarium Research Institute are working to map where such landslide might occur.

Greene worked with UC Santa Cruz geophysicist Steven Ward to model possible tsunami heights in Monterey Bay. A large earthquake could send a surge of water at least 16 feet high onto the Seaside and Marina shores, the duo predicted. The risks are lower in Santa Cruz, Greene says, but residents should know to move away from the coast if a big quake strikes.

Note from the editors: Science Spotlights are a joint project of the UCSC Science Communication Program and Hilltromper. This article was published in December 2016.