Fault’s twists may shake up earthquake forecasts

Deep angles along San Andreas mean temblors may be stronger than predicted

A new twist on the San Andreas fault could shake up southern Californians preparing for the Big One.

The southern San Andreas isn’t vertical in most places, as previously thought. Instead, it twists in opposite directions along its length, a new study finds, meaning a rupture might shake some places harder than current forecasts predict.

“We now have a picture of a propeller-shaped San Andreas,” says Gary Fuis, a geophysicist at the U.S. Geological Survey in Menlo Park, Calif. Fuis and his colleagues report the new findings in the February Bulletin of the Seismological Society of America.

This shape could make the San Andreas — parts of which have been locked and building up strain as the Pacific and North American plates try to slide past each other — even more hazardous.

“The dipping geometry may allow for significantly larger earthquakes,” says Roland Burgmann, a geophysicist at the University of California, Berkeley. Tilted faults pack more surface area into a given depth, potentially releasing more energy when they break.

Scientists took advantage of magnetic rocks that abut parts of the San Andreas to help reveal the fault’s orientation. In computer simulations developed by Fuis’ team, only a tilted fault could explain patterns in the magnetic fields produced by these rocks. Measurements of seismic waves created by small quakes and manmade explosions — which travel at different speeds depending on a fault’s orientation — helped to fill in the picture in other places, as did data showing differences in gravity’s pull along the fault.

In its northern reaches, the fault leans to the southwest. As it snakes to the south, the San Andreas becomes vertical in the Mojave Desert. Closer to Mexico, it dips to the northeast, cutting into Earth’s crust at an angle of 37 degrees near San Bernardino, Calif.

Forecasts designed to predict how hard different places will shake when there’s a massive rupture in the southern San Andreas — which hasn’t happened since 1857 — might change when these new angles are taken into account.

A quake on a leaning fault can shake land harder on one side of the fault than the other. The 1989 Loma Prieta earthquake near San Francisco shook the town of Watsonville, which is south of the fault, twice as hard as it hit San Jose, which is the same distance away but to the north.

But exactly how quakes might shake out in southern California won’t be known until the forecasts have been recalculated.

“We’re adding this to our collection of 3-D fault representations … used to characterize seismic hazards in California,” says Andreas Plesch, a geologist at Harvard University.

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