Special Report: Central Coast earthquake forecasting

Scientists at the U.S. Geological Survey in Menlo Park say the largest earthquakes in California would be produced by the infamous San Andreas Fault, which runs just 10 miles east of Salinas. While the San Andreas Fault is one of the most studied geological marvels in the world, scientists are discovering more about another fault zone that runs through the Monterey Bay.

The San Gregorio-Hosgri Fault runs from Bolinas Bay near San Francisco down south to San Luis Obispo County.

This October marks 27 years since the magnitude 6.9 Loma Prieta earthquake struck the Santa Cruz Mountains. Its effects felt far and wide from San Francisco to Santa Cruz to Salinas. According to their research, geologists at the USGS in Menlo Park don’t think lightning will strike the same place twice, at least during our lifetime.

“The probability depends on when the most recent earthquake occurred,” said David Schwartz, a U.S. Geological Survey geologist. “So if the earthquake is very recent on one part, and it was longer ago on another part of the fault, the one that has the longest time has the highest probability.”

The San Andreas is split into three sections. The northern portion spans from the Bay Area to San Juan Bautista, the central area from San Juan Bautista to Parkfield, and from Parkfield south through the Salton Sea.

“I think the highest probability is on the southern section of the San Andreas, where the most recent large earthquake was probably in the late 1600’s,” Schwartz said.

Schwartz said where the San Andreas runs through the Central Coast would only rarely produce a large earthquake. That’s because where the fault runs through the Santa Cruz Mountains, it’s locked into place. But it’s a different story in San Juan Bautista, where there is a change in the fault. Geologists refer to that area as “the creeping section” because it’s moving. One can see the evidence in nearby Hollister where the Calaveras Fault, which is also a creeper, crosses roads, offsets sidewalks and bends curbs.

West of Salinas, deep in the waters of Monterey Bay is the San Gregorio-Hosgri Fault Zone. Once believed to be two separate faults, about a year and a half ago, geologists discovered they’re the same fault. There’s still lots to be learned about it.

“We also don’t know how often it ruptures, we don’t know its recurrence interval of large earthquakes,” said Sam Johnson, a marine geologist at the U.S. Geological Survey. “It’s largely because we’ve only lived in this area for a couple of hundred years and we don’t have a historical record that comes from living in a place like Japan where they’ve been keeping earthquake records for millennia.”

But they do know it is capable enough to produce micro earthquakes several times a year. As for a large earthquake, Johnson says most of the fault is offshore, but could have the same impact at the 1989 Loma Prieta earthquake.

“So the areas that are low-lying, filled with valley sediment or fill that would shake very strongly, that sediment would liquify and collapse and create structural problems,” Johnson said. “Salinas, certainly, sitting in the valley it is in could be prone to that kind of ground failure.”

While the USGS has earthquake instruments and monitoring stations across the state, it still needs help. Scientists rely on information from the public to understand the distribution of shaking. That’s why they ask everyone who feels an earthquake to get onto the USGS website and report it. The website also has a map where people can find out what the probability of a large earthquake in their area.