At 5:12 a.m. April 18, 1906, the San Andreas Fault ruptured. The Great Quake, with a magnitude 7.8, split 300 miles of earth, and was felt from Los Angeles to Nevada and up to the Mendocino Coast, according to seismologist reports. The epicenter, San Francisco, was destroyed and further north in Santa Rosa, a downtown was demolished. Researchers say the death toll, put at 700 then, could have been underestimated by a factor of three or four.
In the decades after, cities were developed, population boomed, the Bay Area urbanized. Then, in 1989, a 6.9 earthquake tore the Bay Area apart again. The Loma Prieta decimated 16,000 homes, racked up an estimated $10 billion in damage and killed about 60 people.
While those earthquakes, the Bay Area’s most destructive in the last 105 years, tore apart the Bay Area’s economy and infrastructure, important scientific discoveries were made about California’s faults. Building codes were reinvented to better withstand the next big one. Man couldn’t predict nature, but we could be ready for it.
Earthquake scientists today say there’s a 62 percent chance of an earthquake with a magnitude 6.7 or greater to strike the Bay Area in the next 30 years — and the worst place to be is on the southern section of the Rodgers Creek Fault — in Petaluma, Cotati and Rohnert Park.
“In our probability studies, the Rodgers Creek Fault has the highest probability of producing the next large Bay Area earthquake of any of the faults we’ve looked at,” said David P. Schwartz, an earthquake scientist and with the U.S. Geological Survey and co-chair of the Bay Area Earthquake Alliance. “Towns like Petaluma and Rohnert Park, which are really very, very close to the fault, are going to suffer a lot of damage.”
“When we have that large earthquake, we’re going to see a certain amount of liquefaction damage, lateral spreading … shaking, it’s just inevitable,” Schwartz said. “What people here should be aware of is that we’re not making these probabilities up.”
Schwartz explained the likelihood of a major earthquake is so great because since 1906 there’s been very little earthquake activity in the Bay Area.
“When 1906 occurred, it was so large that it released all of the stresses in region, and it relaxed the earth’s crust — all of these faults, like the Rodgers Creek — were relaxed,” Schwartz said. “The Bay Area has been very very quiet in terms of earthquake activity ever since. That’s why we’re so concerned.”
According to Schwartz, the Rodgers Creek historically has produced a large earthquake, over a 7.0 magnitude, every 220 years, and it’s been 300 years since the last one.
“The earth took a big breath, and it’s been holding it,” he said.
Maps produced by the Association of Bay Area Governments, or ABAG, put Rohnert Park and Cotati in the “violent” category of damage to the city from earthquake shaking — by far the most destructive category, according to the USGS. Liquefaction maps show Rohnert Park and Cotati at the highest destruction level, at “high hazard.”
"We do take this very seriously," said City Manager Gabe Gonzalez. "We've had [earthquake] drills and developed emergency preparedness plans."
Schwartz said many people aren’t aware of the high risk because seismologists have focused on talking about cities like San Francisco and Oakland.
“I have a feeling we’ve concentrated a lot in speaking about the Hayward Fault, because it’s right in the center of the Bay Area and it’s very very urbanized,” Schwartz said. “Whereas the Rodgers Creek really passes through beautiful countryside, vineyards and pasture land. It’s not as urbanized.”
“But statistically,” he said, “the Rodgers Creek in the next 30 years has a higher probability of producing the Bay Area’s next large earthquake.”
While researchers from ABAG, Sonoma State University and the USGS say it’s nearly impossible to predict the probability of loss of life, it’s inevitable that earthquake will wreak havoc on the local economy.
Natural Environment: Liquefaction and Shaking
Even though most of Rohnert Park sits on clays, which aren’t susceptible to liquefaction, there’s also coarse sands, silts and gravel mixed in. Those types of soils can liquefy during an earthquake.
“It’s exactly like quicksand,” said Stephen Norwick, a professor of environmental studies and planning at , who also has multiple degrees in geology.
In Rohnert Park, about a half-dozen waterways meandering underneath the city that were straightened over time and built over, pose the biggest threat to property and life during a disaster.
“Rohnert Park has many older channels, like Copeland Creek, that are still channeling water. In an earthquake, those places are likely to liquefy,” Norwick said. “The birds and bunnies are gone, but the wetter soils are still there.”
In addition, much of Rohnert Park, Petaluma, Santa Rosa and Healdsburg are built on young, soft soil deposits that magnify shaking during an earthquake, according to Schwartz. The true impact, however, is impossible to predict, as one house could be built on clay, like many in Rohnert Park are, and the house next door could sit on bedrock.
“The geology is so variable, that in some places the shaking could be dampened because the house sits on bedrock, and some places will suffer amplified shaking and thus, increased damage,” Schwartz said. “The house on the saturated deposits can be very very strongly damaged, and the house on bedrock can look like almost nothing has happened.”
But in Rohnert Park, where liquefaction and violent shaking are both high, widespread damage awaits.
Impact on Buildings, Infrastructure
Rohnert Park’s housing stock, much built after 1960, could do fairly well during an earthquake. An investigation by Rohnert Park Patch into the city’s building records show that the majority of the city’s structures are single-family, one-story homes built from wood.
“People don’t die in one-story or two-story wooden structures, that’s very unlikely,” Norwick said. “But it’s really important to emphasize that disasters do more than kill people — they ruin people’s lives.”
The most dangerous buildings, according to ABAG and county building officials, in order of danger, are unreinforced masonry buildings, soft-story buildings and then single-family homes.
According to the California Seismic Safety Commission, Rohnert Park has no unreinforced masonry buildings. But there may be some threat to multi-family apartment buildings or condos with large openings on the first floor and housing on upper floors, otherwise known as soft-story. Those types of buildings are often houses with a garage on the bottom floor or retail under housing.
Additionally, many single-family homes built throughout the '70s were never adequately bolted to their foundations and lack bracing of walls, which could cause a house to slip off its foundation or lead to a total collapse, according to ABAG.
The problem in Rohnert Park, like hundreds of cities throughout California, is municipalities don't keep track of their building stock, they only are required to make sure new development is built to code, which changes every three years. There’s no law on the books that require cities to keep record of what structures were built when, and how they were built.
Hundreds of buildings could be at risk, said Danielle Hutchings, the Earthquake and Hazards Program coordinator for ABAG., and most retrofits are expensive, time-consuming and voluntary.
“Cities don’t know or keep inventory of buildings that are vulnerable, the way to identify them is city by city, street by street, with volunteers walking the streets,” Hutchings said. “A lot of residents want to know if they’re living in a building that’s unsafe, but in most cases, the government doesn’t know — it’s up to them to find out.”
Hutchings said ABAG is working with cities to identify what buildings are likely to collapse during an earthquake, and help develop incentive programs for cities and individuals to retrofit, because the statewide building code sets the lowest building standards for new development.
“This is a public safety issue,” Hutchings said. “And buildings codes say nothing about preserving the building itself, it’s designed just to get people out. We think the next step should be regaining occupancy after a disaster.”
Cities such as Santa Rosa, Sebastopol, Oakland and San Francisco are aggressively tackling programs that identify soft-story buildings likely to fail.
Locally, the biggest threat is to residences and — Waldo Rohnert, Thomas Page and John Reed.
"The next large earthquake, between a 6.8 and a 7.1 could happen tomorrow, it might not happen for 10 or 20 years, but definitely will happen," Schwartz said. "We're really concerned there will be a tremendous amount of damage throughout the entire Santa Rosa region, including Healdsburg, Petaluma and Rohnert Park."
Rohnert Park building officials said there's not much to worry about. And, the city's general plan states that "there are no known structures in Rohnert Park that would be specifically hazardous during an earthquake."
But researchers say that's deceiving.
"When you read the Rohnert Park plan, they use the wrong words in my opinion; they don't frighten people adequately," Norwick said. "ABAG and the USGS say Rohnert Park is a very dangerous place."
Editor's note: This is the first of a five-part series examining the impact on a major earthquake on Rohnert Park — a city of 40,000, incorporated in 1962 that is between one and five miles from the Rodgers Creek Fault. Check Rohnert Park patch tomorrow for a look in to Rohnert Park's building stock.