On Shaky Ground: Petaluma at Grave Risk When "Big One" Hits

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 reports. San Francisco, the epicenter, was destroyed and further north in Santa Rosa, the 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 built, population boomed, the Bay Area urbanized. Then, in 1989, a 6.9 magnitude 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.

“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.”

Click on the video on the right to hear David Schwartz, a seismologist with the USGS, discuss the Rodgers Creek fault line.

“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 the 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 Petaluma 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 portions of Petaluma at the highest destruction level, or “high hazard.”

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.”

The Petaluma Fire Department has been preparing for “the big one” for many years and has an emergency operations plan that spells out what course of action first responders would take in an event of an earthquake. This includes doing surveys of the damage and checking the stability of structures following an earthquake as well as responding to any medical calls.

Asked about which neighborhood would experience the most damage, Chief Larry Anderson said it would be likely be downtown, where many buildings date to the 1860s.

“We know the whole downtown area, because of when it was built, doesn’t have the same codes for earthquake stability that all the newer buildings have, so we know by default that it’s at risk,” Anderson said.

Some of the older buildings have been reinforced and some have had very specific seismic bracing applied to them, Anderson said. The problem in Petaluma, like hundreds of cities throughout California, is municipalities don't keep track of their building stock. Instead, they are only 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.”

Natural Environment: Liquefaction and Shaking

A significant part of Petaluma’s soil is made up of silt and gravel, which 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.

The young, soft soil deposits, on which the town is built, can magnify shaking during an earthquake, according to Schwartz. The true impact, however, is impossible to predict, as one house could be built on clay, while the neighboring home 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 strongly damaged, and the house on bedrock can look like almost nothing has happened.”

But in Petaluma, where liquefaction and violent shaking are both high, widespread damage awaits, he warned.

Impact on Buildings, Infrastructure

Because most houses in Petaluma are one or two story homes made out of wood, the loss of human life will likely be minimal.

“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 single-family wooden homes. Soft story buildings are multiple story structures, which have windows or other open spaces on the first floor instead of walls, making them more prone to collapse in an earthquake.

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.

"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 Petaluma, Rohnert Park and Petaluma.

Tomorrow: How do I find out if my house is at risk in an earthquake? Patch brings you a user's guide to accessing the information and taking the necessary steps to prepare. Stay tuned!