It’s Petaluma’s fault: Geologists detect seismic past

Long ago, Cotati and the Sonoma Raceway at Sears Point were neighbors. This was actually 10 million years ago, eons before Cotati or the Raceway even existed, but the area north and south of Petaluma - Mecham Hill and Sears Point - are similar enough that geologists have long suspected they were once connected.

Now geologists have confirmed that a long-dormant fault running right under modern day Petaluma was responsible for rending the landscape and creating the Petaluma Valley.

“We looked a little harder and, lo and behold, there it was,” said Don Sweetkind, a geologist with the U.S. Geological Survey. “It’s nice to confirm it. Now we have geological evidence.”

Previous geological work done in 1958 predicted that Petaluma Valley may have been formed by a fault, and a study in 2002 on surface geology added to the evidence. But Sweetkind’s analysis, done as part of a recent USGS survey of the Petaluma groundwater basin, proved that a fault indeed runs from near Cotati, directly under downtown Petaluma, and out to the bay shore.

Sweetkind said he looked at different rock samples from hundreds of well sites in the Petaluma Valley. The evidence shows that the fault was active 10 million years ago and began pulling apart various features surrounding modern day Petaluma. By 6 million years ago, the fault became inactive, leaving the flat plain through which the Petaluma River now flows.

Unlike the active Roger’s Creek fault just to the east, Petaluma’s fault won’t likely cause any temblors, Sweetkind said.

“Now the Petaluma Valley fault is dead,” he said. “There is no seismology. I can’t imagine there’s a threat.”

Knowing Petaluma geologic past can help policy makers more accurately take stock of resources, including Petaluma’s groundwater supply, said Tracy Nishikawa, the lead scientist in the study. He said that the city’s groundwater table is at normal levels, despite several years of drought.

“(The study) is an updated understanding of the basin,” he said. “You can use it for ‘what if’ scenarios - what if we recharge over here, what if we pump over there, what are the climate change implications. It provides the tools for others to use.”

The fault under Petaluma does not impact the groundwater in the basin, Sweetkind said. The fault is so old that groundwater can flow across it unimpeded in most places, he said, and the water generally flows parallel to the fault.

Confirming the existence of a new fault may not be akin to discovering a new species of tree frog, but to a geologist, it doesn’t happen every day.

“I don’t know that it’s so special,” Sweetkind said. “We confirmed the validity of a revised interpretation from 2002 that used surface geological evidence. In once sense, it’s unique.”

(Contact Matt Brown at matt.brown@arguscourier.com.)