The federal agency conducted an airborne study of the Spokane area last spring and found evidence of earthquake faults. Scientists for the agency said additional study is needed to learn more about the possible faults.
"There is some possibility of earthquakes in the Spokane city limits," said Richard Blakely, a geophysicist for the agency.
"Our purpose is not to alarm the public," he added. "We don't have a San Andreas Fault."
The research was sparked by a series of 105 small earthquakes that shook the Spokane area in 2001, which pushed up an area of ground by about half an inch. All of the quakes were less than magnitude 4 and caused little damage. But the swarm prompted scientists to take a closer look at an area that wasn't considered prone to earthquakes.
The initial report released Friday in some ways sparked more questions than answers.
Blakely said scientists need to learn the extent of the faults and how often they produce earthquakes.
The U.S. Geological Survey last spring hired a small airplane to carry a magnetic sensor while flying low-level flights over Spokane. The plane flew some 7,500 air miles over the city, helping create a map of magnetic anomalies that can reveal faults, scientists said.
One alignment of magnetic anomalies crossed Spokane in a northeast direction, passed through the area that saw the swarm of quakes in 2001, and aligned with the possible Spokane fault suggested by an area of "ground deformation" located near Gonzaga University, the USGS said. The ground deformation is a bulge more than a mile across and half an inch high, the USGS said.
"We think we have found evidence for part of the Spokane fault," Blakely said.
Or they may not. There are numerous reasons for magnetic anomalies that do not include earthquake faults, the USGS said.
Spokane County emergency managers aren't taking chances.
Sheriff Ozzie Knezovich, head of Emergency Management for the county, said it is time to start planning to deal with a possible quake.
Officials said that can include urging homeowners to bolt their home to the foundation, strap down hot water heaters, and use special construction methods when building schools and hospitals.
Earthquake risks are much better understood in Western Washington, where people have been worried for years about a major temblor.
Brian Sherrod of the USGS said the Spokane area likely had not had a major earthquake in thousands of years.