North Spokane fault has geologists planning

A series of mild to moderate earthquakes in north Spokane in 2001 was the first sign of an active earthquake fault in the region previously unknown to scientists.

Geologists continue to study the fault, which some researchers have named the Spokane Fault.

“For us, it’s an interesting puzzle to figure out,” Brian Sherrod, a research geologist with the U.S. Geological Survey, said in Spokane on Friday.

For people living above the fault, it may be more than science.

The fault may have potential to produce even larger quakes than the 3.7-magnitude tremor on June 25, 2001, and a 4.0 quake on Nov. 11, 2001, said Richard Blakely, a USGS geophysicist.

The two larger quakes were bookends in a “swarm” of 105 smaller tremors that occurred that year in a relatively shallow layer of basalt.

Historically, Spokane residents have reported small quakes over the years, and some were detected on a seismograph operated by Jesuit priests at the former scholasticate at Mount St. Michael.

Earthquake swarms within the Columbia Basin have also occurred in recent years north of the Tri-Cities and near Maupin, Ore.

Blakely said scientists currently believe the Spokane Fault is a relatively small feature that might produce no more than moderate tremors. But there is a possibility that the studies may lead to discovery of a much larger fault line with potential to produce a strong earthquake in the 6.0-magnitude range.

The shallow nature of the suspected fault – at 0.3 to 2 kilometers in depth – means a larger quake could be very damaging, since shallow quakes cause more shaking than deeper ones.

Sherrod said the USGS plans to conduct high-resolution laser mapping of the area where the quakes occurred in 2001.

That information will be compared with existing evidence that an active fault extends northeastward from Spokane’s North Side.

Spokane County Sheriff Ozzie Knezovich said he wants to use the information for earthquake emergency planning.

If seismic risks are more severe than currently suspected, agencies would urge residents to prepare by strapping down water heaters and securing buildings to foundations, for example.

A higher seismic risk could trigger changes in building codes for greater earthquake safety.

In a study made public earlier this year, scientists used satellite data to detect a bulge of 0.59 inches in the ground following the 2001 quakes.

The oblong bulge measured about 2 miles north to south and was centered at North Central High School. From there it reached east to Ruby Street, south to Boone Avenue, west to Maple Street and north to the North Hill.

The bulge coincided with the locations of the 2001 quakes.

In a second study, a magnetic sensor was flown in a large grid pattern over the suspected seismic zone to map rocks with different levels of magnetism.

That study points to a fault line running along the south side of the area where the bulge occurred and extending to the northeast.

Blakely said the evidence so far points to a thrust fault in which rock to the northwest is pushing upward and over rock to the southeast.

It may be related to a slow clockwise rotation within the North American tectonic plate, the scientists said.

The Spokane Fault is a different feature than the previously documented Latah Fault on the west side of the city.

Scientists are also awaiting results of seismic reflection tests conducted last June. Vibrations from a heavy, truck-mounted machine were sent into the ground on the North Side to give another view of the rock structure below the city.

Part of the problem in diagnosing the Spokane Fault comes from the large amount of Ice Age flood deposits covering the underlying rock. Sherrod said he is prepared to start digging into the ground in search of features that can confirm the size and extent of the fault.

“Our purpose here is not to alarm the public,” Blakely said. “We don’t have a San Andreas Fault.”

Still, he said, “We are really concerned about what might happen in the future.”