It was far from the big one, but scientists confirmed Monday that Spokane was rattled by an earthquake over the weekend.
“This is an unusual event,” said Professor Steve Malone, a seismologist at the University of Washington.
Sunday’s earthquake at 3:16 a.m. had a magnitude of 2.3 and was centered eight miles southwest of Spokane. No damage was reported.
An earthquake generally has to be magnitude 2.0 before it is felt, Malone said.
Sunday’s quake awakened people, particularly along the lower South Hill.
The Spokane office of the U.S. Geological Survey received numerous reports of the quake, a spokeswoman said.
At the Shriners Hospital, the quake shook the building and set off a trouble buzzer on the fire alarm system, said Michael Hoover, director of environmental services at the hospital.
“The night nurses said they heard quite a loud sound,” Hoover said.
Because the quake was small, scientists weren’t aware it had occurred until they received reports from Spokane on Monday morning. They checked their readouts and discovered the temblor and pinpointed its location.
Spokane seldom sees earthquakes of any magnitude. Most temblors felt here occur hundreds of miles away.
The region is surrounded by active fault zones in the Cascade Mountains, Puget Sound, North Idaho and the Pasco-Walla Walla areas.
Spokane has some minor fault lines, but they seldom produce quakes.
By comparison, western Washington sees several earthquakes of 2.0 or greater each week.
The last report of a minor temblor in this area was in 1963.
In 1984, a minor earthquake 190 miles northeast of Spokane was felt. In 1983, a powerful earthquake in southern Idaho shook the city. That quake had a magnitude of 6.9.
A loud concussion was reported by those who felt the quake. It probably was caused when the fast-moving primary wave from the quake reached buildings, said Bill Steele, coordinator of the UW seismology lab.
Slower moving sheer waves, which rock the ground, come after the primary wave.
Scientists said Sunday’s quake occurred relatively close to the surface, probably no deeper then a couple of miles.
Malone speculated the quake occurred along a fault, or facture zone in subterranean rock. One side of the fracture probably moved about a millimeter over a short distance, maybe several hundred yards, he said.
The force of Sunday’s quake would be similar to a blast from a few tons of explosives, Malone said. Because the earthquake occurred underground, its shock wave radiated outward farther than a blast at the surface, he said.
The motion probably was related to the movement of large plates that rest above the molten rock inside the earth, Malone said.
The fault, he said, “had enough stress on it for a little pop.”