Swarm of Small Earthquakes Rattles Nerves in Spokane

The temblors come day and night. Schools hold duck-and-cover drills. Workers in downtown high-rises study evacuation plans.

Since May 24, Spokane has been in the throes of what experts call an “earthquake swarm.” More than 75 temblors have been recorded, and dozens more could not be measured because of a lack of seismographs.

There have been no injuries or major damage--other than bricks falling from chimneys and items off shelves--but nerves are fraying.

“We’ve felt every single one of them,” said Cindy Burrows, who works on the 19th floor of the Bank of America Financial Center, downtown’s tallest building. “The building doesn’t sway. It jumps.”

One quake caused such a jolt that she had to hang onto her desk, Burrows said.

Such a swarm is unusual in a metropolitan area, especially one that was thought to be on solid ground, experts say. Although none of the earthquakes has registered more than a magnitude 4, scientists have no real idea if a big one is looming.

The U. S. Geological Survey has rushed additional equipment and people to Spokane to study the phenomenon.

The Spokane swarms are part of an active earthquake year. On Feb. 28, the magnitude 6.8 Nisqually earthquake in the Puget Sound area injured more than 400 people and caused billions of dollars worth of property damage.

In the last month, hundreds of small earthquakes have been detected near Mt. St. Helens in the Vancouver, Wash., area.

Spokane, a city of 190,000, has had no major quakes in its 120-year recorded history and wasn’t regarded as being in an earthquake zone.

Because of that, there were no seismographs in the city when quakes started hitting in May. Now there are four.

Scientists hope that the devices help them locate an apparent fault line that so far has escaped detection.

Bob Derkey, a geologist with the state Department of Natural Resources, believes that a fault lies along Latah Creek, in a nearly straight line from Steptoe Butte southeast of Spokane to Tum Tum northwest of the city. Others are not as sure.

Scientists are planning to shoot a special laser at the ground from an airplane.

“It was used to discover unknown faults in the Seattle area,” said Bill Steele of the University of Washington seismology lab.

Nearly all the quakes have been centered on the city’s north side, a few miles north of the Spokane River. The quakes have been shallow, sometimes only a mile or two deep, and noisy.

They are announced by loud cracks, sounding like explosions or the pounding of heavy equipment. In the tense atmosphere after the Sept. 11 terrorist attacks on the East Coast, such explosions have been particularly disconcerting.

Ground movement generally lasts only a few hair-raising seconds.

Gonzaga Prep High School, located directly over some of the temblors, is holding earthquake drills for its 970 students.

Steele said some days area residents reported as many as eight earthquakes. His team has recorded more than 75 quakes since May and received reports of many more they were not able to verify for lack of equipment.

Scientists have been reading old newspaper reports of earthquakes in the Spokane area, finding references back to the 1800s, Steele said.

“But nothing as energetic and prolonged as this sequence,” he said.

Spokane sits on hard rock between two major seismic areas: the Puget Sound to the west and the Intermountain seismic belt to the east in Idaho and Montana.

There were small quakes in Spokane intermittently from 1915 to 1962. The largest quake in eastern Washington in the last 100 years was a 5.5-magnitude temblor in 1942 centered 35 miles northeast of Spokane.

Although quakes have been going on for six months, the strongest occurred Nov. 11, a magnitude 4 temblor that was followed by quakes measuring 3.1 and 3.3 the next few hours.

“There’s no way to say if it is a harbinger” of a bigger quake, Steele said. “We can’t rule it out and can’t say if it is.”

But the Spokane quakes are so shallow that even a magnitude 5 shake could cause plenty of damage, Steele said.

Spokane’s downtown includes a large number of historic buildings, built long before modern earthquake codes were developed.

Sharon Johnson works at WorldCom Inc. in the 15-story U. S. Bank Building, which was constructed in 1910.

Johnson transferred from Orange County to Spokane in the last week, just in time for a series of quakes.

“I’m used to earthquakes; I don’t care,” Johnson said.