The number of earthquakes in a nine-week-long swarm of temblors that has shaken Reno has leveled off in recent days, but the threat of a major earthquake still is not over, seismologists said Friday.

John Anderson, director of the seismological laboratory at the University of Nevada, Reno, said it appears the activity that began Feb. 28 on the west edge of Reno has tapered off over the last three days.

"But we don't take very much comfort in that because the cumulative number of earthquakes in this sequence has curled over and flattened out before and then accelerated again," Anderson told reporters at a news conference. "We have no way of predicting which way it's going to go at this time."

The strongest aftershock Friday measured 2.9 and was recorded shortly after 10 a.m. It was among more than 15 for the day.

Since the swarm began, Anderson said, more than 600 quakes have been greater than magnitude-1 and more than 5,000 temblors have registered at some level.

Scientists have urged residents to prepare for a major quake after a magnitude-4.7 quake hit on April 25, the strongest in the swarm so far.

The quake swept store shelves clean, cracked walls in homes and broke a wooden flume that carries water to one of two Reno treatment plants. There were no reports of injuries or widespread structural damage.

Scientists said the swarm was unusual in that the quakes had started out small and built in strength.

The quakes have occurred along one or more unmapped faults around Mogul, an area of upscale homes along the eastern Sierra. The quake area, which is roughly two miles long and a half-mile wide, showed no previous signs of seismic activity.

Before April 24, the temblors were only one or two miles deep, seismologists said. Since then, they have been deeper up to four miles.

Anderson said the quakes have relieved some pressure, but "what we don't know is how much stress was out there in the first place."

Reno is located on the border zone between the Pacific and North American tectonic plates, scientists said, and the Pacific plate is moving northward away from the North American plate.

Glenn Biasi, associate research seismology professor at the Reno university, said he's examining a similar 46-day swarm of quakes that occurred in 1990 around San Ramon, Calif. for clues. The strongest quake in that sequence was 4.6.

"We're looking for analogies," Biasi said. "We're trying to understand better how these quakes work."

Anderson said recent seismic activity across northern Nevada appears to be unrelated. The Reno swarm began a week after a magnitude-6 temblor in the northern Nevada town of Wells, near the Utah border.

Three other quakes larger than magnitude-3 struck areas scattered hundreds of miles apart across northern Nevada within 24 hours after the 4.7 quake in Reno.

"As far as we know, it's just a coincidence," Anderson said.

Nevada is the third most seismically active state in the nation behind California and Alaska. Reno's last major quake measured 6.1 on April 24, 1914.