Magnitude 3.9 and 3.6 quakes struck within a couple minutes of each other shortly before 11 a.m. and were preceded by 3.2 and 3.0 quakes early Sunday, said researchers at the seismological laboratory at the University of Nevada, Reno.

"I was thinking, 'Here we go with the big one,'" said Daryl DiBitonto, a Reno resident who felt the quake at home. "Late in the morning, I thought the whole house was going to come down. Every time I feel a 3, I think it's going to be the big one. Totally, everyone is on edge again."

Diane dePolo, a lab seismologist, said it was the strongest sequence of quakes in the past month and signaled a pickup in activity after a recent lull.

"We had a little pickup in activity on Tuesday and nothing significant until today," dePolo said. "We are monitoring them, but we can't say if it'll get better or worse. There is no way to predict earthquakes."

Seismologists had urged residents of Northern Nevada's largest city to prepare for a bigger event after a 4.7 quake on April 25, the strongest in a swarm that began Feb. 28.

That quake swept store shelves clean, cracked walls in homes and dislodged rocks on hillsides, but there were no reports of injuries or widespread, major damage.

The swarm, which has produced thousands of mostly minor quakes, had prompted some residents in the densely populated quake zone to spend nights outside in campers and trucks.

"We live in earthquake country, and the threat is never over," dePolo said. "We encourage people to be prepared."

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