RENO, Nev. (AP) -- A months-long swarm of earthquakes picked up again Sunday as a string of minor temblors rattled Reno, causing downtown high-rises to sway and knocking items off walls and shelves.
There were no immediate reports of injuries or major property damage after about 20 minor quakes reported on the western edge of Reno over 12 hours ending about noon.
Magnitude-3.9 and 3.7 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.
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 U.S. behind California and Alaska. Reno's last major quake measured 6.1 on April 24, 1914.