RENO " Scientists urged Reno residents to prepare for a bigger event as the city kept rumbling Saturday after the largest earthquake in a two-month-long sequence of temblors.
More than 100 aftershocks were recorded on the west edge of the city after a magnitude 4.7 quake hit at 11:40 p.m. Friday, the strongest quake around Reno since a 5.2 temblor in 1953, said researchers at the University of Nevada, Reno's seismological laboratory.
The latest quake emptied store shelves, cracked walls in homes and dislodged rocks on hillsides. But there were no reports of major damage or injuries.
Seismologists said the recent activity is unusual because the quakes started out small and continue to build in strength. The normal pattern is for a main quake followed by smaller aftershocks.
"A magnitude 6 quake wouldn't be a scientific surprise," John Anderson, director of the seismological lab in Reno, said Saturday. "We certainly hope residents are taking the threat seriously after last night."
But Anderson stressed there was no way to predict what would happen, and the sequence of temblors also could end without a major quake.
Reno's last major quake measured 6.1 on April 24, 1914, and was felt as far away as Berkeley, Calif., said Craig dePolo, research geologist with the Nevada Bureau of Mines and Geology.
A rockslide triggered by Friday night's quake was blamed for causing a 125-foot breach in a wooden flume that carries water to one of Reno's two water treatment plants.
A backup pump was used to divert water to the plant and the breach was not expected to cause any water shortages, said Aaron Kenneston, Washoe County emergency management officer.
The U.S. Geological Survey said Friday night's quake was centered around Mogul, just west of Reno. The area of upscale homes along the eastern Sierra was rattled by more than 100 quakes the day before, the strongest a magnitude 4.2 that caused high-rise casinos to sway in downtown Reno.
The strongest aftershock measured magnitude 3.7 and was recorded at 12:29 a.m. Saturday.
Mike Lentini of Reno said Friday night's quake felt "like a big truck hit the building" and awakened his family.
"It's the unknown. It's shaking and when's it going to stop?" he said Saturday. "And when stuff starts falling off the shelves it's a whole other ballgame."
Jars of mayonnaise, bottles of ketchup and shampoo fell from shelves at a Wal-Mart store in northwest Reno. Overhead televisions swayed at a sports bar in neighboring Sparks, 11 miles east, where bartender Shawn Jones said the rumble was significantly stronger than Thursday's event.
"The bottles were shaking so I sent everybody outside," he said.
Hundreds of mostly minor quakes have occurred along one or possibly more faults since the sequence of temblors began Feb. 28, said Ken Smith, a seismologist at the Reno laboratory. The quakes have occurred along an area about two miles long and a half-mile wide.
"We can't put a number on it, but the probability of a major earthquake has increased with this sequence," Smith said Saturday. "People need to prepare for ground shaking because there's no way to say how this will play out."
Among other things, scientists urged residents to stock up on water and food, to learn how to turn off water and gas, and to strap down bookshelves, televisions and computers.
"It's getting a little bit frightening," Daryl DiBitonto of Reno told the Reno Gazette-Journal. "I'm very concerned about this increase in not only activity but also in magnitude."
The quakes around Reno began a week after a magnitude 6 temblor in the northern Nevada town of Wells, located near the Utah border. The Feb. 21 quake caused an estimated $778,000 in damage to homes, schools and historic downtown buildings, dePolo said.
Scientists said they're unsure whether the seismic activity at opposite sides of Nevada is related.
Nevada is the third most seismically active state in the U.S. behind California and Alaska. The Wells quake was the 15th of at least magnitude 6 in the state's 143-year history.
A 7.4 quake south of Winnemucca in 1915 is the most powerful in state history.