RENO, Nev. (AP) - Reno faces a "small increase" in the chance of a major earthquake after a swarm of temblors caused high-rise casinos to sway and put visitors and residents on edge, scientists said Friday.

More than 125 mostly minor temblors rattled the Reno area over a 24-hour period ending Friday afternoon, the strongest a magnitude 4.2 quake that struck shortly before 4 p.m. Thursday.

No major damage or injuries were reported.

Hundreds of other quakes - all magnitude 3.5 or lower - have occurred since Feb. 28 in the same sequence centered around the community of Mogul, just west of Reno.

"The persistence of this particular earthquake sequence slightly increases the probability for a significant earthquake in west Reno," said a statement issued by the Nevada Seismological Laboratory at the University of Nevada, Reno.

"However, the occurrence of additional earthquake activity in the Mogul area cannot be predicted or forecast," it added.

Chances of a temblor with a magnitude of 6 or greater striking somewhere around Reno within the next 50 years is 34 percent to 98
percent, according to a study released in 2006 by the Nevada Bureau
of Mines and Geology.

Ken Smith, a seismologist at the university lab, said the recent activity around Reno is unusual in that the quakes started out small and continue to build in strength. The normal pattern is for a main quake followed by smaller aftershocks, such as the Feb. 21 quake in the northern Nevada town of Wells.

The magnitude 6 temblor in Wells, 350 miles east of Reno, has been followed by hundreds of smaller aftershocks. The quake caused an estimated $778,000 in damage to homes, schools and historic downtown buildings there.

"If the pattern continues we may be looking at a larger event" in the Reno area, Smith said Friday. "We wouldn't be surprised to see it (swarm) end at any time and it also wouldn't be surprising to see a large earthquake. The bottom line is we don't know what will happen."

More than 70 aftershocks were reported Friday, including separate magnitude 3.3 quakes that struck shortly after 1:30 a.m. and 10:30 a.m.

Reno's last major quake occurred at 12:34 a.m. on April 24, 1914. The 6.1 temblor awakened people as far away as Sacramento, Calif., and Winnemucca, prompted people to rush out of Reno saloons and hotel lobbies into streets and toppled chimneys, said Craig dePolo, research geologist with Nevada Bureau of Mines and Geology.

A magnitude 5.1 quake in Reno in February 1914 caused more
damage, cracking walls and breaking windows around the city, dePolo
said.

Seismologists are unsure of the cause of the recent Reno sequence and whether it's related to the increased activity around Wells and some other Nevada locations.

"Seismic activity fluctuates over the years and we really have no idea whether they're coincidences," Smith said. "Not enough is known about Nevada's faults and their history and what their role is."

The Reno quakes have occurred in an area along the eastern Sierra riddled with faults, both mapped and unmapped. Seismologists continue to monitor the area's ground movement with the help of five stations.

"The quakes have been very shallow, which is somewhat unusual," said Tom Rennie, an analyst at the seismology lab. "You can still have a major quake even if it's shallow."

Seismology lab director John Anderson said aftershocks should continue for several days and the next 10 days will be important.

"The farther we get into that 10-day time window, the faster the chances decrease (for a major quake)."

The recent quake activity has caught the attention of locals, who were urged by local officials at a news conference Friday to prepare for the possibility of a major quake.

Nevada is the third most quake-prone state in the nation, behind California and Alaska.

"It's getting a little bit frightening," said Daryl DiBitonto of Reno. "I'm very concerned about this increase in not only activity but also in magnitude."

Earthquake magnitudes are calculated according to ground motion recorded on seismographs. An increase in one full number - from 5.5
to 6.5, for example - means the quake's magnitude is 10 times as great.

A quake with a magnitude of 6 can cause severe damage, while one
with a magnitude of 7 can cause widespread, heavy damage.

(Copyright 2008 by The Associated Press. All Rights Reserved.)