Earthquakes, typically a West Coast phenomenon, showed up across the country during the past few days and even rattled a few window panes near Reading Saturday evening.

Although the jolts near Reading were somewhat unusual and were accompanied by some more typical tremors in mostly unpopulated areas of Alaska, both were dwarfed by the quake that hit California yesterday morning.

The quake that hit near Reading Saturday measured 4.6 on the Richter scale. The Southern California epicenter, near the site of the major San Andreas fault --a center of seismic activity -- was rocked by a series of tremors that registered 6.6 on the scale.

The California quake caused a bridge to collapse and stalled major traffic and communication lines. That quake was at least the sixth to hit Southern California since 1987. Quakes during the past seven years have killed at least 80 people.

"These are all major earthquakes," said Charles Langston, professor of geophysics.

Earthquakes are caused by the shifting of the earth's tectonic plates due to a buildup of molten material beneath the plates, Langston said. Southern California, located at a point where two such plates meet, experiences frequent and violent earthquakes.

Langston said the quake came as no real surprise because earthquake activity has been increasing lately. "California was quiet in the '60s and late '70s," he said. "You can expect to have more activity like this."

Shelton Alexander, professor of geophysics, said there is also a history of minor earthquakes in certain parts of Pennsylvania, most notably in the Wyomissing, Lancaster and Philadelphia areas. But the Pennsylvania and California quakes are too far apart for them to be related, he said, adding, "That is out of the realm of possibility."

Alexander said although earthquakes in this part of the country are rarer, they carry a higher potential for damage because the bedrock here is more uniformly lateral, allowing the shock of a quake to pass through the earth more easily.

Langston said the last earthquake near State College occurred near Centre Hall in August, 1991, but was not very large. "It just surprised people," he said.

Whether or not the sudden seismic activity is a sign of more earthquake activity to come is uncertain. "Seismology as a science is relatively new," said Bruce Presgrave, a geophysicist with the U.S. Geological Survey's National Earthquake Information Center in Golden, Colo.

Presgrave said that because U.S. records on earthquakes do not date back much before 1900, predicting future activity is difficult.

"We don't really have a lot of history to go on as to what may happen in the future," he said.

The Associated Press contributed to this report.