A 5.8 magnitude earthquake rocked the Washington region Tuesday afternoon, causing damage to several Washington icons -- including the National Cathedral and Washington Monument -- and clogging the region's roads and transit systems as government agencies and businesses sent workers home early.

The quake's epicenter was Mineral, Va., 84 miles from Washington and southwest of Fredericksburg. The shock waves began at 1:51 p.m. and shook buildings and sidewalks for roughly 15 seconds, triggering fire alarms throughout the region and sending thousands of evacuated employees into the streets.

Several buildings suffered damage but no major injuries were reported by Tuesday evening.

The Washington National Cathedral appeared to endure the greatest trauma of major Washington structures, as three of its four pinnacles on the central tower -- the highest point in Washington -- fell off. At Union Station, pieces of plaster fell off the ceiling near the Massachusetts Avenue entrance and at other parts in the building, according to an Amtrak spokesman.

The Smithsonian Castle suffered minor cracks and broken glass and the Washington Monument was closed indefinitely while officials check to make sure it is structurally sound for visitors.

Building damage was reported at 37 Prince George's County schools and two District schools, while other county schools reported minor building damage. Some minor injuries were reported at area schools, but no one was listed as seriously hurt. Some afternoon and evening activities were canceled.

Many Washingtonians, more trained to be on alert for terrorism threats, at first thought a bomb had been detonated.

"We ran like crazy down the stairs," said Kelly Poe, who had evacuated her downtown advertising firm. "We were surprised that it was an earthquake because this is D.C. -- you wouldn't think ..."

While magnitude 2.0 or 3.0 earthquakes are "fairly common" for the East Coast, the last time a comparable quake rocked the region was in 1897, said Kate Hutton, a seismologist at Caltech in Pasadena. Tuesday's quake was caused by built-up internal strain from the shifting movements of the East Coast plate, she said. There is also a minor fault line at Mineral.

Even without major injuries, the region's lack of experience with earthquakes resulted in chaos.

Cell phone service was temporarily down, the National Park Service closed all museums and national monuments, and most federal agencies and many downtown businesses closed for the day. The early release strained the roads and rail systems, which switched to a rush hour schedule. However, Metro and commuter trains ran at 15 mph into the night while workers inspected the tracks and signal outages knotted intersections for hours. Amtrak also ran trains at reduced speed between Baltimore and Washington.

Two nuclear reactors in Louisa County, where the epicenter was located, were automatically taken off line as a precaution, although the Calvert Cliffs nuclear plant on the Chesapeake Bay in Maryland stayed open.

Virginia Gov. Bob McDonnell said the reported damage "has been very, very minor" and he kept polls open for the state's primary elections.

The potential for aftershocks have officials on high alert. Gas leaks and water main breaks are also a possibility. However, Hutton said the likelihood of a larger aftershock is small.

"Normally, 95 percent of the time, they're smaller than the main shock and occur strung out over a period of time [within two or three weeks]," she said.

Examiner reporters Liz Essley, Lisa Gartner and Freeman Klopott contributed to this report.