Strong Quake Rocks Northwest : Seismology: The magnitude is estimated at 5.3 to 5.7. Minor injuries and damage are reported in Oregon, and the Capitol rotunda in Salem is cracked.

A moderately strong earthquake, variously put at magnitude 5.3 to 5.7, caused minor injuries and some pockets of property damage in Oregon on Thursday, reminding the Northwest that it, like California, is vulnerable to the great grindings of the continental plates along the Pacific Coast.

The 5:34 a.m. temblor rivaled--or exceeded--the largest earthquake ever recorded in northwestern Oregon. It was felt from Coos Bay on the Oregon coast to Seattle in Washington.

The rotunda of Oregon’s Capitol in Salem was cracked and parts of the building were evacuated. In the Cascade foothills logging community of Molalla, near the epicenter of the quake, walls and a chimney of a 68-year-old high school were damaged. The school was unoccupied because of spring break. In Newberg, south of Portland, the roof of a theater partially collapsed and several older buildings were closed for structural inspection.

Oregon Highway 18 was closed at Dayton, just west of Newberg, when a bridge across the Yamhill River shifted. Numerous other bridges in the Willamette Valley were blocked off for inspection and then reopened.

Authorities said a number of people were treated for cuts and other injuries, and for exposure to dangerous chemicals knocked off store shelves.

Geologists fixed the epicenter about 30 miles south of Portland in the Cascade foothills, near the community of Scott’s Mills. The area is well to the west of the arc of volcanoes that runs north-south through the region. Scientists said no volcanic activity was associated with the earth movement.

The National Earthquake Center in Golden, Colo., measured the quake at magnitude 5.3, a mid-size West Coast earthquake. The University of Washington seismic laboratory, which maintains instruments throughout the region, calibrated it as a significantly larger magnitude 5.7.

The Oregon Department of Geology and Mineral Industries announced the quake “was as large as any historic earthquake to occur in Northwestern Oregon. It was felt throughout the region and caused damage in the epicentral region. The actual duration of strong shaking was probably less than 10 seconds.”

According to the National Earthquake Center, two other significant quakes have occurred in the same area--a 5.4 in 1964 and a magnitude 5 in 1962. Officials said they suspected that all three quakes were associated with a known fault line in the region, called the Mt. Angel Fault.

In the last year, seismologists have rattled residents with a warning that the region is vulnerable to a “superquake” along a 1,000-mile offshore zone where the ocean floor is pushing its way underneath the North American continent. This activity involves two massive continental plates colliding off the coast. The oceanic mass, known as the Juan de Fuca Plate, is moving east under the North American Plate along a so-called subduction zone off the coast of Oregon, Washington and northernmost California.

As the Juan de Fuca Plate is forced down, it melts and provides the red-hot magma that fuels the Cascade volcanoes. It also pushes upward on the 18-mile-thick crust of the land mass of the northwestern United States. Last year, seismologists reported that this pressure brought on a magnitude 8.4 to 9 quake 300 years ago, producing a tsunami 30 to 40 feet high.

“The earthquake was the wake-up call for many people this morning but (it) should be a societal wake-up call to prepare for even larger earthquakes that await in our region’s future,” a statement from Oregon officials said.

They said that because of the infrequency of quakes in the region it was difficult to guess about the likelihood or interval of significant aftershocks. And, as is always the case with earthquakes, officials left open the possibility that Thursday morning’s activity was a foreshock to a larger quake building beneath the surface.

Balzar reported from Portland and Reich from Los Angeles.