Northwest US sits in quake zone

The quake that struck the US Northwest on Wednesday was the strongest to hit the region in more than 50 years.

The state capitol Olympia and the surrounding Washington area, including Seattle, were shaken by a tremor that measured 6.8 on the Richter Scale. You have to go back to 1949 to find an occasion when the region was hit by a bigger geological punch - a magnitude 7.1 earthquake.

Scientists said the focus of Wednesday's quake was deep beneath the surface and this helped to blunt the impact on communities.

"It's like throwing a pebble into a pond and watching the waves dampen out as they extend," said Tony Crone, associate chief scientist of the US Geological Survey's (USGS) hazards team.

"The earthquake's energy had to travel 30 miles (48 kilometres) in every direction from its point of origin before it hit the surface."

Building pressures

This was a deep-subduction-zone quake, the rumbling result of the spreading sea floor diving and grinding its way under the Americas.

Scientists picture the exterior of the Earth as a jigsaw of rigid "plates" of rock floating on a more "plastic" pool of material. In this case, the relatively small Juan de Fuca Plate is pushing eastwards against the very much larger North American Plate.

It is the Juan de Fuca Plate that gives, in the sense that it has to dip under its opponent. As it dives, pressures build that every so often are released in waves of energy that shake the surface of the Earth.

Wednesday's quake - timed at 1055 PST (1855 GMT) - was felt as far away as Salt Lake City in Utah. That is 1,000 km (620 miles) distant. Its epicentre was given by the USGS as latitude 47.2 degrees north, longitude 122.7 degrees west - about 15 km (10 miles) from the state capitol, Olympia.

Construction standards

This put the quake in the same general area as the magnitude 7.1 tremor that occurred on 13 April, 1949. Both quakes lasted about 30 seconds, but in terms of the energy released, Wednesday's tremor was only about one-third as strong as the 1949 event.

And that may, in part, explain why the damage this time was not as great as it could have been. Particularly hard-hit in both quakes were brick and masonry structures in areas that have been built on fill, such as Seattle's historic Pioneer Square district.

Eight people were killed in 1949; one person has died this time. In 1965, a quake of magnitude 6.5 occurred between Seattle and Tacoma. Six people lost their lives in that event.

Scientists say better design and construction standards have undoubtedly helped to limit the impact of the latest quake.