Experts: Northwest quake under way _ taking weeks, not seconds

Experts: Northwest quake under way _ taking weeks, not seconds Recently discovered silent quakes, which can only be detected with sensitive instruments, aren't as harmless as they may seem.

Scientists say they may be adding to the tremendous pressure in an area where the brittle rocks of two tectonic plates are locked offshore.

Evidence shows that every few hundred years, the jammed plates release that stress in huge magnitude 8 or 9 earthquakes that can rattle the entire Northwest coast and generate lethal tsunamis. The last such powerful subduction-zone quake occurred about 300 years ago.

"These slow slips aren't reducing the stress on the locked zone," said Herb Dragert, a research scientist with the Geological Survey of Canada in Sidney, British Columbia. "They're actually, in little pulses, adding a tiny bit of stress to the locked zone."

About every 14 to 15 months, the slow-motion earthquakes are generated about 15 to 30 miles deep at the interface of the lower Juan de Fuca tectonic plate and the upper North American plate.