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Nisqually Quake

Three kinds of Pacific Northwest earthquakes

The region is susceptible to three kinds of potentially damaging earthquakes. Wednesday's was stronger than most but not as devastating as the one scientists say we can expect in the future.

Subduction zone: the area where a plate of the oceanic crust pushes under the continental crust.

1. Shallow fault quakes
Within a few miles of the surface, these quakes can cause significant damage locally and, because they involve cracks in the Earth called faults, usually are followed by aftershocks. These happen frequently but aren't always felt.

2. Subduction-zone quakes
Offshore, where the Juan de Fuca and North America plates converge. These quakes can be violent and last up to several minutes, causing widespread damage and tsunamis. The last major one probably occurred 300 years ago. This is the kind scientists fear most.

3. Deep-subduction-zone quakes
Fifteen to 40 miles below the surface, where the Juan de Fuca Plate is submerging. They can be widely felt and cause considerable damage, but their force is absorbed somewhat by hotter rock near the mantle. The last significant one was Wednesday.

Source: University of Washington Geophysics Program

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