Three Kinds of Earthquakes

Here in the Pacific Northwest, there are three kinds of earthquakes: subduction zone earthquakes, shallow fault earthquakes, and deep earthquakes. All of these earthquake types can be Big Ones.

Shallow fault earthquakes A fault is a break in the rock beneath our feet. Shallow fault quakes occur very close to the surface. Only recently, we’ve discovered a fault beneath Seattle and right across Puget Sound. There was a major quake on this Seattle fault about 1,100 years ago. Because shallow fault earthquakes are so near the surface, even small ones cause a lot of damage from shaking. Subduction zone earthquakes The largest earthquakes ever recorded are subduction zone earthquakes. They can last several minutes. Subduction zone shaking can occur along the whole subduction zone. In the Pacific Northwest, these major quakes seem to occur every few hundred years. The last known subduction zone earthquake along the Oregon and Washington coast was January 26, 1700. In addition to causing huge shifts in land level here, this quake sent huge waves— tsunamis— racing across the Pacific. Deep earthquakes Deep earthquakes occur in the subducting ocean slab, deep beneath the continental crust. In the Pacific Northwest, deep quakes start about 50 km (30 mi) beneath the surface. Large ones have shaken the Pacific Northwest in 1949, 1965, and 2001—about every 30 years. The last big deep quake—the 2001 Nisqually earthquake—occurred under the southern end of Puget Sound.

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