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,
weve 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
- 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 2001about every 30 years. The last big deep quakethe
2001 Nisqually earthquakeoccurred under the southern
end of Puget Sound.