Types of Earthquakes Help

Types Of Earthquakes

The earth’s crust is divided into eight major chunks (plates) and a lot of minor plates. In the Pacific Northwest, there are two main plates that affect earthquake activity significantly. Oregon, Washington, and California sit on the North American Plate. The oceanic, Juan de Fuca Plate, is located in the Pacific Ocean, west of the Pacific Northwest coastline. The margin between these two plates is called the Cascadia Subduction Zone and is found about 80km offshore. When the Juan de Fuca Plate clashes with the North American Plate, it subducts underneath it down into the earth’s mantle. The collision of the two plates produces three different types of earthquakes.

Subduction Zone Earthquakes

Sometimes as an oceanic plate is shoved underneath a continental plate it sticks instead of sliding smoothly. This sticking causes stress to build up, which may be released suddenly as a large earthquake.

No large earthquakes have been recorded in the Cascadia Subduction Zone since records began in 1790, but the Cascadia has had magnitude 8–9 earthquakes in the past. If an earthquake took place in the subduction zone, it would probably be centered off the coast of Washington or Oregon where the plates converge. The 1989 volcanic eruption of Mt. St. Helens might be a signal of growing activity in the region.

Deep Earthquakes

Deep earthquakes take place within the Juan De Fuca Plate as it sinks into the mantle. These earthquakes are found roughly 25–100km in depth. Because of their great depth, aftershocks are not usually felt. Few earthquakes hit east of the Cascade Mountains. The last one took place in 1965 between Seattle and Tacoma, Washington with a magnitude of 6.5.

Shallow Earthquakes

Shallow earthquakes are found within the North American Plate. These earthquakes are thought to be caused by stress forwarded from the Cascadia Subduction Zone into the North American plate. Shallow earthquakes are recorded to depths of 30km or less and have taken place throughout the northwestern United States. In 1993, an earthquake of magnitude 5.6 occurred inland in the Willamette Valley of Oregon and two earthquakes, magnitudes 5.9 and 6.0, occurred near Klamath Falls, Oregon.

Practice problems of this concept can be found at: Earthquake Practice Test