Pacific Northwest Geography & Tectonics: Flash

Scroll over the bathymetric relief map to learn about the geographic provinces of the Pacific Northwest, including the subducting plate, the subduction zone, coast mountains, central valley, and cascades.

 

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Keypoints:

All these geographic features are related to the subduction zone:

  • Offshore plate boundary (often accompanied by a trench when older cold oceanic plate dives beneath another oceanic plate or continent. The Juan de Fuca Plate is young and warm, so doesn't result in such a trench as it doesn't dive as steeply)
  • Coast Range due to compression of the plate
  • Central valley, which is caught between the Coast Range and the
  • Arc Volcanoes, which grow above the point where the subducting plate reaches about 100 km depth. Melting of the rock requires a particular pressure/temperature regime.


Level: Novice

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The subduction zone iswhere two tectonic (lithospheric) plates come together, one subducting (diving) beneath the other. The plates are locked together and periodically overcome the friction causing the leading edge of the overlying plate to surge back, lifting a wall of water producting a tsunami.

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Oblique view of a highly generalized animation of a subduction zone where an oceanic plate is subducting beneath a continental plate. (See sketch below for parts.) This scenario can happen repeatedly on a 100-500 year cycle. The process which produces a mega-thrust earthquake would generate a tsunami, not depicted here.

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Subduction zones show that there are 3 distinct areas of movement in the overlying plate:

  1. constant movement above the locked leading edge,
  2. see-saw pattern of back-&-forth movement above a zone that alternately locks then slips, and
  3. no movement far inland above the deeper part of the diving oceanic plate.
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Native American oral history, tsunami geology along the Pacific Northwest coastlines, dating of "Ghost Forests", and Sumurai records indicate that a Great magnitude 9 earthquake occurred off the coast of Oregon and Washington on January 26, 1700 at 9:00 pm. This was pre-European contact.

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This UNAVCO animation compares Japan's subduction zone at the location of the 2011 earthquake with a mirror-image subduction zone in the Pacific Northwest. There are many similarities.

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This rollover compares the an earthquake of 1700 in the Pacific Northwest with the 2004 Sumatra earthquake and tsunami. The tectonic settings are similar.

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The Pacific Northwest is host to three kinds of earthquakes revealed in this Flash rollover. Subduction zone great earthquakes, shallow crustal quakes, and earthquakes within the subducting plate.

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Learn how the Pacific Northwest tectonic setting and megathrust earthquake of January 1700 is similar to the catastrophic earthquake in Japan in 2011 by touching icons on this interactive map.

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