Pacific Northwest vs. Sumatra: Flash rollover

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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Keypoints:

What do these subduction zones have in common?

  • Both have volcanoes lined up parallel to the plate boundary
  • Both have had Great earthquakes
  • Both have faster moving oceanic plate diving beneath a slower plate
  • Both have generated tsunamis that traveled across the Pacific Ocean


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:

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  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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GPS can record the movement of the leading edge of the overlying continental plate in a subduction zone. The plates are locked and the overlying plate is forced back. When friction is overcome and strain is released, the GPS receiver will snap back toward its original position. 

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 GPS records the movement of the leading edge of the overlying continental plate in a subduction zone. The plates are locked and the overlying plate is forced back. When friction is overcome and strain is released, the GPS receiver will snap back toward its original position. 

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Oblique view of a highly generalized animation of a subduction zone where an oceanic plate is sliding beneath a continental plate with no friction just to show the relative motion between the plates.. 

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