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Geology of the Pacific Northwest

Geology-208

Week 6:  EARTHQUAKES IN WASHINGTON

Types of Quakes

Types of earthquakes are:

  1. Deep quakes (subducting slabs) — least destructive

    The Seattle-area earthquake this year (2001) was centered 30 miles beneath the earth's surface, underneath Anderson Island, east of Olympia. All our strongly-felt earthquakes have "occurred as one plate of the earth's crust slowly slid farther beneath another." (Christian Science Monitor, March 2, 2001).

  2. Shallow faults — more destructive

  3. Plate boundary quakes — catastrophic


Tectonics and Quakes

Plate Tectonics and Types of Earthquakes

Divergent boundaries: adjacent plates move away from each other

  • Shallow earthquakes (more destructive)
  • Occurs mid-ocean at depth of less than 5 km below the sea floor
  • Limited volcanic activity near edges of diverging plates (not explosive)

Convergent boundaries:  adjacent plates move toward each other

  • Greater range of depths for earthquakes and volcanoes
  • Marked by trenches, island arcs (i.e., Aleutian Islands), volcanoes
  • Subduction zones
  • Deeper earthquakes (up to 700 km deep; less destructive)

Transform boundaries: adjacent plates slide side by side past each other

  • Shallow to moderately deep quakes (i.e., San Andreas Fault, California)
  • Little or no volcanic activity

Hot spot: intraplate; volcanism and quakes are caused by the continental plate passing over a “hot spot” in the earth’s mantle

  • Non-explosive volcanoes
  • Small quakes are common; some are strong enough to ring church bells (Boston)
  • Quakes are caused by motion along old faults in the lithosphere


RESOURCES

Major Faults in the Puget Sound (fault map) (University of Washington)
PNSN Webicorder Displays (Real Time seismic data) (University of Washington)
Recent Washington Earthquakes in the Pacific Northwest (University of Washington)
• Paton, Dean, 2001, Why Seattle's Big Quake Had Little Impact, Christian Science Monitor, v93, n67, p3
• Slattery, William, 1996, Earthquakes, Volcanoes, and The Information Superhighway, Science Activities, v33, n3, p8-13

GLOSSARY



Send comments to Rob Viens (e-mail: rviens@bcc.ctc.edu) or call him at his Bellevue Community College office at (425) 564-3158. Office hours are by appointment.


© 2001-2002, Rob Viens, Bellevue Community College. All rights reserved.
Page updated on 5 September 2002  • 
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