2008 GSA/AEG Richard H. Jahns Distinguished Lecturer in Engineering Geology

Sponsored by :

Geological Society of America

Association of Environmental and Engineering Geologists

 

Earthquake Hazards and Risk in the Pacific Northwest

Dr. John Calgue
Simon Fraser University, Burnaby, British Columbia

 

Abstract:

Ten moderate to large earthquakes (moment magnitude, Mw, 6-7.5) have struck southwest British Columbia and adjacent Washington State in the last 130 years, most recently in 2003 near Olympia, Washington. Some of the earthquakes occurred on faults within the crust of North America. Other historic earthquakes have had much deeper sources, in the Juan de Fuca plate, which is subducting beneath North America at the Cascadia subduction zone. Geological and geophysical evidence demonstrates that a third, much larger (Mw 8-9) type of earthquake has occurred in this region at the boundary between the Juan de Fuca and North America plates. These plate-boundary, or ‘subduction,’ earthquakes are much rarer than Mw 6-7 crustal and interplate events, but by virtue of their size affect a much larger area. The last great plate-boundary earthquake, in January 1700 exceeded Mw 9 and produced a tsunami similar to the one that devastated coasts bordering the Indian Ocean in December 2004.

Future quake damage in the Pacific Northwest will result from fire, strong ground motions, tsunamis, landslides, liquefaction, and possibly coseismic land-level change. Considerable progress has been made in recent years in identifying potentially active faults where earthquakes can be expected and ground accelerations are likely to be highest. The intensity of ground shaking, however, is critically depending on local geology and topography and can vary by a factor of three or four over relatively short distances. High-frequency seismic waves generated by Cascadia plate-boundary earthquakes attenuate over the 100-150 km that they travel from their source to major cities in the Pacific Northwest, thus ground accelerations at Vancouver, Seattle, and Portland would be less those of a strong local quake. The shaking of a great earthquake, however, would last much longer (up to 2-3 minutes) than that of a large local quake. A Cascadia plate-boundary earthquake would generate a large tsunami that would strike Vancouver Island and the Pacific coasts of Washington, Oregon, and northern California soon after the shaking stops. Seismic microzonation mapping and geological studies have identified areas in and around major cities in the Pacific Northwest that are susceptible to liquefaction. A great plate-boundary earthquake might cause part or the entire Pacific coast from central Vancouver Island to northernmost California to subside 1 m or more, resulting in some flooding of low-lying areas. Coseismic uplift or subsidence might also occur within the epicentral area of a large crustal earthquake.

Speaker Bio:  

John J. Clague is Shrum Professor of Science at Simon Fraser University and Emeritus Scientist, Geological Survey of Canada. He has published over 200 papers in 34 journals on a range of earth science disciplines, including glacial geology, geomorphology, stratigraphy, sedimentology, and natural hazards. John and his graduate students are currently conducting research on natural hazards and late Holocene climate change in western Canada. His other principle professional interest is improving public awareness of earth science by making relevant geoscience information available to students, teachers, and the general public. John is a Fellow of the Royal Society of Canada, former President of the Geological Association of Canada, and Past-President of the International Union for Quaternary Research. He is recipient of the Geological Society of America Burwell Award, the Royal Society of Canada Bancroft Award, the Association of Professional Engineers and Geoscientists of British Columbia’s 2001 and 2005 Innovation Editorial Board Awards, the Geological Association of Canada’s (GAC) 2006 E.R.W Neale Medal, and GAC’s 2007 Logan Medal. He is the 2007-2008 Richard Jahns Distinguished Lecturer.

Education

Ph.D., Geology 1973 University of British Columbia, Vancouver, B.C. M.A., Geology 1969 University of California, Berkeley, California A.B. magna cum laude, 1967 Occidental College, Los Angeles, California

Research Interests

· Quaternary geology of western Canada, with emphasis on glacial stratigraphy and history
· Neoglacial climate and environmental change in high mountains
· Earthquakes and tsunamis
· Natural dam formation and failure
· Landslides
Crustal deformation and sea-level change