Earthquake Hazards in Western Washington The mega-thrust at the coast Shallow earthquakes in the...
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![Page 1: Earthquake Hazards in Western Washington The mega-thrust at the coast Shallow earthquakes in the crust of Puget Sound Deep earthquakes in the subducted.](https://reader030.fdocuments.in/reader030/viewer/2022032612/56649eb25503460f94bb835f/html5/thumbnails/1.jpg)
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Intraslab Seismicity
• Largest intraslab earthquakes are in south sound region at base of microseismicity, including 1949 M7.1; 1965 M 6.5 and 2001 M6.8
• Deepest earthquakes are downstream from large events
• Intraslab seismicity is virtually absent north and south of arch
• Intraslab focal mechanisms are widely scattered but generally are in-plane tension
• Should we prepare for M7 or M8 intraslab earthquakes?
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Discussion
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The largest intraslab earthquakes tend to occur at the base of the seismic zone.
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3-D Tomographic Inversion
Active-Source Data
~92,000 first ~92,000 first arrivals from arrivals from active source active source experiments (e.g. experiments (e.g. SHIPS)SHIPS)
1200 wide-angle 1200 wide-angle reflection times reflection times from SHIPSfrom SHIPS
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Previous Tomography (Preston et al. 2003)
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USGeology
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US Topography
Topographic action mark tectonic activity
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USA seismicity• Notice that all the action is in the West
– most dramatic topography in the West• Yosemite, Cascade Mts., Big Sur coast, etc.
– few earthquakes in the East
• Some faults as far east as Yellowstone
• No action to west in Pacific plate either– Hawaii is special case - hot spot volcano
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US Seismicity
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Geologyof western US -Fabric related todeformation
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Western boundary of North American Plate
• Type of boundary depends on orientation• Plate boundaries move and change in time• Past 30 My saw major changes & development
of San Andreas Fault• From 80 to 30 My Farallon plate subducted
under west coast.– Juan de Fuca & Cocos plates are remnants of
Farallon plate
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Cartoon ofWestern
N. Americatectonics
Vidale
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Details: US and Mexico coast
• Three little plates subducting offshore Oregon, Washington, and B. Columbia– Juan de Fuca Plate– Gorda Plate– Explorer Plate
• Spreading ridge splitting Gulf of California– Separating Baja from N. America– Oblique because ridges are combined with transform
faults
• Cocos Plate subducting to the south
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Exoticterrain
placement
Accretion of fragmentsto continent
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Exotic terrainsWestern N. Am (Cordillera)assembled from small pieces over past 200 My
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Changes in west coast• A mid-ocean ridge subducted
– Before that, just subduction on coast– First hit near LA 25 Mya– San Andreas fault system started then
• As ridge is subducting, two triple junctions are moving apart on coast– Mendocino Triple Junction moving north
• NA-Pacific-Juan de Fuca junction
– NA-Pacific-Rivera junction off Baja California
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Note present San Andreas fault
Show movie #02
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Oregon-Washington
• M 9 every 1000 years, last in 1700
• Recent quakes– M 7.0 in 2000, 6.5 in 1965 in Seattle– M 7.1 in 1949 in Olympia
• Main fault is subduction zone
• Also volcanoes (like Mt. St. Helens)
• Not adequately prepared
• We’ve looked at these faults before– But biggest surface offshore
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• Cascadia subduction zone
• 4 cm/yr convergence rate
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Pacific - Juan de Fuca spreading
Subduction of J. de F. under N. Am. Iacopi, 27
Note:Plate motionsTriple Junctions
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Cartoon view
Show movie #09
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Also smaller faults in Seattle
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Oregon-Wash.
Close-up
Yeats, 5-2
Note:VolcanoesSan AndreasTriple Junctions
Mt. HoodFrom Portland
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Mt Rainier and Seattle
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Subduction under Pacific Northwest
• 4 cm/yr of plate convergence– like small convection cell - up at ridge down at
subduction zone
• Site of great underthrusting earthquakes– None in historic record– But evidence for magnitude 9 quake in 1700
• Tsunami sands in buried marsh
• Tsunami in Japan in 1700
• Volcanic arc & eruptions– Mt. St. Helens, Mt. Rainier, Mt. Shasta
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Oregon-Washington X-sectCascadia cross-section
Keller, 7-5
Ridge
OlympicMts
CascadeMts
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Tsunami sand associated with the 1700 earthquake covering a Native fire pit on the banks of the Salmon River in Oregon. Fire-cracked rocks, charcoal and ashes delineate the fire pit that was dug into the sandy beach.
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Last 30 Mya of west coast tectonicsCapeMend.
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ETSEvery
15 months
Creager & Wech
*
BS Array
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Tremor Migration for
four ETS events
Wech, Creager and Melbourne, JGR, submitted
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Tremor and Slip summed over 4 ETS events
Wech, Creager and Melbourne, JGR, submitted
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Episodic Tremor and Slip (McCausland et al., GRL 2005)
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Cascadia
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