Transcript Lecture 17: Pacific Northwest Earthquakes

Giant Earthquakes in the Pacific
Northwest
I. Introduction
A. Earthquakes 
• ______________ of the crust from the rapid
release of stored _____________
• ____________, ruptures in crust allow
release
I. Introduction
B. EQ’s & Tectonic Zones
1. Divergent boundaries  spreading centers
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I. Introduction
2. Transform boundaries 
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I. Introduction
3. Convergent boundaries
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I. Introduction
C. Great or Giant Earthquakes
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• most often associated with _______________
• _________________ EQ’s
Subduction Zone EQ’s
II. Cascadia
A. Cascadian Subduction Zone
1. Evidence for subduction
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II. Cascadia
2. No Giant Quakes in Cascadia?
• apparent absence 
• historically, no biggies in last 300 years
• Why?
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II. Cascadia
B. Ancient Giant Quakes in PNW
1. Evidence
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• esturaries & salt marshes  _____________
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Layered Cycles
Draw the picture here
II. Cascadia
2. Dating of the Cycles
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II. Cascadia
3. Human Record
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II. Cascadia
C. Understanding the Modern
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II. Cascadia
Results of Surveys
• horizontal shortening 
• vertical bulging 
II. Cascadia
D. Locked Subduction Zone
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• what’s causing the clog?
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II. Cascadia
E. Locked & Loaded
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LOCKED & LOADED
1. Buildup of stress = strain to leading edge 
_____________________________
• Vertical (& horizontal) ____________________
2. Rupture  massive __________ rupture 
_____________ (surge of plate) ________ 
__________________________
3. Subsidence behind _____________________
II. Cascadia
F. Future Quakes
• energy released (magnitude) depends 
• extent of mega-thrust rupture 
• B.C. to northern California? 100,000 km2
• 1700 quake  evidence = ________________
Maule, Chile, 2010
• Magnitude = 8.8
• Depth ~ 35 km
• NazcaS. America convergence ~ 7m/100 yr
• .07m/yr = ~7cm/yr
• 100km x 500km
• 12 m of slip
Chile vs. Haiti
• Chile, 2010  80,000 km2 fault area
movement
• Haiti, 2010  600 km2
• Chile = 12 m slip
• Haiti = 5 m slip
• Chile  __________________ more energy
III. Quakes and Faults in Cascadia
A. Mega-thrust related
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III. Quakes and Faults in Cascadia
B. Shallow crustal faults
• Seattle Fault
2001 Spokane Earthquake
Sequence:
a whole ‘lotta shaking going on!
2001 Spokane Earthquakes
Summary
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3 main “swarm” sequences
June, July & November sequences
Considered “unusual” by seismologists
Lack of seismic stations=poor data
2001 Spokane Earthquakes
June Sequence
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Mainshock = 3.9M (6/25/01)
Total of 21 EQ’s recorded
11 reported “felt” EQ’s (3.4 M, <3.0M)
Foreshocks? 5/24-6/24 (2.0M)
2001 Spokane Earthquakes
Late July Sequence
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July 28-August 1
14 recorded EQ’s
3.2 maximum magnitude
4 EQ’s on Sept. 28 (1.6-2.8M)
2001 Spokane Earthquakes
November Sequence
• November 11th to January 2002
• 21 reported EQ’s (19 felt)
• 4.0M (11/11/01) maximum magnitude
2001 Spokane Earthquakes
Common Characteristics
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Relatively low magnitudes
Explosion-like sounds = shallow depths
Clustered “swarms”
NW-trending linearity to aftershocks?
2001 Spokane Earthquakes
What is going on here?
Historical Seismicity
Local Seismicity
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1915*, 1920, 1922*
1941, 1942, 1948*, 1952*
1961 & 1962
All “low” magnitudes events
*Similar locations to 2001
Historical Seismicity
Regional Events
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1872 Lake Chelan, WA (7.3M)
1936 Milton-Freewater, WA (6.4)
1959 Hebgen Lake, MT (7.5M)
1983 Borah Peak, ID (7.3M)
1987 Othello, WA “swarm” (3.3M)
Northwest Regional Geology
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Rocky Mountains to east
Columbia River Flood Basalt Plateau
Cascade Volcanic Range to west
Cascadian subduction zone
Cascade Subduction Zone
• Convergent plate boundary
• Volcanic mountains
• Compressional stress = crustal uplift
Regional Tectonic Stress Fields
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Compressional stress in Cascadia
Northward movement of CA-OR blocks
S. Columbia Plateau compression
E. Columbia Plateau extension & slip?
Inland Northwest Faults
• Lewis & Clark faults (northern ID-MT)
• NNW-trending Latah Creek Fault?
• Other NW-trending structures?
Latah Creek Fault
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Real or not?
Robert Derkey, Washington DNR
Definite NNW-trending lineament
Intersects with NNW-trend Spokane River
Geologically active 8-16 MY ago
Other NW-trending structures
NW-trending structures
Future Seismicity
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Future Seismicity
Cascadia:
Puget Sound - Western Washington
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Subduction continues=compression
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Nisqually 2001 = NOT THE BIG ONE!
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Seattle Fault
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Shallow “giant” subduction EQ’s
Future Seismicity
Eastern Washington
• Yakima Fold Belt = Rattlesnake Hills
• Spokane & Latah Fault
• Very little data = ?
Spokane Seismicity & Hazards
• M4 or less: minor damage potential
• M4-M6: moderate to significant damage
to older & unreinforced masonary
structures
• >M6: severe damage
Potential of event magnitudes?