Giant Earthquakes in the Pacific Northwest

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Presentation transcript:

Giant Earthquakes in the Pacific Northwest

I. Introduction Earthquakes  ______________ of the crust from the rapid release of stored _____________ ____________, ruptures in crust allow release

I. Introduction B. EQ’s & Tectonic Zones Divergent boundaries  spreading centers

I. Introduction 2. Transform boundaries 

I. Introduction Convergent boundaries

I. Introduction C. Great or Giant Earthquakes most often associated with _______________ _________________ EQ’s

Subduction Zone EQ’s

II. Cascadia Cascadian Subduction Zone Evidence for subduction

II. Cascadia 2. No Giant Quakes in Cascadia? apparent absence  historically, no biggies in last 300 years Why?

II. Cascadia B. Ancient Giant Quakes in PNW Evidence esturaries & salt marshes  _____________

Layered Cycles Draw the picture here

II. Cascadia 2. Dating of the Cycles

II. Cascadia 3. Human Record

II. Cascadia C. Understanding the Modern

II. Cascadia Results of Surveys horizontal shortening  vertical bulging 

II. Cascadia D. Locked Subduction Zone what’s causing the clog?

II. Cascadia E. Locked & Loaded

__________________________ LOCKED & LOADED 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 Mega-thrust related

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 3 main “swarm” sequences June, July & November sequences Considered “unusual” by seismologists Lack of seismic stations=poor data

2001 Spokane Earthquakes June Sequence 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 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

Common Characteristics 2001 Spokane Earthquakes Common Characteristics 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 1915*, 1920, 1922* 1941, 1942, 1948*, 1952* 1961 & 1962 All “low” magnitudes events *Similar locations to 2001

Historical Seismicity Regional Events 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 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 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 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 ?

Puget Sound - Western Washington Future Seismicity Cascadia: Puget Sound - Western Washington Subduction continues=compression Nisqually 2001 = NOT THE BIG ONE! Seattle Fault 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?