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Earthquake Predictions & Tectonic Environments
Natural Disasters Earthquake Predictions & Tectonic Environments
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Predicting Earthquakes
Forecast Involve statements of where and how frequent an event is likely to occur and how large it might be Predictions Involve statements about specifically when and where and earthquake is expected to occur
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Forecasts Reasonable Forecasts:
Most earthquakes will be on plate boundaries By knowing the type boundary, we can sense the type of motion along faults Convergent thrust fault Divergent normal fault Transform strike-slip Longer faults produce larger earthquakes
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Earthquake Predictions
Earthquake Precursors Seismic Gaps Migrating Earthquakes Earthquake Regularity Paleoseismology Water as a trigger
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Earthquake Precursors
Foreshock Change in surface elevations Change in water table Change in radioactivity Peculiar behavior in animals
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Earthquake Precursors
Tangshan, China Haicheng, China (1975) Successful prediction of 7.3M EQ Evacuated 1 million people; saved >100,000 90% of buildings severely damaged Tangshan, China (1976) No precursors; did not predict 7.6M EQ 250,000 died; 500,000 injured
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Seismic Gaps A fault segment with few or no historic earthquakes
Upper plot (purple) shows a lack of seismic activity (seismic gap) near Loma Prieta; Lower plot shows seismic gap filled in 1989
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Seismic Gap Some seismic gaps slip continuously
Stress is relieved w/o an earthquake Hollister fault (purple) continuously creeps
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Migrating Earthquakes
In Turkey ( ), EQ moved sequentially westward Each rupture juxtaposed each other Seismic gaps later filled with lesser EQ
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Earthquake Regularity
Fault movement regulated by calendar i.e. Parkfield, CA M EQ w/ average 22-year interval Method uncommon and unreliable
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Paleoseismology Study former EQ by measuring offset of rock layers below ground Trenches across active faults determine ancient fault movement Amount of offset proportional to magnitude
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Water as a Trigger Addition of fluids increases pore pressure
Decreases friction between sediments trigger EQ
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Long-Term Forecasts and Risk Maps
Based on past activity, frequency, and magnitude Long-term forecasts requires knowledge of past EQ along fault
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Surviving Earthquakes
The largest EQ do not kill people, poor construction does Notice beneath stucco walls, house is built by poorly-cemented rocks
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San Andreas Fault San Andreas Fault Zone is composed of:
San Andreas fault is main strand Many parallel faults ~50km width Dominant EQ zone in U.S. Continental Transform Fault Pacific Plate moves northwest relative to North American Plate 1,200 km length in California Just south of Mexico border to Cape Mendocino in northern CA Moves at a rate of 3.5 cm/year
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San Andreas Fault San Francisco Bay Area San Francisco (1906) – 7.8M
Shaking seconds; brick buildings collapsed 500 deaths (early estimate); 3,000 deaths (recent estimate) Fires burned for days; destroyed 28,000 buildings dynamite used to stop fire
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San Francisco Bay Area The Next Big One?
Hayward, Rogers Creek, Calaveras faults Pose serious threat; last major quake in 1868 62% chance of >6.7M; 80% chance M Problem – lie beneath heavily populated area Kobe, Japan (1995) similar Mag. & pop. density 6,000 deaths
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San Andreas Fault Los Angeles area Northridge, CA (1994) 6.7M
61 deaths 10,000 bldgs closed; 7 freeway collapse; 170 bridges damaged Many buildings collapsed due to weak 1st floors
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Los Angeles Area The Next Big One?
Aside from San Andreas fault, there are many potentially dangerous faults closer to L.A. Metro area Sierra Madre-Cucamonga, Santa Monica Mtns., Palos Verdes faults Capable of causing M EQ Probability of ~6.7M has recurrence interval <10 years; 7.5M 300 years
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Tectonic Environments & Major Earthquakes
Transform Boundary Convergent Boundary Divergent Boundary
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Transform Boundary Plates slide past one another
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Transform Boundary Izmit, Turkey (1999) 7.4M
Similar to San Andreas fault Arabian & African Plates slide northward against Eurasian Plate Slippage rate cm/year; 900 km length
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Convergent Boundary Ocean-continent boundary
Oceanic crust is subducted beneath continental crust
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Ocean-Continent Chile (1960) 9.5M
Subduction zones produce the largest EQ’s Large foreshock saves 1,000’s of lives; 2,000 died Tsunami hit Japan 22 hours after 120 deaths Expect similar results for the next Cascade Range EQ (Pacific NW U.S.)
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Convergent Boundary Continent-continent boundary
Build up of huge mountain ranges
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Continent-Continent Bam, Iran (2003)
Arabian Plate collides with Eurasian Plate Caucasus Mtns. 3.0 cm/year convergence 6.7 M similar to Northridge 61 deaths (Northridge); 26,000 deaths (Bam) The increase in deaths due to poor construction
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Convergent Boundary Ocean-ocean boundary
Older, colder oceanic plate is subducted
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Ocean-Ocean Kobe, Japan (1995) 7.2M Very shallow EQ
5,000 deaths, 190,000 buildings destroyed
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Divergent Boundary Spreading Zones
Extensional forces pulls plates apart
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Basin & Range Basin & Range Basin & Range in Nevada & Utah
Numerous north-trending normal faults Faults separate dropped valleys (basins) and uplifted mountains (ranges)
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