Transcript Natural Hazards - Cal State LA

Natural Disasters
Earthquake Predictions & Tectonic Environments
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
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
Earthquake Predictions
Earthquake Precursors
Seismic Gaps
Migrating Earthquakes
Earthquake Regularity
Paleoseismology
Water as a trigger
Earthquake Precursors
• Foreshock
• Change in surface
elevations
• Change in water table
• Change in
radioactivity
• Peculiar behavior in
animals
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
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
Seismic Gap
• Some seismic gaps
slip continuously
• Stress is relieved w/o
an earthquake
• Hollister fault
(purple) continuously
creeps
Migrating Earthquakes
• In Turkey (1939-1999), EQ moved sequentially westward
• Each rupture juxtaposed each other
– Seismic gaps later filled with lesser EQ
Earthquake Regularity
• Fault movement regulated by calendar
– i.e. Parkfield, CA
– 5.5 - 6.0M EQ w/ average 22-year interval
• Method uncommon and unreliable
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
Water as a Trigger
• Addition of fluids increases pore pressure
– Decreases friction between sediments  trigger EQ
Long-Term Forecasts and Risk Maps
• Risk Maps
– Based on past activity, frequency, and magnitude
• Long-term forecasts requires knowledge of past EQ
along fault
Surviving Earthquakes
• The largest EQ do not kill people, poor construction does
• Notice beneath stucco walls, house is built by poorlycemented rocks
San Andreas Fault
• San Andreas Fault Zone is composed of:
– San Andreas fault is main strand
– Many parallel faults ~50km width
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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
San Andreas Fault
• San Francisco Bay Area
• San Francisco (1906) – 7.8M
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Shaking 45-60 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
San Francisco Bay Area
• The Next Big One?
• Hayward, Rogers Creek, Calaveras faults
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Pose serious threat; last major quake in 1868
62% chance of >6.7M; 80% chance 6.0-6.6M
Problem – lie beneath heavily populated area
Kobe, Japan (1995)  similar Mag. & pop. density  6,000 deaths
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
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 7.2-7.6M EQ
• Probability of ~6.7M has recurrence interval <10 years; 7.5M
 300 years
Tectonic Environments & Major Earthquakes
Transform Boundary
Convergent Boundary
Divergent Boundary
Transform Boundary
• Plates slide past one another
Transform Boundary
• Izmit, Turkey (1999) 7.4M
• Similar to San Andreas fault
– Arabian & African Plates slide northward against Eurasian Plate
– Slippage rate 1.8-2.5 cm/year; 900 km length
Convergent Boundary
• Ocean-continent boundary
• Oceanic crust is subducted beneath continental crust
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.)
Convergent Boundary
• Continent-continent boundary
• Build up of huge mountain ranges
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
Convergent Boundary
• Ocean-ocean boundary
• Older, colder oceanic plate is subducted
Ocean-Ocean
• Kobe, Japan (1995) 7.2M
• Very shallow EQ
• 5,000 deaths, 190,000 buildings destroyed
Divergent Boundary
• Spreading Zones
• Extensional forces pulls plates apart
Basin & Range
• Basin & Range
• Basin & Range in Nevada & Utah
– Numerous north-trending normal faults
– Faults separate dropped valleys (basins) and uplifted mountains (ranges)