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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 • • • • 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 – – – – 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 – – – – 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)