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Download PDF file of Chapter 11 lecture - Earthquakes
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Dr. James Wittke [email protected] Vibrations produced by rapid release of energy (rupture and slippage along faults) Origin of break at focus (hypocenter) Epicenter located directly above focus Energy in form of seismic waves radiates in all directions from source Most of motion along faults can be explained by plate tectonics theory ◦ Extension (normal) ◦ Compression (reverse, thrust) ◦ Shear stress (transform) Type of fault can be determined from earthquake data Faults kept from moving by friction build up elastic strain Earthquakes occur when stresses overcome friction rocks on both sides “rebound” Break starts at focus and rupture spreads outward across fault surface (travels at 2-3 km/s) Rupture reaches surface surface breaks Final displacement called fault slip (typically 1 m, maximum 20 m) Surface breaks (ruptures) can be 100s km long ◦ San Francisco (1906) break ran 430 km Some faults do not reach surface, e.g., Northridge (1994) Boys adjacent to young surface break due to relatively recent, but probably prehistoric, earthquake (near Minjar River, Russia). Most studied fault system in world Displacement along discrete segments 100 to 200 km long Some sections show slow, gradual displacement (no earthquakes fault creep) Other segments slip regularly causing small earthquakes Other segments store energy for 100s of years before rupturing in great earthquakes Small foreshocks may precede major earthquake by days or as much as several years Adjustments after major earthquake can cause smaller earthquakes called aftershocks Sumatra 2004 Aftershocks (star indicates location of main earthquake) Alaska 2002 Aftershocks Seismometers record ground movement caused by body (P, S) and surface waves Different types of seismometers needed to record vertical and horizontal ground motion Records obtained called seismograms (above) Travel through Earth’s interior Two types based on how they propagate… ◦ Primary (P) waves ◦ Secondary (S) waves P waves faster than S waves Travel through solids, liquids, and gases Push-pull (compress and expand) motion, changing volume of intervening material Shaking motion at right angles to their direction of travel Travel only through solids Travel along surface of Earth Greatest amplitude and slowest velocity Cause greatest destruction Three station recordings needed to locate epicenter Each station determines time interval between first P wave arrival and first S wave arrival Travel-time graph used to determine each station’s distance to epicenter Circle with radius equal to distance to epicenter drawn around each station Point where all three circles intersect is earthquake epicenter Intensity ◦ Qualitative measure damage and effects at a given location Magnitude ◦ Quantitative measure of energy released at source of earthquake ◦ Based upon amplitude of largest seismic wave recorded ◦ Accounts for decrease in wave amplitude with distance from epicenter Qualitative report of earthquake's intensity or destructiveness Roman numerals (I to XII) describe what people felt or saw Damage/experiences may not reflect earthquake’s size accurately ◦ Affected by how close to focus, local geology, type of earthquake, etc. Northridge 1994 Japan 1925 Introduced by Charles Richter in 1935 ◦ Based on amplitude of largest seismic wave ◦ Accounts for decrease in amplitude with distance ◦ Intended for local shallow southern CA earthquakes Magnitudes <2.0 not felt by humans Each unit of magnitude increase… ◦ 10x increase in amplitude ◦ About 32x increase in energy Developed because Richter scale does not adequately indicate size of very large earthquakes Total strain energy based upon… ◦ Displacement along fault ◦ Strength of rock ◦ Area of rupture surface Examples… ◦ Alaska 1964: ML = 8.3, MW = 9.2 ◦ San Francisco 1906: ML = 8.3, MW = 7.9 ◦ Chile 1960: MW = 9.5 (largest known) About 95% of energy released by earthquakes originates in few relatively narrow zones that wind around globe Earthquakes with magnitudes >7.5 since 1990 Shallow: 0-70 km deep Intermediate: 70-300 km deep ◦ In crust (most destructive) ◦ Divergent plate boundaries ◦ Convergent plate margins (subduction) Deep: >300 km ◦ Convergent plate margins (subduction) ◦ Almost all in circum-Pacific belt (landward of deepocean trenches) Earthquake foci at subduction margins form WadatiBenioff zone (depths increase away from trench) Damage depends on... ◦ Magnitude ◦ Duration of vibrations ◦ Nature of material upon which structure rests ◦ Type and design of structures Deaths depend on… ◦ Amount of structural damage ◦ Population density ◦ Time of day Damage in Anchorage due to Alaska 1964 earthquake Amplification depends on nature of ground Soft sediments amplify waves more than bedrock Predicted amplification (Los Angeles area) Unconsolidated materials saturated with water turn into mobile fluid Liquefaction hazard in Alameda, Berkeley, Emeryville, Oakland, and Piedmont for magnitude 7.1 earthquake on the Hayward fault Sudden vertical displacement along ocean-bottom fault, or Large undersea landslide triggered by earthquake Want: warning of location and magnitude of earthquake within narrow time frame Methods: monitor phenomena before forthcoming earthquake ◦ Ground uplift or subsidence ◦ Gas releases ◦ Strain (deformation) in rocks No reliable method exists Based on idea that earthquake occurrence is repetitive or cyclical Provides a probability of certain magnitude earthquake occurring on long time scale (30100+ years) Probability of earthquake of listed size occurring between 1988 and 2018 Earthquake Prediction: Geographic Progression