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Transcript
CAUSES OF EARTHQUAKES • How do earthquakes happen? • What kinds of seismic waves are produced by earthquakes? Ch. 3, pp. 75-76, Plummer Canadian First Edition What is an earthquake? • Sudden release of elastic energy (snap) • Induced by stress buildup • Released when stress > strength of fault • Energy released as – seismic waves – displacement along faults – heat and other energy • Elastic rebound theory Turkey, 1999 • Elastic deformation – – – – relatively small forces small displacements materials shape restored when force is removed energy can pass as waves • Plastic (or ductile) deformation – like putty – materials respond to forces by changing shape – no storage of energy • Brittle deformation – material stores energy put in by the force. They accumulate stress. – at some point the material will break – catastrophic release of energy – repeated breaking of pre-existing weak surfaces (faults) Cold rock is brittle (for example, ocean lithosphere and most of continental lithosphere) Hot rock is ductile (for example, the asthenosphere and some parts of continental plates) Both lithosphere and asthenosphere act elastic if stress change is sudden and small (e.g., seismic waves) In ductile materials, elastic stress does not build up and earthquakes don!t happen A rock Apply a force (stress) Deformation (strain) Release the force (elastic rebound) Elastic materials store and release elastic energy If there is too much strain, the rock will 1) deform or flow (plastic or ductile deformation) 2) break, or slip along an existing fault (brittle deformation) - earthquake! - rapid release of elastic strain energy, some in the form of vibrations or waves http://civilx.unm.edu/laboratories_ss/mechmat/ bendwoodfracture.jpg Brittle materials store elastic strain energy but break if a threshold stress is exceeded Elastic materials store and release elastic energy (bounce back) Brittle materials containing faults: the fault will slip if a threshold stress is exceeded Plastic or ductile materials flow and do not really hold elastic strain energy One idea about how faults work at depth rock formerly in the lower crust, now exposed next to a highway in western Massachusetts plate 1 plate 2 elastic upper and middle crust ductile lower crust (for continents) Fig 3.2 uppermost crust fault stuck between earthquakes ductile creeping fault zone at depth - no earthquakes shear zones can merge at depth and many ductile layers and zones can be present Elastic Rebound Theory Fig. 3.3 Elastic stresses build up as rock deforms slowly over time Rupture occurs when elastic stresses exceed what the fault can bear (friction). Rocks along fault spring back to undeformed state (“elastic rebound”) What happens along the fault during an earthquake? 1994 M = 7.4 Landers, California earthquake !!!!! epicenter hypocenter Maximum slip is not usually at the hypocenter Rupture propagates away from the hypocentre at about 2-3 km / sec (slower near surface) Rupture begins at the hypocenter and travels away (!unilateral! = one-way, "bilateral! = both ways) Rupture is usually discontinuous (or absent) at the surface, but likely continuous (smooth) at depth Peyrat et al., 2000 Animation of the Landers earthquake rupture SEISMIC WAVES Plummer, Canadian First Edition, Ch. 3 pp. 76-77 from the USGS Earthquake Hazards Team website First, Two Questions: Seismology • Earthquake seismologists – Describe, explain, assess, predict(?) earthquakes • What is seismology? • What do seismologists do? • Global seismologists – Map deep Earth structure using seismic waves generated by earthquakes – Goal is to understand whole Earth dynamics • Exploration seismologists – Make their own earthquakes (explosives, etc.) – Search for valuable resources (water, oil, gas) – Image much smaller regions than global seismologists Two Kinds of Seismic Waves P (Primary) wave: particles move in line with wave direction body wave: travels through the inside the Earth surface wave: travels along the surface of the Earth body wave Body Waves S (secondary) wave: particles move perpendicular to wave direction body wave • P Waves (Primary, or Compressional) # - change in volume of the material # - particle motions are parallel to the direction the # wave travels # - the wave spreads out in all directions from # the earthquake in 3D (spherical spreading) # - fastest seismic wave • S Waves (Shear, or Secondary) # - change in shape of the material # - particle motions are perpendicular to the direction # the wave travels # - spherical spreading # - slower than P wave Rayleigh wave: particles move elliptically as in ocean waves surface wave Love wave: particles move perpendicular to wave direction, and horizontal surface wave Surface waves • Love waves: – like S-waves with no vertical displacement – Ground moves side-to-side horizontally – destroy buildings, major seismic hazard – circular speading from a point (2D), like a pebble in a pond • Rayleigh waves: – like ocean rolling waves with vertical and horizontal displacement – elliptical particle motion – circular spreading from a point (2D)