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Elastic Rebound Theory This theoryy was discovered byy making g measurements at a number of points across a fault. Prior to an earthquake it was noted that the rocks adjacent to the fault were bending bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake. Elastic Rebound Theory Elastic Rebound Sequence of elastic rebound: Stresses Sequence of elastic rebound: Bending Sequence of elastic rebound: Rupture Sequence of elastic rebound: Rebound Sudden Slip by Elastic Rebound Stresses (force/area) are applied to a fault fault. Strain (deformation) accumulates in the vicinity of friction friction-locked locked faults faults. Strain accumulation reaches a threshold and d ffaultlt slips li suddenly dd l Rupture (slip) continues over some portion of the fault. Slip is the distance of displacement along a fault. Fault A ffracture t (crack) ( k) in i the th earth, th where h th the ttwo sides id move past each other and the relative motion is parallel to the fracture. 90˚ dip = vertical fault plane 0˚ strike = north parallel fault plane Surface Trace of a fault Different Fault Types n) shear) Normal DipDip-slip fault hanging wall moves down A Normal dip slip fault Reverse DipDip-slip fault Hanging wall moves up Thi iis also This l called ll d a Thrust Th t Fault. A reverse dip-slip fault Strike--slip fault Strike Displacement in horizontal directio A strike-slip fault Strike--Slip Strike p Fault – Left Lateral Strike--Slip Strike p Fault – Right g Lateral Oblique--slip fault Oblique Displacement in both vertical and horizontal directions An oblique-slip fault Blind/Hidden faults Sequence of Events 1) Tectonic loading of faults 2) Earthquakes 3) Seismic S i i waves 4) Shaking (ground motion) 5) Structural failure Seismic Waves (Earthquake’s energy is transmitted through the earth as seismic waves) Two types of seismic waves Body waveswaves- transmit energy through earth’s interior Pi Primary (P) wavewave- rocks k vibrate ib t parallel ll l to t direction di ti of wave Compression and expansion (slinky example) Secondary (S) wave wave-- rocks move perpendicular to wave direction Rock shearing (rope (rope--like or ‘wave’ in a stadium) Surface waves waves-- transmit energy along earth’s surface Rock moves from side to side like snake Rolling R lli pattern like lik ocean wave Primary waves P-waves, compressional or longitudinal. Typical crustal velocity: 6 km/s ( ~13,500 mph) Travel T l th through h solids, lid liliquids, id or gases Material movement is in the same direction as wave movement Behavior: Cause dilation and contraction (compression) of the earth material through which they pass. Arrival: They arrive first on a seismogram seismogram. Even for P waves (which can travel all the way through) we see some changes in the path at certain points within Earth. This is due to the discontinuities present at different boundaries in earth structure Secondary waves S waves (secondary) Typical crustal velocity: 3 km/s ( ~6,750 mph) Behavior: Cause shearing and stretching t t hi off the th earth th material t i l through which they pass. Generally cause the most g; very y severe shaking; damaging to structures. Travel through solids only shear waves - move material perpendicular to wave movement Arrival: Second on a seismogram seismogram. S-wave velocity drops to zero at the coremantle boundary or Gutenberg Discontinuity Shadow Zone - no earthquake waves Variation of P and S wave velocities within the earth M disc M-disc G disc G-disc M-Disc : The Mohorovicic discontinuity G-disc: The Gutenberg di discontinuity ti it Surface Waves Travel a e just be below o or o along a o g the t e ground’s surface Slower than body waves; rolling and side side--to to--side movement Especially b ildi buildings damaging to Two most common types of surface waves Rayleigh Waves Typical yp velocity: y ~ 0.9 that of the S wave Behavior: Causes vertical together with back-and-forth horizontal motion. Motion is similar to that of being in a boat in the ocean when a swell moves past. Arrival: They usually arrive last on a seismogram. Love Waves Typical yp velocity: y Depends p on earth structure, but less than velocity of S waves. Behavior: Causes shearing motion (horizontal) similar to S waves. Arrival: They usually arrive after the S wave and before the Rayleigh wave. Primary Waves Secondary Waves Movement of Earth Locating an Earthquake’s Epicenter Seismic wave behavior P waves arrive first, then S waves, then L and R After an earthquake earthquake, the difference in arrival times at a seismograph station can be used to calculate the distance from the seismograph to the epicenter (D). If average speeds for all these waves is known, use the S P (S minus P) time formula: a method to compute the S-P distance (D) between a recording station and an event. Distance Velocity P wave has a velocity VP ; S wave has a velocity VS . VS is less than VP . Both originate g at the same place--the p hypocenter. yp They travel the same distance but the S wave takes more time than the P wave. D Time for the S wave to travel a distance D: TS ; VS D Time for the P wave to travel a distance D: TP . VP The time difference Time TS TP 1 V VS D D 1 D D P VS VP VS VP VP VS Now solve for the Distance D: V V D = P S TS TP VP VS Seismic TravelTravel-time Curve: If the speeds of the seismic waves are not known, use Travel Travel--Time curve for that region to g get the distance 1. Measure time between P and S wave on seismogram i 2. Use traveltime graph to get distance to epicenter Global Travel Time Curve 3-circle method: 3-circle steps: 1) Read S-P time from 3 seismograms. north D1 D2 2) Compute distance for each h event/recording t/ di station pair (D1, D2, D3) using S-P time formula. 3) Draw each circle of radius Di on map. 4) Overlapping point is the event location. D3 Assumption: Source is relatively shallow; epicenter is relatively close l tto h hypocenter. t