第一章地震波的运动学.ppt

Making P Waves Heres how you do it: Tie one end of your Slinky? to a hook on the wall or a door knob (close the door first). Holding the other end of the Slinky?, walk away from the wall or door. Stop walking when the Slinky? isnt sagging anymore. Dont pull the Slinky? tight; just take up the slack. Quickly jerk your end of the Slinky? toward the wall then back. Dont let go of it. Youll see waves similar to P waves moving back and forth along the Slinky? like in the picture below. (From Lutgens Tarbuck, 1989) Making S Waves This is probably even easier than making P waves. Tie one end of your rope to a hook on the wall or a door knob (close the door first). Holding the other end of the rope, walk away from the wall or door. Stop walking when the rope is barely sagging. Dont pull the rope tight; just take up most of the slack. Quickly jerk your end of the rope up and down once. Dont let go of it. Youll see waves similar to S waves moving along the rope like in the picture below (From Lutgens Tarbuck, 1989) SEISMIC WAVES S-wave: P-wave: primary wave longitudinal particle vibration compressional and dialtion quadrants first arrival on seismogram undulating wave amplitudes are larger than P wave, to conserve energy the amplitude builds up comes in later and slower than p wave secondary wave P and S are BODY WAVES Love wave an interfact wave horizontal motion Rayleigh wave very high amplitude wave retrograde elliptical motion: motion is opposite direction of propagation v(Rayeligh)=.92*v(s-wave) Love and Rayleigh are SURFACE WAVES COMPARISON 1. P-waves primary waves (first arrivals) longitudinal sound waves, so why dont we hear them? travel at frequencies humans cant hear speeds: crust, 6 km/s mantle, 8 km/s core, center of earth 11 km/s (fastest waves) 2. S-waves secondary waves shear waves no volume change speeds: crust, 4 km/s mantle, 4.5 km/s outter core, 0 because it is a liquid and shear waves cannot travel in liq