ESS 202 - Earthquakes What is a wave?! A wave is a disturbance that travels far through a medium while particles of the medium move a small amount back and forth and do not experience a net translation.! Examples: ripples on a pond, the wave at sports events Profs. Vidale & Creager Bolt, 5-17 Seismic wave radiation! Radiation - waves that travel outward and carry energy " Examples # Light energy from space heater Travels too fast to see go, 300,000 km/sec # Water waves from a splash, few m/sec # Sound waves from a speaker, 300 m/sec! Seismic waves (motions) are just vibrations of the ground, like sound waves are vibrations of the air Doppler shift Lower pitch For example, approaching siren has a higher pitch than a receding siren 1 2 3 1 2 3 Higher pitch Moving wave sources zebu.uoregon.edu/~js/space/ lectures/lec05.html Breaking sound barrier Doppler shift Sonic boom http://www.fly.faa.gov/carf/
Shuttle sonic boom The Thrust SST, 1997 Breaking sound barrier on land! 763 mph, 1997 X-15 world records Old speed record - Mach 6.7 4520 miles per hour! 4520 miles per hour - Mach 6.7 " 2 km/sec, no longer champ! 354,000 ft off the ground " 100 km high! Flew for just 2 minutes at a time! Intended to prototype moonships " NASA vs Air Force turf war, Air Force lost
X-43A - new record in 2004! Mach 9.6 (7000 mph), off California coast! Unpiloted, 12-foot long vehicle! Ramjet powered (air breathing) " Burned for just 10 seconds! $250M research project Types of Seismic Waves! P waves! S waves! Surface wave train " Love " Rayleigh body waves Walt on Disneyland train Amplitude Wavelength Period
Frequency Types of Seismic Waves! Body waves Earthquake Station * Frequency:! Surface waves Earthquake Station How many waves pass a point in a given amount of time * For sound: frequency=pitch P waves Raypaths and wavefronts! Longitudinal - material moves back and forth (vibrates) in same direction that wave travels, produces compression/dilatation cycle! Fastest type of wave, so arrives first " termed Primary wave! Typical velocities in crust: 5-7 km/sec! Travels through solids or fluids P waves Another view of P wave motion
Strike-slip P-wave radiation pattern This is left lateral strike-slip faulting in map view L N L Different First Motions in different directions Up Down Vertical ground motion Strong in some directions: Lobes Weak in other directions: Nodes L N N N L Down Up First direction of motion: beach ball diagrams Up Down Up Raypaths bend as seismic waves travel Down First Motions and Rture modes 3-D view Map views Three primary focal mechanisms Deformation Strike-slip Map view of first motions Normal Thrust or reverse
S waves S waves! Shearing - material moves back and forth perpendicular to the direction the wave travels in a twisting motion.! Slower than P wave, arrives second " termed Secondary wave! Typical velocities in crust: 3-5 km/sec " P waves travel 5-7 km/s! Travels through solids, but not fluids " because there is no restoring force for the perpendicular motions S wave in a solid Surface Waves! Travel on surface of Earth! Two types " Love waves " Rayleigh waves! Travel a bit slower than S waves! Are the largest amplitude waves " so the P wave can serve as a warning to take cover or shut down critical facilities " warning ranges from a few to 100 seconds # Can get 1 s of warning for each 10 km in distance More surface waves! Need a surface to travel along, which is the rock-air interface at the Earth s surface. " Motion is strongest near the surface " Most strongly generated by earthquakes near the surface Rayleigh waves Love waves
P Where is energy? Review of Waves S Love Rayleigh Bolt, 1-9! In waves, energy has two forms " Strain or deformation - like the energy stored by deforming a spring - 1/2 kx 2 " Motion or vibration - kinetic energy in physics - 1/2 mv 2 " Vibration is the most damaging, but either kind of energy can cause damage Bouncing ball on a spring At rest stretched moving compressed Water waves are different! Energy again has two parts " One part is kinetic energy " The other part is gravitational! There is no twisting energy, not much is strain Amplitude of seismic waves! Amplitude is strength of shaking " Depends on magnitude " Determines amount of damage! Amplitude decreases with distance from the earthquake " energy spreading out over larger area! P wave smallest! S waves larger! Surface waves largest Because the waves travel at different velocities! As waves radiate outward from the earthquake, through the Earth, they separate into a predictable pattern with " P waves arriving first " then S waves " then surface waves P Time S surface
P waves, then S waves, then surface waves Motion has 3 components Transverse Love At 80, as drawn Radial S Rayleigh 10 m 20 m 30-50 m Vertical P Body waves Complications for Seismic waves! Reflection! Refraction! Conversion Reflection on mirror - simple Refraction - Object in water (bending of rays)
Refraction (water in fishbowl) Press, 19-1! Refraction of light by water " Because speed of light waves is slower in water than in air! Seismic waves refract too! And can also switch between P and S Reflection - complicated Includes refraction through curved glass www.nashobawinery.com Straw magic Light through prism Glass-air refraction Conversions of seismic waves Reflection & refraction of waves! P waves and S waves can partially convert to each other when they encounter a sharp change in seismic velocity " usually just a small percent of total energy of the wave! The seismic waves quickly get very complicated! P & S waves are reflected, refracted (bent), and converted at sharp changes (discontinuities) in seismic velocity (as occur between rock layers) One incident P or S wave results in to four waves refracted S refracted P (most energy) Slow Fast incident P reflected S reflected P
Simulated reflection Waves bounce * Bolt, 1-10 1992 Landers EQ Simulation of the 1992 Landers EQ
Simulation map Line source: Like shock wave Shawn Larsen, LBL Doug Dreger, UCB Produced more energy in the shaking to the north Seismicity map Accurate simulation Hayward-Calaveras Fault Right-lateral slip on strand of San Andreas fault Note strong directivity First part of fault breaks Middle of fault breaks After 15 seconds After 30 seconds End of fault Breaks Waves keep Going across basin Fault rture over Waves keep Going across basin After 45 seconds After 60 seconds
Fault rture over Reverbs in basins dying away After 75 seconds Fault rture over Only middle of basins still ringing a little bit After 100 seconds 1964 Nigata quake Which waves cause damage?! S and surface waves are much bigger than P waves, and thus cause the most damage.! Most damaged area is close to fault rture. " And damaging quakes rture for > several sec " P, S, and surface waves have not separated, but rather arrive almost simultaneously " So it is hard to isolate each wave in records of ground motion from damaged areas! Waves bounce, and convert between P, S, and surface types, complicating identification.