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ESS 8 - Earthquakes Profs. Vidale & Creager Bolt, 5-17 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 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 1 2 3 1 Lower pitch For example, approaching siren has a higher pitch than a receding siren 2 3 Higher pitch zebu.uoregon.edu/~js/space/ lectures/lec05.html Moving wave sources Doppler shift Sonic boom Breaking sound barrier http://www.fly.faa.gov/carf/ Shuttle sonic boom The “Thrust SST”, 1997 Breaking sound barrier on land! 763 mph, 1997 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Old speed record - Mach 6.7 4520 miles per hour X-15 world records 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 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Types of Seismic Waves P waves S waves body waves Surface wave train Love Rayleigh Walt on Disneyland train Amplitude Wavelength Period Frequency Frequency: How many waves pass a point in a given amount of time For sound: frequency=pitch Types of Seismic Waves Earthquake Station Body waves * Surface waves Earthquake * Station Raypaths and wavefronts P waves 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, fluids, or gas P waves Sun movie Another view of P wave motion P movie Strike-slip P-wave radiation pattern This is left lateral strike-slip faulting in map view N L N Strong in some directions: Lobes Weak in other directions: Nodes L L N N L Different “First Motions” in different directions Up Down Vertical ground motion Down Up First direction of motion: beach ball diagrams Up Down Up Down Raypaths bend as seismic waves travel First Motions and Rupture modes 3-D view Map views Three primary focal mechanisms Deformation Map view of first motions Strike-slip Normal Thrust or reverse 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 waves S movie 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 Review of Waves S Love Rayleigh Bolt, 1-9 Where is energy? In waves, energy has two forms Strain or deformation - like the energy stored by deforming a spring - 1/2 kx2 Motion or vibration - kinetic energy in physics - 1/2 mv2 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 S P Time surface P waves, then S waves, then surface waves At 80°, as drawn 10 m 20 m 30-50 m Body waves Mike’s movie Motion has 3 components Transverse Radial S Love Rayleigh Vertical P Tromp movie Complications for Seismic waves Reflection Refraction Conversion Reflection on mirror - simple Refraction - Object in water (bending of rays) Refraction (water in fishbowl) 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 Press, 19-1 Reflection complicated Includes refraction through curved glass www.nashobawinery.com Straw magic Glass-air refraction Light through prism Conversions of seismic 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 Reflection & refraction of waves P & S waves are reflected, refracted (bent), and converted at sharp changes (discontinuities) in seismic velocity (as occur between rock layers) refracted S refracted P (most energy) One incident P or S wave results in up to four waves Slow Fast incident P reflected S reflected P Simulated reflection Waves bounce * Bolt, 1-10 1992 Landers EQ QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Peyrat et al., 2000 up QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Simulation of the 1992 Landers EQ Peyrat et al., 2000 QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 North up South Simulation of the 1992 Landers EQ Peyrat et al., 2000 Produced more energy in the shaking to the north North up South Simulation map Shawn Larsen, LBL Doug Dreger, UCB Line source: Like shock wave Seismicity map Accurate simulation Right-lateral slip on strand of San Andreas fault First part of fault breaks After 15 seconds Hayward-Calaveras Fault Note strong directivity Middle of fault breaks After 30 seconds End of fault Breaks Waves keep Going across basin After 45 seconds Fault rupture over Waves keep Going across basin After 60 seconds Fault rupture over Reverbs in basins dying away After 75 seconds Fault rupture 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 rupture. And damaging quakes rupture 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 SCEC movie Waves bounce, and convert between P, S, and surface types, complicating identification.