Lecture 7

Lecture 7 - Types of earthquake waves Types • Body waves – P waves(compressional waves Travel at speeds of 6-8 km/s through the earth's

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Unformatted text preview: Types of earthquake waves Types • Body waves – P waves (compressional waves): Travel at speeds of 6-8 km/s through the earth's interior. Particle motion is parallel to direction of propagation. Also called primary waves (thus the P). Similar to sound waves. – S waves (shear waves): Travel at speeds of 3-5 km/s through the earth's interior. Particle motion is perpendicular to direction of propagation. Also called secondary waves (thus the S). Do not pass through liquids. • Surface waves: Travel at speeds less than 3-4 km/s. They consist of dispersed wave trains (different frequencies travel at different velocities) that travel around the earth at its surface. – Love waves: Particle motion is horizontal and perpendicular to the direction of propagation. – Rayleigh waves Particle motion is retrograde elliptical in direction of propagation. Pathways and recording of three types of seismic waves P-wave (a) and S-wave (b) P-wave particle motion • P-wave particle motion involves alternating compression and extension along the direction in which the wave travels. P-waves travel throughout the earth. S-wave particle motion involves back and forth (shearing) motion perpendicular to the direction in which the wave travels. S-waves can not travel through liquids since shear motions (stresses) cannot be transmitted from particle to particle in a liquid. For that reason, S-waves can not travel through earth's outer core. • P-wave and S-wave particle motions P-wave Rayleigh-wave (a) and Love-wave (b) particle motion Rayleigh Water wave Surface Waves and Dispersion Surface • • • Surface wave particle motions die off with depth like water waves. Longer wavelength surface waves penetrate deeper into the earth than shorter wavelength waves. Because seismic velocities increase with depth, longer wavelength surface waves travel faster than shorter wavelength waves. This property of the dependence of velocity on wavelength is known as "dispersion". Because surface waves are dispersed, they arrive at distant stations over a range of times rather than all at once, as in the case of body waves. This leads to the arrival of a "surface wave train" rather than a single pulse or two. • Water wave Ground motions produced by the various types Ground of seismic waves at the earth's surface Particle motions in the various types of seismic waves Particle P-waves S-waves Love waves Rayleigh waves Surface wave particle motions Surface (a) Love waves (b) Rayleigh waves Waves spreading out on a pond from a stone Waves spreading out in the earth from an earthquake Reflection and Refraction (a) A beam of light, when it reaches the boundary between two different media such as water and air, partly reflects and partly refracts. In (a) the refracted beam bends down as it enters the water. (b) A wave that enters a slower medium from a faster one bends away from the boundary (like light entering water from air). (c) A wave that enters a faster medium from a slower one, bends toward the boundary. {Faster = Bigger Angle} i i R i R R Refraction Refraction and Reflection of beams of light in a fish bowl Reflection Seismic waves travel at different velocities through different Seismic materials. This causes the wave paths (rays) to bend or curve. Boundaries between different materials with different seismic velocities. Rays encountering the boundaries at an angle will be “refracted”. Rays in the actual earth bend since the seismic velocities change with depth. ...
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This note was uploaded on 01/11/2010 for the course GEOL 240Lxg at USC.

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