Seismic WavesPart - ILecture-4*

What is a Wave ?A wave is a disturbance or variation that transfers energy progressively from point to point in a medium.Examples:Sound wave: variation of pressure in the airRadio wave: variation of Electromagnetic intensityWater wave: variation of water surfaceSeismic wave: variation of elastic deformation in rocksThe medium doesnt travel along with the wave.Waves like the electromagnetic wave do not even need a medium to propagate.*

Seismic WavesMovement of rocks beneath the Earths surface causes EarthquakesRocks are stressed because of forces found deep inside the Earth. When this stress exceeds the strength of the rock, the rock breaks or changes shape, which releases energy in the form of wavesThese waves are seismic waves, which are the waves produced by earthquakes

*Source: google images

Types of Seismic WavesBody waves- transmit energy through earths interior

Primary (P) waves- rocks vibrate parallel to direction of waveSecondary (S) waves- rocks move perpendicular to wave direction

Surface waves- transmit energy along earths surface

Love waves - Rock moves from side to side like snakeRayleigh waves - Rolling pattern like ocean wave

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Types of Seismic Waves*Source: google images

Primary WavesP waves, also known as primary waves, are longitudinal seismic wavesThey are called primary waves because they travel faster than other seismic waves and therefore arrive at distant points before other seismic wavesP waves are made up of refractions and compressions of the rock inside EarthP waves compress and expand the ground as they move through it

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Primary wavesP-waves, compressional or longitudinal.Typical crustal velocity: 6 km/s ( ~13,500 mph) Travel through solids, liquids, or gasesShakes the ground back and forth in same and opposite direction as the wave directionCause dilation and contraction (compression) of the earth material through which they pass. They arrive first on a seismogram.*

Primary Waves*Source: google images

Secondary WavesS waves (secondary waves) are transverse seismic waves with crests and troughsS waves shake the ground up and down and side to side as they move through itS waves CANNOT travel through liquidSince part of the Earths core is made of liquid S waves do not travel directly through the Earth like P wavesTherefore, S waves CANNOT be detected on the side of the Earth opposite of the Earthquake

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Secondary wavesS waves (secondary) Typical crustal velocity: 3 km/s ( ~6,750 mph)Cause shearing and stretching of the earth material through which they pass. Generally cause the most severe shaking; very damaging to structures.Travel through solids onlyshear waves Shake the ground perpendicular to wave directionArrive second on a seismogram.*

Secondary Waves*Source: google images

Surface WavesWhen P waves and S waves reach the Earths surface they can create surface wavesA surface wave is a combination of longitudinal and transverse waves that travels along the surface of a mediumSurface waves move more slowly than P and S wavesSurface waves cause more severe ground movements than P and S waves because they combine up and down and side to side motions that can make the ground roll like ocean waves*

Surface WavesTravel just below or along the grounds surfaceSlower than body waves; rolling and side-to-side movementEspecially damaging to buildings*Source: USGS public domain

Two most common types of surface waves*Source: google images

Rayleigh WavesTypical velocity: ~ 0.9 that of the S waveCauses 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. They usually arrive last on a seismogram.

*Source: google images

Love WavesTypical velocity: Depends on earth structure, but less than velocity of S waves. Cause shearing motion (horizontal) similar to S waves. They usually arrive after the S wave and before the Rayleigh wave.*Source: google images

Wave Motion*

Wave Motion

Love waves are pure S-type surface waves. They vibrate the ground from side-to-side with no vertical movement.Rayleigh waves are the P and S mixture. They vibrate the ground in an elliptical pattern.

*Source: USGS public domain

Movement of earth*Source: USGS public domain

Wave ParametersVelocity (v) is the speed at which the wave travelsPeriod (T) is the time for a point to go through one cycle of disturbance.

Frequency ( f) is one over the period. It has units of one over time (s-1), also called Hz.Wavelength () is the distance that the wave travels in one period. = v T = v/f has units of length (m).Amplitude, A, is a measure of the height of the wave (peak to peak). TimePeriodAmplitude*

P waves: travel at 4-7 km/sS waves: travel at 2-5 km/sBlue primary waves followed by red secondary waves move outward in concentric circles from the epicenter of an earthquakeEach wave travels with a characteristic speed, and vibrates the ground in a specific manner.Wave Parameters*

Wave ParametersP-waveS-wave*Source: USGS public domain

Detecting Seismic wavesTo detect and measure earthquake waves scientists use seismographsA seismograph records the ground movements caused by seismic waves as they move through the earthThe frame of the seismograph is attached to the ground, so the frame shakes when seismic waves arriveSeismographs use to have pens attached to the frame that would wiggle back and forth on a roll of paper as the ground shook, but now electronic seismographs are usedP waves arrive at the seismograph first followed by slower moving S wavesScientists can tell how far away the earthquake was by measuring the time of arrival between P and S wavesScientists can tell where an earthquake occurred by using three seismographs at three different locations

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Kramer (1996) Geotechnical Earthquake Engineering, Prentice Hall. http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/waves_and_interior.html (Accessed on 27 March 2012)Stein, S. and Wysession, M. (2003). An introduction to seismology, earthquakes, and earth structure, Blackwell Science.Graff, K.F. (1991) Wave motion in elastic solids, Dover Publicationshttp://web.ics.purdue.edu/~braile/edumod/waves/WaveDemo.htm (Accessed on 27 March 2012)

References*