Lecture-15 1 Lecture #15- Seismic Wave Overview

Lecture-15 2 Seismograms F Seismograms are records of Earth’s motion as a function of time.

Lecture-15 3 Seismograms F Seismograms record ground motion in terms of Fdisplacement Fvelocity Facceleration F Normally a seismometer samples ground motion about 20 times per second (20 Hz), but this number can be as high as 500 Hz.

Lecture-15 4

Lecture-15 5 Seismograms F Ground motion is a vector (whether it is displacement, velocity or acceleration), so it takes 3 numbers to describe it. Thus, seismometers generally have three components: FVertical (up is positive) FNorth-South (north is positive) FEast-west (east is positive) } horizontals

Lecture-15 6 Izmit, Turkey Earthquake recorded in Great Britain.

Lecture-15 7 Components of Motion There are simple mathematical operations that allow seismologists to rotate (abstractly) the horizontal components: N EW S earthquake seismometer Original Coordinate System

Lecture-15 8 Components of Motion There are simple mathematical operations that allow seismologists to rotate (abstractly) the horizontal components: seismometer earthquake N W E S R T Modified Coordinate System The new components are called: (1) Radial, R (2) Transverse, T

Lecture-15 9 Oaxaca, Mexico earthquake recorded by seismometer in Alaska.

Lecture Seismic Waves F The wiggles on a seismogram are caused by seismic waves which are generated by the movement of the rocks along a fault. F The waves emanate from the “source” or earthquake, and travel: F through the body of the Earth, and Fover the surface of Earth.

Lecture Waves in a pond F The idea is analogous to waves caused by tossing a stone in a pond.

Lecture Sound Wave Analogy F Seismic waves represent acoustic (sound) energy and so are analogous to speech: (1)Vocal cords vibrate (2) Sound waves propagate thru atmosphere (3) Ears record these vibrations (4) Brain processes the recordings Speech Earthquakes (1)A locked fault segment fails (ruptures) (2) Sound waves propagate thru the Earth (3) Seismometers record these vibrations (4) Seismologists process these recordings (seismograms)

Lecture What is a Wave ? F A wave is a disturbance that transfers energy. F Waves are common in nature: –Light is a wave –Sound is a wave F Waves are periodic in both space and time (they have wavelengths and periods)

Lecture Wave Terminology F Wavelength is a measure of the spatial width of a wave. It has units of length (m). F Period is a measure of the duration of a vibration. Period has units of time (s) F Frequency is one over the period. It has units of one over time (s -1 ). F Amplitude is a measure of the height of the wave. It has units of displacement (m).

Lecture Wavelength and Period Amplitude Distance from Source Wavelength At a given instant in time, the displacement is periodic in space(distance). Amplitude Time Period At a given fixed place, the displacement is periodic in time.

Lecture Wave Speeds F The speed that a wave propagates at is not a dynamic quantity – it is a fixed material property. (like density) F No matter how big an earthquake is, the seismic waves it produces will always travel at the same speed. F The seismic wave speed of a material depends mainly its upon: – Temperature – Pressure – Composition

Lecture Elastic Waves F Seismic waves are also called elastic waves, because they deform the Earth elastically - the rock returns to its original shape and position after the seismic wave passes through. F An example of a non-elastic wave is a shock wave. This type of wave fundamentally changes the medium thru which it propagates.

Lecture Sources of Seismic Waves F Earthquakes generate seismic waves, but so do many other processes: –Volcanic eruptions –Explosions –Wind –Sonic Booms (planes, shuttle, meteorites) –Humans

Lecture A Jet and an Earthquake

Lecture Multiple-Frequency Signals F Most interesting signals are composites of waves with many different frequencies. The range of frequency is sometimes called the “band” and we speak of bandwidth. Light is a usually a multiple frequency signal, and the different frequencies correspond to what we call colors.

Lecture Sometimes we can use the observed frequencies to identify different sources of vibrations. Which has higher frequency content, the sonic boom or the earthquake?

Lecture Seismic Wave Types F Seismic waves can be labeled by the paths they take in the Earth. Surface Waves: (1) Love Waves (2) Rayleigh Waves Body Waves: (1)P waves (2)S waves

Lecture Seismic Wave Types Surface Waves FLarge amplitude FLong wavelength FWide range of frequencies (large bandwidth) FTravel slowly FNot produced by deep earthquakes Body Waves FSmall amplitude FShort wavelength FNarrow frequency band FTravel more quickly FProduced by all earthquakes

Lecture Seismic Wave Types F A second way we distinguish between waves is by the type of deformation (strain) they induce: –Compressional waves cause changes in volume: – Rayleigh wave (compressional surface wave) – P wave (compressional body wave) –Shear waves cause changes in shape: – Love wave (shear surface wave) – S wave (shear body wave)

Lecture Compressional Body Waves ( P-waves ) F “P” stands for primary, because they travel the fastest and are the first waves to arrive. –They also travel through all types of materials including solids, liquids, gasses. –Within Earth, P-waves travel at speeds between 1 and 14 km/s (kilometers per second). The precise velocity depends on the rock type.

Lecture Compressional Wave Vibrations The motion produced by a P-wave is an alternating compression and expansion of the material. The ground is deformed along the direction that the wave is traveling. P-waves are sound waves, but most seismic P-waves are at too low a frequency for humans to hear.

Lecture-15 27

Lecture Shear Body Waves (S-Waves) F “S” stands for secondary, and these waves travel second fastest. S-waves are often called shear waves. –S-waves also travel through solids but not through liquids or gasses. –Within Earth, S-waves travel at speeds between 1 and 8 km/s (kilometers per second). The precise velocity depends on the rock type.

Lecture Shear-Wave Vibrations S-waves vibrate the ground in a shearing motion, with movement perpendicular to the direction that the wave is traveling. They are often the largest waves close to an earthquake, and they usually do the most damage.

Lecture-15 30

Lecture Shear Surface Waves (Love Waves) F Loves waves are the faster of the two surface waves. –They move at speeds on the order of 3 to 6 km/s. –They vibrate the ground from side-to-side with no vertical movement.

Lecture-15 32

Lecture Compressional Surface Waves (Rayleigh Waves) F Rayleigh waves are the most complex wave, and they are also the slowest. F They travel at speeds of 2 to 5 km/sec and vibrate the ground in an elliptical pattern.

Lecture-15 34

Lecture Summary F Seismic waves are traveling vibrations that transport energy from the earthquake “source” region throughout the Earth. F We distinguish between 4 types of waves, the body waves P and S, and the surface waves, Love and Rayleigh. F Each wave travels with a characteristic speed, and vibrates the ground in a specific manner.