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Earthquakes

Presented by Lind Gee
Berkeley Seismological Laborartory

Wave types

• P - compressional or longitudinal - "primary" e.g., sound is a compressional wave in air which explains why people "hear" earthquakes and why the space shuttle creates seismic waves! involves a volume change
• S - shear or transverse wave - "secundo" or "secondary" e.g., involve a shape change do not propagate in liquids or in gases

The existence of P and S waves was predicted long before they were observed!

• Compression and shear waves are the only wave types in a homogeneous solid
• Surface waves - initially called "long" or "L-waves" or "principal section" arise from the interaction of P and S waves with the free-surface. The motion of surface waves decays exponentially with depth.
• Rayleigh waves - have elliptical particle motion in the horizontal and vertical plane. . Rayleigh waves extend to depths of 150-200 km.
• Love waves - have only horizontal motions require a layered medium similar motion to shear waves. For Love waves, motions are primarily limited to the upper 50-60 km

What else generates waves in the Earth?

Volcanoes
Explosions
Wind, waves, falling trees, ....
Humans (cars, axes, ....)

Seismometers, seismologists. .....
A few words abot Etymology

• Seismic - of or having to do with earthquakes
• Seismicity - earthquake activity
• Seismologist - a scientist who studies earthquakes
• Seismometer - an instrument generating a record of earth motion
• Seismoscope - an instrument for detecting an earthquake, but which does not produce a true record of motion
• Seismogram - a record of an earthquake
• Seismograph - a seismometer + timing system + recording device
• Seismometry - the instrumental aspects of seismology

Seismology is a young science

• Japanese folklore - other examples myths in India (turtle), Greece (Zeus' thunderbolts)
• Lisbon earthquake in 1755 - John Michell associated earthquakes and seismic waves recognized that waves spread out away from earthquakes and that timing of these waves could be used to locate earthquakes.

Seismoscopes

• An instrument which responds to earth motion, but makes no recording
• First seismoscope was developed in China in 132 AD by Chang Heng reported to both detect and indicate the azimuth of an event although modern seismologists have not be able to reproduce the behavior
• Other examples in Europe in the early 18th century a bowl of mercury - measuring the direction and amount
• Pendulums were used extensively in Italy and elsewhere as part of mechanical systems to detect earthquakes. Fairly complex systems were developed to try and determine time and direction of motion.

Seismograph

• 1875 - Ceechi - first machine built to measure the relative motion of the Earth and a pendulum as a function of time
• 1880s - Milne, Ewing and Grey working in Japan - first network Mechanical systems ---> electromagnetic systems for amplification Importance of time Drum recording systems
• Modern seismographic stations

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updated January 28, 2002

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