Home | Session 2 |
Earthquakes Page 1, 2
by Lind Gee
Berkeley Seismological Laborartory
- 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
- Compression and shear waves are the only wave types in a homogeneous
- 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
- 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?
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
- 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.
- 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.
- 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