How It Works: Science and Technology - Vol. 15

By Wendy Horobin | Go to book overview

Seismology

Puu Oo crater erupting
in Hawaii’s Volcanoes
National Park. Volcanoes
and earthquakes both
tend to occur in the same
regions, commonly where
tectonic plates meet or
diverge or, as in the case
of the Hawaiian Islands,
within a plate at an
intraplate hot spot.

Seismology is the study of earthquakes. Because humans first lived in earthquake-prone areas such as Ethiopia, Mesopotamia, and Indonesia, they have always known and feared earthquakes. Over the past 100 years, however, scientists have learned to use earthquakes as a powerful tool for investigating Earth’s interior.

In 132 B.C.E., a device for registering seismic activity was made by Chang Hâng in China. It had a number of metal balls arranged around the rim of an urn so that an earth tremor would disturb a central column and thus operated a mechanism that dropped one of the balls into a metal holder causing a noise. The direction of the earthquake was worked out by observing which balls fell and which did not. From this simple device, today’s sensitive seismometers have developed into complex instruments, and a science has grown up with them. There is now an industry involved in using artificially generated seismic shocks to search Earth’s crust for minerals. The data collected from these shocks have so far provided a wealth of knowledge on conditions near Earth’s surface. A technique called seismic tomography, developed in the 1980s, uses seismic waves to construct three-dimensional images of Earth’s interior. A similar technique is employed on the Solar and Heliospheric Observatory (SOHO) space probe, which collects data on solar waves that pass through the Sun and uses the information to create internal images. This technique has resulted in a new science called helioseismology.


Seismographs

In the past 100 years, there have been many advances in the design of seismographs, which record the movements of Earth’s crust at a given location. There are now seismic recording devices fixed in countries throughout the world making continuous recordings. Portable equipment is available that can be installed for temporary projects, while remote locations may be monitored using telemetric devices that record and transmit information to a central collecting station. Any device that measures seismic activity is a seismometer; those that record the activity, for example, by means of a pen recorder are known as seismographs.

It has not proved possible to design a seismograph to measure faithfully the movement of Earth’s crust in every direction at once in response to seismic waves. What invariably happens is that each seismic observatory has at least two seismographs, one each for the horizontal and vertical parts of the movement. Often there are three—the horizontal component is measured in two directions.

The horizontal-component seismograph consists of a pendulum with a heavy weight. Just as the French physicist Léon Foucault’s pendulum, which is extremely long, always swings in the same direction in space, thus showing the rotation of Earth beneath it, the seismograph pendulum tends to stay in the same place by virtue of its inertia while the ground vibrates beneath it. The difference is that the seismograph pendulum is not set swinging—it is restrained from swinging by means of damping of some kind. The pendulum has a resonant period—the time it would normally take to complete a whole swing—and if the period of the earthquake waves happens to coincide with it, the result will be meaningless.

-2046-

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How It Works: Science and Technology - Vol. 15
Table of contents

Table of contents

  • How It Works® Science and Technology 2017
  • Title Page 2019
  • Contents 2020
  • Salvage, Marine 2021
  • Satellite, Artificial 2024
  • Schlieren Techniques 2030
  • Screw Manufacture 2032
  • Seaplane and Amphibian 2035
  • Sea Rescue 2038
  • Security System 2042
  • Seismology 2046
  • Self-Righting Boat 2050
  • Semiconductor 2052
  • Servomechanism 2055
  • Sewing Machine 2058
  • Sextant 2062
  • Sheet Metal 2064
  • Ship 2067
  • Shutter 2074
  • Silicon 2076
  • Silicone 2078
  • Silver 2079
  • Sine Wave 2081
  • Siphon 2083
  • Ski and Snowboard 2084
  • Skin 2087
  • Skyscraper 2090
  • Slaughterhouse 2096
  • Sleep 2099
  • Smell and Taste 2103
  • Soap Manufacture 2107
  • Soft-Drink Dispenser 2109
  • Soil Research 2110
  • Solar Energy 2114
  • Solar System 2118
  • Solenoid 2124
  • Sonar 2126
  • Sorption 2129
  • Sound 2131
  • Sound Effects and Sampling 2136
  • Sound Mixing 2138
  • Soundproofing 2140
  • Sound Reproduction 2142
  • Soundtrack 2146
  • Space Debris 2150
  • Space Photography 2152
  • Space Probe 2156
  • Index i
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