Seismometer


  Seismometer.pdf (Size: 993.94 KB / Downloads: 8)

Seismometers are instruments that measure motions of the ground, including those of seismic waves generated by
earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the
interior of the Earth, and locate and measure the size of these different sources.

Zhang Heng's seismoscope

Replica of Zhang Heng's seismoscope Houfeng
Didong Yi
In AD 132, Zhang Heng of China's Han dynasty invented the first
seismoscope (by the definition above), which was called Houfeng
Didong Yi (literally, "instrument for measuring the seasonal winds and
the movements of the Earth"). The description we have, from the
History of the Later Han Dynasty, says that it was a large bronze
vessel, about 2 meters in diameter; at eight points around the top were
dragon's heads holding bronze balls. When there was an earthquake,
one of the mouths would open and drop its ball into a bronze toad at
the base, making a sound and supposedly showing the direction of the
earthquake. On at least one occasion, probably at the time of a large
earthquake in Gansu in AD 143, the seismoscope indicated an
earthquake even though one was not felt.

Early designs

After 1880, most seismometers were descended from those developed by the team of John Milne, James Alfred
Ewing and Thomas Gray, who worked in Japan from 1880 to 1895. These seismometers used damped horizontal
pendulums. After World War II, these were adapted into the widely used Press-Ewing seismometer.
Later, professional suites of instruments for the world-wide standard seismographic network had one set of
instruments tuned to oscillate at fifteen seconds, and the other at ninety seconds, each set measuring in three
directions. Amateurs or observatories with limited means tuned their smaller, less sensitive instruments to ten
seconds. The basic damped horizontal pendulum seismometer swings like the gate of a fence. A heavy weight is
mounted on the point of a long (from 10 cm to several meters) triangle, hinged at its vertical edge. As the ground
moves, the weight stays unmoving, swinging the "gate" on the hinge.

Modern instruments

Modern instruments use electronic sensors, amplifiers, and recording devices. Most are broadband covering a wide
range of frequencies. Some seismometers can measure motions with frequencies from 500 Hz to 0.00118 Hz (1/500
= 0.002 seconds per cycle, to 1/0.00118 = 850 seconds per cycle). The mechanical suspension for horizontal
instruments remains the garden-gate described above. Vertical instruments use some kind of constant-force
suspension, such as the LaCoste suspension. The LaCoste suspension uses a zero-length spring to provide a long
period (high sensitivity).[5] [6] Some modern instruments use a "triaxial" design, in which three identical motion
sensors are set at the same angle to the vertical but 120 degrees apart on the horizontal. Vertical and horizontal
motions can be computed from the outputs of the three sensors.