Sunday, June 22, 2008

Sunday seismometer #3

Wiechert horizontal seismograph (1904)

The Wiechert horizontal seismograph (built in Göttingen, Germany, in 1904) has an unusual and striking design: it is essentially an inverse pendulum weighing 1 ton, in unstable equilibrium about a universal pivot at its base. Its natural period is 8 seconds.

The horizontal motion of the mass with respect to the casing is decomposed into its two perpendicular components, North-South and East-West, as we are used to seeing in modern-day instruments. The particularity of the Wiechert is its use of the 2D motion of a single mass to measure the two horizontal components of ground motion.

The motion of the mass is damped by air pistons (see schematic drawing). Damping is used in all modern seismometers to permit recording and interpretation of seismic energy after the first arrival. In undamped instruments, the later arrivals are drowned out by the oscillations caused by the first arrival.

The recording system is mechanical: two fine points scratch out the seismograms for each component on a roll of smoke blackened paper that rotates and translates in order for a full day of recording to be contained on a single sheet. This system is not dissimilar to the drum recordings used by the World-wide Standard Seismograph Network (WSSN) in the 1970s and 1980s.

The Wiechert seismograms contain minute marks made by lifting the two recording styluses in response to an electrical impulse that could be given by a contact switch on a precision pendulum, or by any other time-keeping device.

The Wiechert horizontal seismograph was kept working in Strasbourg Seismic Observatry from 1904 to 1968. It is now visible in the Seismology Museum, which is housed in the original observatory building.

Should you wish to see a working Wiechert seismograph, you should visit the Wiechert Earthquake Station in Göttingen.

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