seismograph

Seismograph recording the activities of the Pinatubo volcano ( Philippines )

Modern broadband seismometer of the type Fahrtisen STS-2

Inner workings of a seismometer

Replica of the seismometer of the Apollo 11 mission in the Science Museum

A seismograph or graph ( Greek σεισμος seismós , 'vibration' and γράφω grapho 'write') or seismometer (Greek: μετρέω metréo 'measure') is a device used in seismology that can register ground vibrations from earthquakes and other seismic waves . In principle, it consists of a mass mounted on a spring suspension. While the ground movement is transferred to the housing of the instrument, the mass remains at rest due to its inertia . The relative movement of the floor can thus be measured as a change in length over time.

functionality

The seismoscopes , which were predominantly used up to around 1900, were merely vibration indicators, indicating the occurrence of a quake and sometimes its strength and direction of impact, but not the temporal course of the ground movement. However, no significant findings could be obtained from this.

The first functional seismographs (from 1875) worked purely mechanically. The movement of the ground relative to the inert seismograph mass was magnified with levers and recorded as a curve on an endlessly revolving, sooty paper tape (hence seismograph, from -grápho ' to write'). Electrodynamic seismometers (since 1904) measure the relative movement via an induction voltage in that a coil connected to the seismic mass moves in a magnetic field. The electrical output signal was originally recorded photographically using a mirror galvanometer ^[document?] , Later it could be amplified electronically. Modern broadband seismometers (since 1976, today generally used for scientific purposes) derive the output signal from the force that is necessary to carry the moving seismograph mass with the housing. This avoids large mechanical deflections and the associated problems. The electrical output signal is digitized and the data is usually immediately forwarded to a data center.

Broadband seismometers cover a wide frequency range (around 15 octaves compared to the 10 octaves of the human hearing range). The highest frequencies are in the lower audible range at around 50 Hz and the lowest at around one oscillation per hour. The tides of the solid earth with periods of 12 and 24 hours are still recorded, although not as precisely as with a gravimeter . The smallest detectable periodic acceleration of the earth is a few trillionths (10 ⁻¹² ) of normal gravity. At the same time, these instruments are able to record regional earthquakes up to magnitude 7 without distortion.

evaluation

A network of seismographs is used to determine the exact location and time of earthquakes. The time differences are evaluated for precise localization ( bearing principle ). Since seismic waves propagate inside the earth at speeds of several kilometers per second, earthquakes across continents are registered and localized after a short time. In addition to natural earthquakes, the following can also be registered:

induced tremors , e.g. B. earthquakes generated by dams
Microquake, microseismic unrest
Blasting (e.g. in quarries)
atomic explosions
Traffic vibrations

Depending on the construction of the instrument, the

acceleration
speed
Deflection (ground movement)

From the records one can deduce:

the magnitude of the earthquake
the place of origin ( epicenter and hypocenter )
the frequency spectrum
the temporal, spatial and energetic distribution

A seismogram is the graphical record of a seismograph. The ground movements can be calculated from the seismogram. Further evaluation can u. a. Determine the natural vibration frequency of buildings or, in another frequency range, the natural vibration frequencies of the earth caused by strong earthquakes.

history

Replica of Zhang Heng's seismoscope

The oldest seismoscope in the world comes in all likelihood from the Chinese Empire . It was invented by Zhang Heng , Imperial Astronomer of the Han Dynasty , around the year 132 AD - about 1,600 years before the first seismographs were developed in the Western world . The device was a movement-sensitive vessel made of bronze with eight outward-pointing dragon heads that pointed in eight different directions. Each of the dragons held a copper ball in its mouth, which was spat out when the earth moved in the direction of the quake. Around the base of the vessel, under each dragon, crouched a porcelain frog with its mouth wide open. Even the slightest seismic tremors caused a pendulum hidden in the vessel to vibrate, which released the ball from its holder. The ball fell from the dragon's mouth into that of the frog, so that the direction of the earthquake could be deduced.

Around 1856 Luigi Palmieri developed electromagnetic seismographs in which an impact closed an electrical contact and thus enabled further actions such as recording the time or starting other devices. In 1904 Boris Borissowitsch Golizyn (Prince Galitzin) developed an electrodynamic seismograph in Saint Petersburg , which after some technical improvements became the model for modern devices and replaced mechanical devices.

The first seismogram of a long-distance quake was registered in Potsdam in 1889 rather by chance. The astronomer Ernst von Rebeur-Paschwitz recorded the surface waves of an earthquake near Japan with an apparatus that was actually intended for measuring vertical deviations due to astrophysical influences. Von Rebeur-Paschwitz is therefore considered one of the fathers of seismology today.

Emil Wiechert's air-cushioned seismograph with high magnification, which for the first time enabled continuous recording of global earthquake activity, remained the model for most of the instruments used in earthquake monitoring stations around the world for decades.