Seismometer

A seismograph or - graph (Greek σεισμος SEISMOS, shake ' and γράφω grapho to write ') or seismometer (Greek: μετρέω Metreo measure ') is a device that can record ground motion from earthquakes and other seismic waves. It consists in principle of a spring mounted on a suspension composition. While the ground motion to the housing of the instrument, the mass is at rest, due to their inertia. The relative movement of the floor can thus be measured as the change in length over time.

Operation

The Seismoskope predominantly used until around 1900 were only shake indicator, although anzeigten the occurrence of an earthquake, and sometimes his strength and direction of impact, but not the time course of the ground motion. This, however, no significant findings can be summarized.

The first working seismographs ( from 1875) worked purely mechanically. The movement of the earth relative to the inertial seismograph mass was increased with levers as curves on a continuously revolving, berußtes paper tape recorded (hence seismograph of - grapho to write '). Electrodynamic seismometers (since 1904) to measure the relative movement via an induction voltage by means connected to said seismic mass coil moving in a magnetic field. The electrical output signal was originally recorded on a mirror galvanometer photographic, later it could be electronically amplified. Modern Breitbandseismometer ( since 1976, now commonly used for scientific purposes ), however, are derived from the output signal from the force that is needed to carry the portable seismograph mass with the housing. In this way large mechanical excursions and associated problems are avoided. The electrical output signal is digitized and stored locally in order not to lose it and interference. Knowledgeable users can retrieve via the internet archived in data centers signals of many seismograph stations worldwide.

Breitbandseismometer cover a wide frequency range ( about 15 octaves compared to the 10 octaves of human hearing ). The highest frequencies are in the lower audible range at about 50 Hz and the lowest at about one cycle per hour. The tides of the solid Earth with periods of 12 and 24 hours are still covered, although not as accurate as with a gravimeter. The smallest detectable periodic acceleration of the ground is several trillion fifteen digits (10-12 ) of the normal gravity. At the same time, these instruments are able to record regional earthquakes up to magnitude 7 undistorted.

Evaluation

A network of seismographs used to determine the precise location and the time of earthquake. For precise localization, the time differences are evaluated ( Peilungsprinzip ). As seismic waves propagate inside the Earth at speeds of several kilometers per second, earthquakes are registered across continents after a short time and localized. Apart from natural earthquakes can also be registered:

• Induced quake, tremor, for example, generated by dams
• Micro-earthquakes, microseismic unrest
• Blasting (eg in quarries )
• Atomic explosions
• Traffic vibrations

Be recorded depending on the design of the instrument, the

• Acceleration
• Speed
• Deflection ( ground motion )

From the records, one can derive:

• Magnitude of the earthquake
• The place of origin ( epicenter and hypocenter )
• The frequency spectrum
• Temporal, spatial and energy distribution

A seismogram is the graphic recording of a seismograph. The ground motions can be calculated from the seismogram. Further evaluation can be, inter alia, the natural frequency of vibration of structures determined, or, in a different frequency domain, the induced by strong earthquakes natural modes of vibration of the earth.

History

The oldest seismoscope the world comes in all probability from the Empire of China. It was around the year 132 AD by Zhang Heng, imperial astronomer of the Han Dynasty, invented - about 1600 years before the first seismographs were developed in the Western world. It was said device is a motion sensitive vessel made ​​of bronze with eight outward dragon heads that reported in eight different directions. Each of the Dragons held a copper ball in its mouth, which were spewed out at earth moving in the direction of the earthquake. To the base of the vessel around crouched under each dragon a frog made ​​of porcelain with a wide open mouth. Already by lightest seismic vibrations in a hidden vessel pendulum was oscillated, which solved the ball out of its holder. From the mouth of the dragon, the ball fell into the frog and produces a characteristic sound, it could be concluded from the direction of the earthquake.

Circa 1856 Luigi Palmieri developed electromagnetic seismograph, in which a shock closed electrical contact and thus further actions such as holding the time, or launch other devices enabled. Boris Borisovich Golitsyn developed in 1904 ( Prince Galitzine ) in Saint Petersburg an electrodynamic seismograph that after some technical improvements to the model for modern appliances and mechanical equipment was replaced.

The first seismogram of a distant earthquake was registered in 1889 by accident in Potsdam. The astronomer Ernst von Rebeur - Paschwitz drew the surface waves of an earthquake in Japan with an apparatus which was actually intended for the measurement of deflections of the vertical by astrophysical influences. From Rebeur - Paschwitz therefore today is considered one of the fathers of seismology.

Emil Wiechert seismograph air cushioned with high magnification, which allowed a continuous record of global earthquake activity for the first time, was the model for most of the instruments used in the seismic stations around the world for decades.