Guralp Ocean Bottom Seismometer captures unique ringing on the Earth

September 12

After each big earthquake, the Earth rings like a bell. The sudden, extremely rapid movement of large blocks of rock within the Earth’s crust during a quake hits the rest of our planet like a clapper striking the body of a bell. As a consequence, the whole structure begins to hum. Inside the Earth, these vibrations are called normal modes or free oscillations and the may shake the whole planet for hours if not days. Researchers have found two types of this kind of ringing: The spheroidal mode changes the actual shape of the Earth while the toroidal mode twists the body of the Earth back and forth (see figure 1). Unlike strong earthquake waves, such free oscillations cannot be felt by humans. The reason: The periods of these vibrations are very long. They reach from 100 seconds for the shortest period up to 54 minutes.

The Earth's two normal modes

Figure 1 – The Earth’s two normal modes


Such periods are well below the pass-band of modern broad band seismometers. Nevertheless, if the installation is done well, say in a properly insulated seismic observatory built on an outcrop of hard rock, free oscillations have been recorded around the globe by many Guralp seismometers. However, these free oscillations have been rarely – if at all – registered by seismometers located on the ocean bottom (OBS). Usually, the strong noise generated by the ocean waves masks the weak amplitudes of the bell-like vibrations.

A very unique recording of free oscillations has now been found in the data collected by a very broadband Guralp OBS (CMG-3T, 360 sec – 100 Hz) after the huge magnitude 8.6 earthquake off the coast of Sumatra on April 11th 2012. The OBS is located in 2500 meters water depth at the bottom of the Mediterranean off the coast of Toulon, France. The sensor was installed (see figure 2) in November 2010 and has been operating flawlessly ever since.

CMG-3T OBS instrument deployed in the Mediterranean

Figure 2 – CMG-3T OBS instrument deployed in the Mediterranean


Figure 3 (courtesy of Anne Dechamps from the Laboratoire Geoazur, Nice, France) shows the spectrum of the free oscillations captured by the OBS (blue line, labelled ASEAF) as compared to the seismic land-station SSB (red line). The nomenclature describes the various modes of free oscillations in the period band (bottom axis) between 500 and 2000 seconds (8 to 33 minutes).

Spectrum of free oscillations

Figure 3 – Spectrum of free oscillations

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