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| Titel |
First observation of rotational motions from Earth's free oscillations |
| VerfasserIn |
Dieter Kurrle, Heiner Igel, Joachim Wassermann, Ana Ferreira, Ulrich Schreiber |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250034324
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| Zusammenfassung |
| At most seismological observatories, earthquake induced ground motions are measured using
broadband seismometers. These seismometers record three components of translational
motions, usually in the vertical, North-South and East-West direction. However, for a
comprehensive understanding of seismic waves, it would be of advantage to measure in
addition six components of strain and three components of rotation. While at several places
strainmeters have been operating for decades, the techniques for measuring rotations are still
under development.
One of the areas of research where measurements of rotations could be helpful is the study of
the Earth’s free oscillations. Since the Earth’s free oscillations allow putting constraints on
the large scale structure of the Earth and provide the only data that can give direct
information about 3D density variations, any improvement or complement to existing normal
mode observations is desirable. However, the measurement of low-frequency free
oscillations requires sensors with a high sensitivity, and until recently, it was not
possible to make direct observations of free oscillations with the rotation sensors
available.
At present, the most sensitive instrument for the measurement of seismic rotations in a broad
frequency range is the G ring laser system at the geodetic observatory at Wettzell in Southeast
Germany. This instrument acts as a single component rotation sensor and measures the
rotation rate around a vertical axis. Due to this special orientation, it is only sensitive to
horizontally polarized shear waves or, in terms of free oscillations of the Earth, toroidal
modes. Thus, the Wettzell ring laser can serve as a sensor for toroidal free oscillations of the
Earth.
Here we present the first observations of free oscillations with this instrument, made after the
MW = 8.1 earthquake near the Samoa Islands on September 29, 2009. We show that several
toroidal modes can be detected in the ring laser data and that our observations are
in reasonable agreement with synthetic seismograms computed by normal mode
summation.
The detection of normal modes in a ring laser recording shows that this is a promising
technology for low-frequency seismology in the future. Given this new observation, it seems
even possible to use vertically mounted ring lasers for the first measurement of true tilt
without any contaminations from translational motions. |
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