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| Titel |
Six-degree-of-freedom local seismic records from micro-earthquakes and anthropogenic events |
| VerfasserIn |
J. Brokešová, J. Málek |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250062045
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| Zusammenfassung |
| Full ground motion description, including both translational and rotational components,
attracts still more attention in modern seismology. There is a need for portable rotational
sensors, easily installed and operated in the field, sensitive enough to provide 6DOF records
in a wide range of amplitudes excited not only by strong but also weak motion events. A new
prototype of our rotational seismic sensor system (Rotaphone) designed to measure three
rotation rate and three ground velocity components is presented. Our method is based on
measurements of the differential motions between paired low-frequency geophones attached
to a rigid skeleton, the size of which is much smaller than the wavelength of interest. This
approach is conditioned by a precise calibration of each paired geophone in the
system. A method of our in-situ calibration is explained and demonstrated. The
device was thoroughly tested at the Albuquerque Seismological Laboratory, U.S.
Geological Survey, with focus on linearity and cross-axis errors. Results of these
tests confirm its proper function. The Rotaphone operates in the frequency range
2-40 Hz and its dynamic range is at least 120 dB. It was successfully applied to
measure 6DOF ground motions from local shallow micro-earthquakes in the West
Bohemia swarm area (Czech Republic), at Hronov-PoÅíÄí fault zone in East Bohemia
(Czech Republic), and in the vicinity of the salt-works of Provadia (Bulgaria). It
was also used to obtain records from anthropogenic sources (blasts, rockbursts).
Three-axial rotation rate records for these types of events are shown and discussed. Their
peak amplitudes reach values from 10-7 rad/s to 10-4 rad/s. To characterize the
significance of rotation we define the rotation to translation ratio (RTR) relating peak
amplitudes of rotation rate and translational velocity. The RTR factor for different types
of local events is discussed in view of its dependence on hypocentral distance,
source type and probably on radiation pattern and geological structure along the
wavepaths. Thanks to our method of retrieving rotation rates, their records are not
contaminated by translational motions. Opposite is not true. Special attention is
devoted to the problem of contamination of translational records by rotations. Our
approach enables to compensate for this contamination. This procedure is especially
important for horizontal translational components that contain a non-negligible
contribution from gravity in the case of tilting of the instrument. Possible application
of Rotaphones in local seismic networks and seismic prospecting is discussed. |
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