Horizontal rotation signals detected by “G-Pisa” ring laser for the M w = 9.0, March 2011, Japan earthquake
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We report the observation of the ground rotation induced by the M w = 9.0, 11th of March 2011, Japan earthquake. The rotation measurements have been conducted with a ring laser gyroscope operating in a vertical plane, thus detecting rotations around the horizontal axis. Comparison of ground rotations with vertical accelerations from a co-located force balance accelerometer shows excellent ring laser coupling at periods longer than 100 s. Under the plane wave assumption, we derive a theoretical relationship between horizontal rotation and vertical acceleration for Rayleigh waves. Due to the oblique mounting of the gyroscope with respect to the wave direction of arrival, apparent velocities derived from the acceleration/rotation rate ratio are expected to be always larger than or equal to the true wave propagation velocity. This hypothesis is confirmed through comparison with fundamental mode, Rayleigh-wave phase velocities predicted for a standard Earth model.
KeywordsLaser gyroscopes Rotational seismology Seismic instrumentation
We acknowledge F. Bosi, A. Gebauer, R. Hurst, and U. Schreiber for the useful discussions. A. Velikoseltsev acknowledges the partial support from the Federal Targeted Program Scientific and scientific pedagogical personnel of the innovative Russia in 2009–2013 of the Ministry of Education and Science of the Russian Federation.
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