Horizontal rotation signals detected by “G-Pisa” ring laser for the Mw = 9.0, March 2011 Japan earthquake

Abstract

We report the observation of the ground rotation induced by theMw=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 100s. 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 con_rmed through comparison with fundamental{mode, Rayleigh wave phase velocities predicted for a standard Earth model.