OBSERVATIONS OF TOROIDAL FREE OSCILLATIONS WITH A RING LASER: THE M9 TOHOKU-OKI EVENT

Rotational seismology is a recent and promising field in active development that focuses on the study of all aspects concerning rotational ground motions induced by earthquakes, explosions, and ambient vibrations. The understanding of rotational ground motions is relevant to several disciplines: seismology (strong-motion studies, seismic instrumentation), earthquake engineering, exploration geophysics, and even the decoupling of sensitive structures from ground motions (e.g., gravitational wave detection). In this study we explore the potential ofmeasuring Earth’s free oscillations using ring laser rotational measurements. Many different instruments have been developed to measure the rotational contribution in seismic events. So far the most successful results have been achieved using Ring Laser technology based on the Sagnac effect. In 2009, a marked improvement of the signal-to-noise ratio for the broad-band frequency range of seismic observations could be achieved after some technical improvements to the ring laser system, located at the Geodetic Observatory Wettzell (Germany), measuring the vertical component of rotation rate. This led to the first direct observation of rotational ground motions induced by toroidal free oscillations of the Earth. Here we show the observations following the M9 Tohoku-Oki event off the coast of Honshu of March 2011.We compare the observations with synthetic data and demonstrate the sensitivity of the observations to source parameters.