Observations of Earth's toroidal free oscillations with a rotation sensor: The 2011 magnitude 9.0 Tohoku-Oki earthquake

We report for the first time observations of Earth's toroidal free oscillations recorded on a ring laser system that is sensitive to rotational ground motions around a vertical axis. Because of the high noise level on the horizontal translational components in classical seismometers, long‐period toroidal modes are amongst the most challenging observations in seismology. In addition, pure uncontaminated observations of long‐period motions are difficult as translational sensors are sensitive to rotational motions also. We show that the toroidal modes associated with rotational motions are complementary to those associated with translations and confirm the relatively spatially compact character of the Mw 9.0 Tohoku‐Oki earthquake. The observations presented here complement the translational, strain, and gravitational records. We expect these observations to provide additional constraints on long‐wavelength deep Earth structure and earthquake sources. Key Points Ring lasers are sensitive enough to observe free oscillations Pure toroidal motions can be observed with ring laser systems The observations are sensitive to source and structure