Rotational motions induced by the M8.1 Tokachi-oki earthquake, September 25, 2003

We report the first consistent observations of rotational motions around a vertical axis induced by distant large earthquakes. It is standard in seismology to observe three components (up-down, N-S, E-W) of earthquake-induced translational ground motions using inertial seismometers. However, only recently ring laser technology has provided the required sensitivity for observations of the theoretically predicted rotational part of ground motion generated by seismic waves in a wide distance range and frequency band. Here we show that the rotations observed are consistent in waveform and amplitude with collocated recordings of transverse accelerations recorded by a standard seismometer. This suggests that rotations may become a new observable for seismology and related fields with the potential of providing complementary information on earthquake source processes, structural properties, and ground shaking.