Opposite sense ground rotations of a pair of Cavendish balances in earthquakes

Among the intermingled translational and rotational effects occurring in earthquakes, the translational effects are rather well understood. Recent experiments have been performed to investigate the rotational effects which have been observed for centuries that remain intriguing and less well understood. Although rotational seismology is of interest in a wide range of disciplines, rotational ground motions remain challenging to detect directly, especially their sense of rotation. To avoid a possible random response of a single balance, we locate two Cavendish balances in an ultra-low-noise laboratory. For the two successive 2012 Italian earthquakes in Emilia detected in exactly the same direction, opposed counterclockwise and clockwise responses of the two balances are recorded at the same site. Despite the complex combinations of Rayleigh and Love surface waves in the far-field, the two circular fundamental eigenmodes of the gravity-free torsion balances permit the determination of the ground rotation senses, which are indirectly confirmed by the phase shifted acceleration components in the balance frequency bandwidth, as well as the corresponding opposite driving angular impulses. The versatility of the Cavendish balances suggests that they could be used as inexpensive rotational sensor arrays in seismic areas to follow the propagation of ground rotations from the epicentres