Deep Underground Observation of Rotational Motions Induced by M6.1 Hualien Earthquake

In recent years, rotational seismology has developed rapidly due to the improvement of the rotational sensors and the progress of seismic observations. Compared to the conventional translational motions, the extra information of wave field brought by rotational motions is helpful in a broad range of applications, such as phase velocity estimation, seismic exploration, structural health monitoring, seismic source tracking, and so on. In this research, we first installed the fiber-optic rotational seismometer in the deep underground tunnel of 870 m depth, where the ultralow ambient noise can significantly improve the quality of seismic records. The underground rotational seismometer recorded the 2021 Hualien M6.1 earthquake with an epicentral distance of over 1000 km. By using the rotational records together with the 3 C broadband records, we estimated the back azimuth (BAz) of the epicenter and the phase velocity of the Love wave. The estimated BAz is very close to the actual location released by the earthquake network, and the phase velocity of the Love wave is about 1.0–3.5 km/s in the frequency band of 0.3–0.7 Hz.