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An effort is made to examine the properties of rotational (torsional and rocking) ground motions using Chiba dense array data. The Chiba array system, located 30 km east of Tokyo, Japan, is composed of 15 boreholes with separation distances varying from 5 to 320 m. This provides a unique opportunity to examine the characteristics of rotational components. For this purpose, 17 events are considered and rotational ground motions are evaluated using spatial derivatives of translational ones. The effects of seismological parameters and separation distances between stations on properties of rotational motions are examined, showing a sudden increase in rotational motions for the earthquakes with large magnitude or PGA and decrease of these motions with increasing separation distance. While the duration of torsional motion is found to be larger than translational ones, there is no significant difference between durations of rocking and vertical motions. The effects of separation distance and earthquake magnitude on rotational response spectra are also investigated. The normalized rotational response spectra are found to be strongly affected by separation distance. The spectral ratios of rotational and translational motions are not linearly proportional to period as suggested by the previous studies. Finally, the torsional motion is predicted from translation ones for different separation distances at the site. The comparison of the predicted and the calculated torsional motions reveals a weak estimation in close separation distances (<30 m) and satisfactory predictions in other cases.structures [1], and second because sensitive measuring devices were not available until quite recently. The benefits of the determination of rotational motion in seismology and engineering are still under investigation (e.g. [2,3]). In seismology, rotational motions can provide accurate data for arrival times of SH waves and, in the near-source distance range, rotational motions might provide more detailed information on the rupture processes of earthquakes [3]. Rotational motions could also be used to better estimate the static displacement from seismic recordings, identifying translational signals caused by rotation [2].In engineering, dynamic response estimation of structures subjected to earthquake-induced base excitations is often simplified by ignoring the rotational components. This has been a widely accepted practice in engineering community, mainly caused by the lack of recorded strong motion accelerograms for these motions. Many structural failures and the damage caused by earthquakes can be linked to differential and rotational ground motions. Torsional responses of tall buildings in Los Angeles, during the San Fernando earthquake in 1971, could be ascribed to torsional excitation, while rotational and longitudinal differential motions may have caused the collapse of bridges during San Fernando (1971), Miyagi-ken-Oki (1978) [4] and Northridge (1994) [5] earthquakes. For the first time, Newmark [6] established a simple relationsh...

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