It is widely accepted that the effects of rotational motions on seismic waves can be neglected when ground motion is small and might be underestimated in near-fault and/or with extremely large ground motions. However, quantitative validation of these assumptions is still needed. During the 2012 Wutai earthquake in Taiwan (6.4 ML), a set of small rotation rate-strong motion velocity data was recorded at the HWLB (Hualien) seismographic station. This station is 161 km away from the epicenter and the recorded peak ground acceleration and rotation rate are about 7 cm s-1 s-1 and 0.1 mrad s-1, respectively. This data set is useful for evaluating the lower bound of ground motion that can be neglected regarding the effects of rotational motions.
In this study, we apply the algorithm developed by Chiu et al. (2012) and derive the time histories of the rotation angle, the centrifugal acceleration and the gravity effect due to the rotational motions. The results show that all three-component rotation angles are less than 3 × 10-4 degrees. The maximum centrifugal acceleration and effect of gravity are about 5 × 10-5 and 5 × 10-3 cm s-1 s-1, respectively. Although the amount of induced ground motions due to the rotational motions is small, the effect on the waveforms is significant; the maximum waveform difference before and after rotation-motion correction is about 10% of the Peak Ground Acceleration. We also found that the effects of rotational motions are always present and are proportional to the ground acceleration except that centrifugal acceleration and the vertical component of gravity effect decrease faster compared to the decrease in ground accelerations.