Ground rotational motions of the 1999 Chi-Chi, Taiwan earthquake as inferred from dense array observations

[ 1] Large rotational motions excited by the 1999 Chi-Chi, Taiwan earthquake are inferred from a dense acceleration array near the northern end of the rupture fault where large surface slips along the fault are observed. The estimated major frequency content is from 0.1 to 1.0 Hz with a peak near 0.2 Hz. It is found that in the footwall region the observed strong rotational ground motions were most probably excited by the slip of the northern rupture ending segment of the Chi-Chi earthquake fault and that its thrust slips induced a major rock motion in the radial component. The analysis here indicates that during the Chi-Chi earthquake significant rotation seismic energy was radiated from the bent northern end of the earthquake fault. Further analysis of rotational ground motions may provide useful constraints for its rupture mechanisms.