Strong‐Motion Directionality and Evidence of Rupture Directivity Effects during the Chi‐Chi Mw 7.6 Earthquake

AbstractThis article investigates the spatial distribution, predominant direction, and variations in the intensity measures (IMs) with orientation for classified pulse‐like and nonpulse motions during Chi‐Chi Mw 7.6 earthquake. The results show evidence of high polarization for long‐period spectral accelerations at relatively large source‐to‐site distances (50–100 km) north of the Chelungpu fault. The polarization of long‐period motions shows a clear correlation with the directivity parameters’ isochrone directivity predictor and ξ, indicating a connection between directionality and rupture directivity. The variation in strong‐motion directionality with the period is also studied. The discrepancy in directionality caused by strong directivity increases with the period from 1 to 10 s, which confirms a clear correlation of period‐dependent directionality with directivity effects. This study finds stronger directionality of pulse‐like motions than nonpulse motions for long periods over 3 s with higher maximum‐to‐median and maximum‐to‐minimum IM ratios. For periods over 3 s, the maximum‐to‐median ratios of pulse‐like motions are higher than the mean prediction of the Shahi and Baker (2014a) model, whereas those of nonpulse motions are lower than the prediction. However, this study does not find simple and clear results for the directions of the maximum component at different periods for pulse‐like and nonpulse motions. Despite clear differences between the unidirectional fling‐step and bidirectional forward directivity pulses, the effects of fling‐step and forward directivity are actually coupled in the waveforms.