Quantitative evaluation of velocity pulse-like ground motions using spatial rotation technique

Strong pulse-like ground motions excited by a causative fault with a rupture propagation close to the shear wave velocity can induce significant earthquake hazards. The single original and horizontal rotation components of pulse-like ground motion were mainly considered in the last years, especially the generation mechanism of velocity pulse and its influence on engineering structures. Conversely, less attention is paid to the vertical component in such seismic events, so that the identification of pulses in arbitrary direction of space from multi-component motion is neglected. Furthermore, although extensive seismic record data have been obtained with the improvement of observation equipment and analysis technology, there are still few strong motion records carrying velocity pulse waveform. In order to obtain more pulse records and expand the range of pulse identification within limited strong motion data, we describe a spatial rotation technique to determine the velocity pulse in arbitrary direction from the three orthogonal components of ground motion. In this paper, the strong ground motions of 46 seismic events are processed, and the strongest velocity pulse is identified and extracted based on continuous wavelet transform. The extracted time history of the long-period velocity pulse is well matched with the rotated seismic record. To better represent the seismic hazard, we quantify the spatial pulse and spectral parameters that characterize pulse-like ground motion. The results indicate that velocity pulse-like motion exhibits marked systematic distribution characteristics, the spatially rotated component of ground motion is significantly larger than the strongest original and horizontal records, and the spatial orientation of velocity pulse is affected by various geological factors. This study supplements the long-period velocity pulse data and increases the pulse peak threshold range. The acceleration amplification factor is 1.2 times the seismic code value, especially the higher magnification values of velocity and acceleration occur in stiff soil and soft rock sites. The peak ratio of seismic ground motion increases with increasing hypocenter distance, which reflects that the attenuation of ground motion velocity is slower than that of motion acceleration. Thus, by combining the moment magnitude, source-site geometry, and site conditions, we provide a quantitative framework to better assess and simulate pulse-like ground motion in seismogenic regions.