Numerical modelling of the near-field velocity pulse-like ground motions of the Northridge earthquake

The seismic records acquired during the 1994 MW6.7 Northridge earthquake provide important data for studying the pulse-like ground motions in the vicinity of reverse faults. We selected 106 horizontal records from 468 strong ground motion records in the near-field region and rotated the original records into fault-parallel and fault-normal orientations. Large velocity pulses were simulated by the 3D finite difference method using a kinematic source model and a velocity structure model. Regression analysis was performed on the simulated and observed amplitudes of the velocity time history and response spectrum using the least-squares method. Our results show that the released energy and rupture time of asperities in the source model have important effects on the near-field velocity pulses, and the asperity near the initial rupture contributes more to the velocity pulses than does the asperity near the central region. The unidirectional and bidirectional characteristics of large velocity pulses are related to the thrust slip and rupture direction of the fault. The pulse period and the characteristic period are positively correlated with the rise time, and the pulse peak is regulated by multiple parameters of the subfaults. The distributions of the simulated PGV and Arias intensity agree well with the observed records, in which the contours exhibit asymmetric distribution and irregular elliptical attenuation in the near-field region, and the distributions exhibit a significant directivity along the fault. Moreover, the attenuation rate decreases with increasing distance from the fault. In addition, the fault-normal component is larger than that on the fault-parallel component, and the former decays faster. Velocity pulses larger than 30 cm/s are most likely to be distributed within approximately 15 km from the fault plane of the Northridge earthquake. Thus, the revealed pattern of the near-field velocity pulse-like ground motions indicates their close relation with the most severe earthquake effects.