Directivity Effect Of Near-fault Ground Motion And Super-shear Rupture

The nature of rupture directivity caused by a predominately unilateral propagating fault with similar rupture speed to the local shear-wave velocity is a key factor in characterizing the near-fault ground motions. This study focuses on the fault related parameters that contribute to the directivity effect, including the fault rupture speed, rupture direction, fault dip, focal depth, rupture model and the local parameters on the fault plane. Among all the factors mentioned above, the super-shear rupture speed is the one that this study paid special attention to. So is there still exist rupture directivity under super-shear rupture speed of the fault? And in what extent will the super-shear rupture affect the strong ground motions? To answer these questions, three aspects of studies were conducted, including the theoretical analysis, numerical modeling and analysis of the real near-fault recordings and simulated ground motions. Through analysis of peak ground motion, response spectra and significant duration of the fault-normal, fault-parallel and vertical components of ground motions, some conclusions were drawn.1. The directivity effect, Doppler effect and rupture propagation effect are three but closely related concepts in characterizing the phenomenon of a rupturing fault. Firstly, the classical Doppler effect describes the frequency shift between the radiated and received waves of a moving source with single frequency. This is caused by the relative movement of the receiver and source, and it is only a changing of received numbers of wave in a given time but no wave superposition and interference. Secondly, for rupture propagation effect, the slipping of a point on the fault plane still continues after the surpass of the rupture front, so it may lead to a constructive interference of wave radiated from the multiple wave source, thus it is different in nature from the finite moving source and Doppler effect. Thirdly, the rupture process of a real earthquake fault is so complex that it is difficult to generate a frequency shift similar to the Doppler effect.2. Analysis on strong ground motions of the Chi-Chi earthquake indicates that the rupture directivity has an orientation modulation effect on the motion parameters. To be specific, firstly, directivity increases the amplitude of motion and decreases the attenuation rate in the forward direction, while it decreases the amplitude of motion and increases the attenuation rate in the backward direction. Secondly, the response spectra with periods larger than 2 sec are much more sensitive to the rupture directivity compared with that of the short periods, and the bigger the period, the larger the directivity effect appeared. Thirdly, the significant duration is getting longer with increasing epicenter distance in the forward direction. Lastly, the fault-normal component is the most sensitive to be affected by the directivity effect.3. Analysis on the related source parameters indicates that the focal depth, hypocenter location, rupture velocity, fault dip and rupture model play important roles in affecting the strong ground motion's directivity.(1) Modeling results of a typical unilateral rupture fault indicate that there is an asymmetry distribution of amplitudes, spectral ordinates and durations in the ground surface. The peak ground motion and response spectral ordinates in the forward direction get much higher than that of the backward direction with similar rupture distance, while the duration parameter is in the opposite.(2) Variation of focal depth also has an obvious effect on the directivity. With increasing of focal depth, the amplitude of ground motion is getting more and more smaller, while the significantly affected areas in the forward direction is getting more and more far from the epicenter, and the directivity affected areas are getting more and more bigger.(3) The hypocenter location has a similar effect with the focal depth in the study, and in general, the more closely the hypocenter located to the ground surface, the bigger the amplitude and spectral ordinate is. And also the hypocenter parameter has a different effect on fault-normal and fault-parallel components.(4) The uniform rupture velocity with different value has an effect on the amplitude, spectral ordinate and duration. It is concluded that larger rupture velocity increases the amplitude and spectral ordinate but decrease the duration. And for different rupture velocity in the study, the significantly affected areas show little difference.(5) Dip angle of the fault model has a special effect on the ground motion. In a word, big dip angle leads to a distinct directivity effect in the fault-normal component, while small dip angle leads to a distinct directivity effect in the vertical component.(6) Nonuniform rupture speed along the fault length also generates directivity on the ground motions. Compared to the constant rupture speed, the variable rupture speed leads to relatively smaller amplitude and spectral ordinate, but a higher duration in the same rupture distance.(7) Bilateral rupture of the fault can also generate directivity effect in the two end of the rupture propagating. Analysis to the simulated ground motion indicates that the directivity effect generated by a bilateral rupture is more complex than that of a unilateral rupture. And it also has different effect on the fault-normal and fault-parallel components.4. The concept of super-shear rupture and its validation were introduced, through analysis of ground motion under various super-shear rupture speeds, results indicate that super-rupture also leads to directivity effect but there appear some difference compared with the sub-shear ruptures. Firstly, the amplitude of fault-normal component decreases with increasing super-shear rupture speed, while the fault-parallel component increases with increasing super-shear rupture speed. And the significantly affected areas in the forward direction decrease with increasing super-shear rupture speed. Secondly, as for the response spectra ordinate, it bears some similar features with the amplitude aspect. Thirdly, in general, the duration in the forward direction for fault-normal component decreases with increasing super-rupture speed, while the fault-parallel component is in opposite.5. A preliminary analysis of directivity effect on strong ground motions in Wenchuan earthquake is conducted. Firstly, in general, the peak ground acceleration in the forward direction is bigger than that of the backward direction, and the difference between the forward and backward depends on the closest rupture distance. Secondly, the average response spectra in the forward direction is higher than that of the backward direction, especially for periods higher than 2 sec, and the maxim ratio of forward and backward average spectra can reaches 4. Thirdly, for strong ground motion duration, the difference between the forward and backward direction some times reaches 2~4 times on average. Lastly, the distribution of building collapse ratio and human mortality ratio in direction along and perpendicular to the fault strike are adopted to additionally prove the directivity effect.