The results of a series of tests performed with different amounts of explosive at short distances above and below ground level, as well as on the soil surface are briefly described. After an introductory description of both the main features of the blast wave and the mechanics of crater formation, a brief review of empirical methods for crater size prediction is presented. Next, the experimental design and the results obtained are described. The crater dimensions for underground explosions coincide with those found in the literature. For explosions at ground level the results are qualitatively described by empirical equations. For explosive charges situated above ground level, the dimensions of the craters are smaller than those observed in underground and near the surface explosions. Two new single prediction equations for this case are presented.
[1] As there is no evidence that cracks are created directly in modes II and III, shear cracks probably occur only along a weakness plane, as in a preexisting fault. Mode I fracture therefore may be an important factor in crack formation during shallow earthquakes. A three-dimensional shear dynamic rupture process was simulated on the assumption that shear slip occurs only in a preexisting fault and the possibility of introducing new internal cracks that propagate under tensile stress as a consequence of the dynamic process of shear slip propagation. The discrete element method (DEM) was used to solve this problem because it can introduce internal tensile cracks. The simple slipweakening model was used as the friction law on the preexisting fault for shear rupture propagation. For new tensile cracks, fracture follows classical Griffith theory when the critical value for tensile fracture surface energy is reached. The proposed model was used to simulate the rupture process of a strike-slip shallow fault. Results show that the generation of new cracks is affected by rupture directivity in terms of the hypocenter and asperity location as well as by fault geometry with respect to the free surface. Cracks develop a flower-like structure that surrounds the preexisting fault. When the asperity is located at less than a certain depth, the flower-like structure that originates from the top of the fault reaches the free surface. We consider that this is the mechanism for forming the flower structure near surface during a strike-slip shallow earthquake.
INDEX TERMS:7209 Seismology: Earthquake dynamics and mechanics; 7260 Seismology: Theory and modeling; 8010 Structural Geology: Fractures and faults; KEYWORDS: rupture dynamic, shear and tensile cracks, fault mechanics, fault branching, earthquakes Citation: Dalguer, L. A., K. Irikura, and J. D. Riera, Simulation of tensile crack generation by three-dimensional dynamic shear rupture propagation during an earthquake,
The 1999 Chi-Chi, Taiwan, earthquake, that originated on a low-angle reverse fault, showed complexity and uncommon characteristics. The records show that the hanging-wall side is characterized by larger particle motions than the footwall, and the ground motion is stronger in the northern part than in the southern part of the causative fault. Although the strongest ground motion occurred near the northern part of the trace, structural damage was heavier in the southern part. In order to get a better understanding of the complex damage distribution caused by this earthquake, the dynamic rupture process was numerically simulated. Because of the differences between the observed features of the rupture process in the northern and southern parts of the fault, each part was modeled independently by using a 2D discrete element model (DEM). The principal results of the simulation show that the velocity ground motions in the northern part, in the frequency range of 0.5-2 Hz (natural frequency range of standard structures), are small near the surface break, thus, light structural damage might be predicted near the surface rupture. Moreover, in the northern part the fault rupture propagation reaches the surface with a very slow velocity (about 1.2 km/sec); however, in the southern part the rupture propagation reaches the surface with higher velocity (about 3.0 km/sec). These differences between the models could explain why the ground motion near the surface rupture in the northern part caused less damage in structures than the ground motion in the southern part.
The dynamic and static stress changes during the 2000 Tottori earthquake have been recovered from the results of waveform inversion. We use the DEM to solve the elastodynamic equation specifying the slip along the fault obtained by a kinematic fault model. The resulting shear stress distribution suggests an explanation of the foreshock and aftershock distributions. We conclude that the fault zone heterogeneity is strong and most of the foreshocks and aftershocks were located in the zone of negative stress drop and mainly in the area surrounding the asperity. This suggests that the asperity behaved as a barrier during the foreshocks and after the main shock the stress in the area surrounding the asperity increased and triggered most of the aftershocks. The foreshock distribution was confined to a finite localized zone in the central part of the fault, suggesting that this zone was bordered by barriers.
The numerical fracture analysis of non-homogeneous rock or concrete dowels subjected to shear and compression is described in detail. The method of analysis allows the consideration of scale and rate effects due to material non-homogeneity and fracture. The proposed approach is verified by comparing numerical predictions with experimental results reported in the literature for a series of small rock samples, since experimental evidence for large bodies is not yet available (2007). Results generated by Monte Carlo simulation using the so-called discrete element method to model the dowels suggest that a simple three parameters law can be used to predict the relationship between tangential stress at the base and lateral distortion. It is observed that the larger the size of the cubes, the smaller both the peak tangential stress and the rupture distortion. Size effects are also evaluated in samples with vertical restraint. The influence of loading rate is likewise numerically assessed for two sample sizes. The effect is compatible with experimental evidence available for concrete using small samples.
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