Hanging Wall Effect In Wenchuan Earthquake

Along with the increasingly development of strong earthquake observation technology, increasingly extended construction of network station for strong earthquake observation, accumulation of earthquake records especially of those from near fault area, people started to find out the absolute different performance of the near fault ground motions in comparison with ground motions in far field. Near fault ground motions possess many special characteristics, such as rupture directivity, hanging wall effect, concentration of ground motions, surface rupture and flint-step, large velocity pulse. These characteristics cause the near fault ground motions to behave differently, other than what supposedly similar to the far field ground motions. For example, in the near fault area, ground motions in some areas are strong while mild in other areas, or ground motions in long periods are fierce while mild in short periods, or even ground motions have pulse like waves. So, it is necessary to study comprehensively these characteristics or effects of near fault ground motions in order to revise the old attenuation relationships to incorporate influence of these effects on ground motions. The revised the attenuation relationships are supposed to predict near fault ground motions more accurately.Wenchuan earthquake is the biggest one happened in densely populated area in our country. It caused such huge damage and heavy casualty. However, due to the rapid construction of strong-motion instrument in recent years, a large set of data has been obtained in this earthquake. This is the biggest set of strong ground motion data ever since, and enable an in-depth research into the aspects of this earthquake. This study focuses on the hanging wall effect of Wenchuan earthquake, and the main work includes:1. Selected near fault data of Wenchuan earthquake and data were processed, collected and classified. Based on the selected data, the Wenchuan earthquake specific attenuation relationship was regressed. Then, residuals of chosen data were obtained by subtracting attenuation relationship from original data. The hanging wall effect in Wenchuan earthquake was identified after the distribution of residuals on hanging wall and footwall was analyzed.2. After the recognition of hanging wall effect in Wenchuan earthquake, it is necessary to model the hanging wall effect for quantitative analysis of the hanging wall effect in the earthquake. There are two models AS1996 and KS2003 as denoted in this study put forward by previous researchers to the hanging wall effect in Northridge earthquake and Chi-Chi earthquake respectively. However, after a comprehensive analysis of the two models, some drawbacks are identified with the models. Specifically, firstly, the two models are complex and used with inconvenience; the other, the two models introduced large portion of subjectivity, to lead to big uncertainty as a result.3. After the recognition of the drawbacks of previous models, a new model was put forward in this study to avoid all problems with previous models. So, the new model or proposed one has advantages over previous models. It is easy to use and can reduce or even completely avoid subjectivity during the process of determination of models. Finally, the new model makes full use of all data, and error can be reduced as low as possible. The new model therefore was applied to model the hanging wall effect in Northridge earthquake and Chi-Chi earthquake, results of which were compared with each that of AS1996 and KS2003. Satisfying results were obtained as the comparisons show.4. The new model was applied to model hanging wall effect in Wenchuan earthquake, including PGAH and SAH. Meanwhile, the modeling results were drawn in comparison with the AS NGA's prediction. It showed that AS NGA predicted pretty high level of hanging wall effect in Wenchuan earthquake. Besides, it failed also to predict the existence of hanging wall effect in a little bit far distance still.