Dynamic Simulation of Seismic Excitation on Dipping.Faults: the,Importance ofDynamic Source Effects on Strong Ground Motion Prediction

摘要

Earthquakes on dipping faults generate large differences between the near-source ground motion at the hangingwall andrnat the footwall, the ground motion in the hanging wall being larger than in the footwaU. This effect was distinctly observed inrnthe recent 1999 Chi-Chi (Tatwan) earthquake (Ms = 7,6)in which the rupture of the causative fault of the earthquake reachedrnthe surface and propagated along 80 km. Unusually large vertical displacements, between t.0 arm 4.0m, as well as horizontalrndisplacements of up to 8.0m, were registered along the fault trace on the hanging wall. No earthquake with suchrncharacteristics had previously been recorded, so the event had an immediate impact in the fields of seismology andrnearthquake engineering, In order to show that the ground motion on dipping faults that break the free surface is affectedrnmainly by dynamic source effects, the rupture process of dipping faults with different asperity size but with the same seismicrnmoment is modeled. The results suggest that the effectof the asperity size on ground motionis important in the generation ofrnhigh frequency components, The smaller the asperity, the larger arethe high frequency components generated. Based on thesernresults, we simulate the rupture process of the southernand northern parts of the 1999 Chi, Chi (Taiwan)earthquake in orderrnto explain the damage distribution on buildings causedby the earthquake in which, although the strongest ground motionrnoccurredat the northern part of the causative fault, structural damage was heavier in the southern part. In addition, the rupturernprocess of the thrust fault near the hypocenter area of the 1999 Chi-Chi (Taiwan) earthquake was compared with anotherrnsimilar model in which the rupture is forced to stop 3 km before it reaches the surface. From the results, it can be clearlyrnseen that the effect of the rapture reaching the surface cannot be disregarded in predictions of the ground motion in thernvicinity of the surface trace. A 2D Discrete Element Model (DEM) was employed to perform a dynamic simulation of thernrupture process of the fault and the near-fault ground motions. The resulting equations of the elastodynamic proble m werernsolved for a region around the causative fault, using explicit time integration. The dynamic model presented herein shows thernimportance of an adequate consideration of the source process in gro0nd motion prediction and the potential of the approachrnto complement seismic risk studies.