Dependence of Ductility Response Spectra on the Seismogenic Depth from Finite Element Earthquake Rupture Simulations

Ongoing research in the development of design philosophies for earthquake resistant structures over the past few decades was initially based on the strength and elastic analysis. Later, design philosophies recognized the deformation to be an important parameter to be considered in design following nonlinear analysis. The maximum design lateral force, for a particular earthquake, acting on structures having multiple natural time periods can be obtained from inelastic response spectrum. Scenario earthquakes characterize the spatio-temporal evolution of fault rupture, which when solved together with the elastodynamic equations can give the acceleration time-history at any point on the surface. For any tectonic regime, based on the past seismicity, a seismogenic depth could be defined based on the depth below which no occurrence of earthquakes was observed in the past. Fixing a certain magnitude, we prescribe the slip on a vertical fault based on statistical relations that exist in literature, and simulate ground motion. The ruptured region is varied, is initially assumed to be closer to the free surface, and is later lowered deeper in intervals of 10 km to emulate larger seismogenic depths. Using the simulated ground motion, we compute the fundamental entity of earthquake engineering: the response spectrum for five depths of hypocenter. Earthquake resistant design of structures is mostly done to allow for large inelastic deformations, giving ductile detailing. Choosing ductility ratios of 2, 4, 6 and 8, this paper describes dependency of elastic and inelastic horizontal spectral acceleration on seismogenic depth, by considering kinematic rupture description of a Mw 6.5 earthquake on a vertical strike slip fault.