Characterization of response spectra for near field conditions by earthquake ground motion simulation

Abstract

Strong ground motion estimation tools such as empirical ground motion prediction equations rely mostly on recorded data. Good quality strong motion data became available mostly in recent years and are still sparse for near field conditions. Strong ground motion records on the other hand contain them several characteristics such as source, path and especially local site effects. These features can sometimes be hardly differentiated when one is willing to study only one of them. As such, realistic simulation of the 3D wavefield generated by a fault rupture in a heterogeneous medium becomes critical for the study of strong ground motion. Particularly features such as intra-event and inter-event spatial and temporal variability of ground motion can be studied in detail. With increasing computational power and newly emerged methods such simulations become increasingly feasible. In this study I use kinematic simulation with 3D Staggered Grid Finite Difference method to study the characteristics of near source effects of strong ground motion. I also aim to prove that kinematic simulation with 3D Staggered Grid Finite Difference method (3DFD) is a useful tool to analyze and estimate the characteristics of long period strong ground motion. For this purpose first a simulation model has been set for the 2004 Mw=6.0 Parkfield earthquake. Making use of the available 3D crustal velocity structure of the region and several slip models of the earthquake over 40 near source recordings produced by the event have been simulated. Encouraged by the very good agreement between observed and simulated waveforms, 3DFD has been used for the characterization of directivity effects in the near source region. Eleven earthquakes covering a magnitude range from 6.0 to 7.4 have been modeled using one or more slip models for each of them. Eight of these events had a strike-slip mechanism whereas the remaining ones were dip-slip events. Resulting spatial distributions of fault parallel, fault normal and average response spectral accelerations at longer periods have been analyzed to derive general formulations for the modification of the response spectral values found from earthquake hazard analysis to include near source directivity effects. With recently emerging structural design methods, engineering studies concentrate more and more on the long period characteristics of strong ground motion. The present study validates kinematic simulation tools, particularly the 3DFD method used herein as a useful method for the simulation of low frequency strong ground motion in realistic earth media for engineering purposes.