Simulation of Strong Motion Time History for the 2011 Tohoku Earthquake With Considerations of Multiple Nonlinear Effects

Abstract eng:
To predict strong ground motions for future large earthquakes including huge subduction earthquakes, it is important to take into account the effects of soil nonlinearity. The author has been developing a simple method to simulate strong ground motions taking into account the effects of soil nonlinearity (e.g., Nozu and Morikawa, 2003). One of the key concepts involved in our method is the multiple nonlinear effects. In general, a seismic ray connecting the source and the site usually crosses the soft soil layers several times except for the direct S wave. Therefore, the seismic wave is affected by soil nonlinearity several times during the propagation from the source to the receiver. This phenomenon is referred to as "the multiple nonlinear effects". The simplified method used in this study considers the multiple nonlinear effects. It uses two parameters to represent the effects of soil nonlinearity; one representing the reduction of averaged shear wave velocity within the sediment (ν 1 ) and the other representing the increase of averaged damping factor within the sediment (ν 2 ). The method has been applied to the 2000 Western Tottori earthquake, etc. (e.g., Nozu and Morikawa, 2003). The method, however, has been validated only for a limited amount of strong motion data, partly because there was only a limited amount of strong motion data affected by soil nonlinearity. Therefore, in this article, making use of strong motion data for the 2011 Tohoku earthquake and the source model developed for the same earthquake by the author (Nozu, 2012), strong motion simulation with considerations of soil nonlinearity was conducted and its effectiveness was studied. In particular, strong motion records with the evident effects of soil nonlinearity were selected and they were simulated using the source model and taking into account empirical site amplification and phase effects (Nozu et al., 2009). Soil nonlinearity was considered using the method of Nozu and Morikawa (2003). Among the parameters involved in the method, ν 1 was basically determined based on Wakai and Nozu (2013) and ν 2 was determined so that the observed ground motion could be simulated as accurately as possible. As a result, it was found that, the duration of strong ground motions tended to be overestimated if the parameter ν 2 was not used for the sites with the effect of soil nonlinearity. In each of the target sites, by using these two parameters, the simulation result was improved. Thus, the effectiveness of strong motion simulation with considerations of soil nonlinearity was confirmed. Based on the results, the application of the method for future earthquakes was also discussed.

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