Semi‐empirical model for site effects on acceleration time histories at soft‐soil sites. Part 2: calibration

Abstract

SUMMARYA previously developed simpliÿed model of ground motion ampliÿcation is applied to the simulation of acceleration time histories at several soft-soil sites in the Valley of Mexico, on the basis of the corresponding records on ÿrm ground. The main objective is to assess the ability of the model to reproduce characteristics such as e ective duration, frequency content and instantaneous intensity. The model is based on the identiÿcation of a number of parameters that characterize the complex ÿrm-ground to soft-soil transfer function, and on the adjustment of these parameters in order to account for non-linear soil behavior. Once the adjusted model parameters are introduced, the statistical properties of the simulated and the recorded ground motions agree reasonably well. For the sites and for the seismic events considered in this study, it is concluded that non-linear soil behavior may have a signiÿcant e ect on the ampliÿcation of ground motion. The non-linear soil behavior signiÿcantly a ects the e ective ground motion duration for the components with the higher intensities, but it does not have any noticeable in uence on the lengthening of the dominant ground period.

Semi‐empirical model for site effects on acceleration time histories at soft‐soil sites. Part 1: formulation and development

“…According to our results, long duration acceleration time histories on soft soil can be simulated even if the duration on ÿrm ground is signiÿcantly shorter. This conclusion applies both to the earthquakes and sites studied here and to those mentioned in Reference [15]. These long durations, as well as the high observed ampliÿcations that lead to the low damping values, can perhaps be physically explained by the contribution of wave trains generated by di raction phenomena and small irregularities at the contact surface between the ÿrm-ground and the soft-soil layers.…”

“…If these parameters were used to simulate ground motions for another earthquake, the ampliÿcation e ects for that earthquake would be assumed to be equal. This problem is explored elsewhere [15]. As mentioned above, an excellent agreement was observed for both the real and imaginary components and the phase spectra of the empirical transfer functions at the signiÿcant frequency bands for the response at each site (including those determined in accordance with the one-dimensional model).…”

“…Each function in the sample can be applied to an earthquake input-motion speciÿed at the reference site by the complex function A(!) in Equation (9), in order to generate an artiÿcial ground motion record at the site of interest [15].…”

“…The study is divided into two parts; this article deals with the formulation and development of the model, while its calibration is presented in a second article [15]. The next section describes some properties of the empirical transfer functions that are applied in the formulation of the proposed model, as well as in the criteria and methods used to identify its parameters.…”

“…This is accomplished by adjusting the properties of the oscillators that determine the moduli of the transfer functions that will be assumed during the simulation process. The second article [15] uses ground acceleration time histories recorded on ÿrm ground and soft soil in Mexico City, during earthquakes of di erent intensities, in order to calibrate the model and verify the accuracy of the procedure proposed to account for non-linear soil behavior.…”