Generation of Synthetic Accelerograms Using Engineering Earthquake Source Model

The strong motion records available during an earthquake can be treated as the response of the earth as a structural system to unknown forces acting at unknown locations. Thus, if the part of the earth participating in ground motion is modeled as a known finite elastic medium, one can model the source location and forces generated during the earthquake as an inverse problem. Based on this analogy, a simple model for the earthquake source is proposed, by assuming the source to be a sequence of impulses acting at locations yet to be found. These impulses and their locations are found using the normal mode expansion along with a minimization of mean squared error. The medium is assumed to be finite, elastic, homogeneous, layered and horizontal with specified boundary conditions. Detailed results are obtained for the Uttarkashi earthquake of 20th October 1991, in India. The impulse locations are shown to be closely associated with the underlying fault mechanism. The proposed model is then used to simulate the acceleration time histories at a few recording stations. The earthquake source expressed in terms of a sequence of impulses acting at different locations is applied to a 2D finite elastic medium. The acceleration time histories found from this model agree well with with the accelerations recorded for the earthquake.