Simulation of strong ground motion for the 2004 Firozabad Kojoor earthquake in northern Iran

In recent years, seismologists have attempted to develop quantitative models of the earthquake rupture process with the ultimate goal of predicting strong ground motion. Simulation procedures provide a means of including specific information about the earthquake source, the wave propagation path between the source and the site and local site response in an estimation of ground motion. Simulation procedures also provide a means of estimating the dependence of strong ground motions on variations in specific fault parameters. Several different methods for simulating strong ground motions are available in the literature. A number of possible methods that could be used to generate synthetic records include the following: (i) deterministic methods, (ii) stochastic methods, (iii) empirical Green’s function, (iv) semi-empirical methods, (v) composite source models, and (vi) hybrid methods.
The Firozabad Kojoor earthquake occurred on May 18, 2004, with a magnitude of 6.2 in the central Alborz region. We have used the empirical Green’s function and stochastic finite fault modeling to study the source parameters and rupture propagation and make a comparison between the observed and simulated records. The empirical Green’s function method synthesizes the ground motion from a large earthquake (target) by exploiting actual small-event ground motions as the Green’s functions of the earth. It is based on the concept of self-similarity, a notion that assumes a constant stress drop for earthquakes of all magnitudes and provides scaling values for determining related faulting parameters of earthquakes of varying size. The simulation of the ground motion from the target event is expressed as a superposition of contributions from the subfaults separated by a time difference that depends on (1) the location of the particular sub-element on the fault relative to the rupture initiation point and (2) the rupture propagation characteristics. It assumes that the rupture propagates radially from the rupture initiation point at a constant fraction of the shear wave velocity.
This study also employs the modified stochastic finite fault modeling of Motazedian and Atkinson (2005), which is based on the dynamic corner frequency. In this method, a large fault is divided into N subfaults, where each subfault is considered as a small point source. The ground motions of subfaults are summed with a proper time delay in the time domain to estimate acceleration time history.
The comparison shows that the length and width of the rupture plane is 15 × 15 km. The coordinate of the nucleation point is estimated as 36.33 ^oN, 51.60 ^oE and 29 km depth, and rupture was determined to have propagated from east toward the west. The estimated focal mechanism is reverse with a minor left-lateral strike slip component. The strike, dip and rake have been estimated as 110^o, 34^o and 71^o, respectively.