Modeling energy release parameters in stochastic simulation of ground motions generated by Vrancea intermediate-depth seismic source

Ground motion time-domain stochastic simulations are performed by generating a white or a Gaussian noise, shaping it with a window, loading its normalized spectrum with the spectrum of the motion at a specific site and transforming it back in time-domain (Boore in Pure Appl Geophys 160: 635-676, 2003). The outcome of the resulted simulations is compared to the target parameters, namely amplitude, frequency content and duration parameters. Using SMSIM (Boore in SMSIM: Fortran programs for simulating ground motions form earthquakes, 2005) and EXSIM (Motazedian and Atkinson in Bull Seismol Soc Am 95(3): 995-1010, 2005. 10.1785/0120030207), Co & x21b;ovanu (J Mil Technol 1(2): 27-32, 2018. 10.32754/jmt.2018.2.05) and Co & x21b;ovanu and Vacareanu (J Seismol 24(1): 229-241, 2020. 10.1007/s10950-019-09892-5) applied the stochastic simulation method for generating ground motions produced by intermediate-depth Vrancea (Romania) earthquakes and found inconsistent results for duration parameters (significant duration and energy distribution in time-domain). In simulations, the duration of the motion results from the source and path duration, but the significant duration (that contains 90% of the energy) depends on the way the energy is distributed in time, and this distribution is controlled through a noise shaping window. The objective of this paper is to provide an appropriate form of the shaping window that can describe the specific energy release of ground motions generated by Vrancea intermediate depth seismic source (the first 50-70% of energy being rapidly released, the rest of it having a very slow release). For this matter, 371 recorded motions were analyzed and 5 types of energy release were defined. The set with the above described pattern that contained almost half of the analyzed accelerograms was used to define a mean normalized cumulative energy that describes the Vrancea specificity. On this basis, the parameters for the SMSIM implemented exponential window were determined and, because the results were inconsistent, a more adequate window function was defined and implemented in SMSIM. For the newly defined two-interval window function an algorithm for calculating the parameters was described, and the parameters of the mean release of energy were determined. By using the two-interval function for describing the Vrancea specific release of energy, more appropriate ground motion simulations resulted.