Physics based ground motion model in seismic hazard assessment

Ground motion prediction equations (GMPE) are traditionally used in site specific seismic hazard analysis to obtain design response spectra. These equations are obtained by regression analysis on the available strong motion data in a given tectonic and geological region. Assuming ergodicity regional GMPE are routinely used in site-specific probabilistic seismic hazard analysis. Since these empirical equations are region specific, However the obtained seismic hazard curves are not specific to the site. Due to lack of data for all possible combinations of magnitude and distances, development of site-specific GMPE is not possible in the near future. The only way to develop a site-specific GMPE is through numerical models. Given a 3D velocity structure, topography and source information these models can simulate site-specific ground motion. Once calibrated with the recorded strong motion data, numerical models can be used to simulate ensemble of ground motions by including the uncertainty in the slip models. In regions lacking strong motion data, these models have an additional advantage compared to GMPE. In the present study, an broad band simulation model is developed for a typical site in peninsular India. Spectral finite element method (SPECFEM) is used to simulate the low frequency ground motion by incorporating the 3D velocity structure in the medium. The high frequency ground motion is simulated from the stochastic seismological model (Otarola and Ruiz, 2016). Statistical kinematic rupture model is used to represent the earthquake source (Dhanya and Raghukanth 2018). The rupture length, width and correlation lengths of the random field are estimated from magnitude. Assuming the phase as random, a total of 30 rupture models are simulated for each magnitude. An ensemble of ground motions is simulated at the site for various possible combination of faults and magnitudes in a region around 500 km from the site. The simulated low-frequency and high-frequency ground motions are combined in the frequency domain to obtained broad band ground motions (0-100 Hz). The mean and standard deviation of the response spectra are estimated from these simulated motions for all possible combinations of magnitudes and distances at the given site. Further, probabilistic seismic hazard analysis is carried out using the simulated data to obtain hazard curves for spectral accelerations at various natural periods. Uniform hazard response spectra (UHRS) for 475yr and 2475 yr is obtained from the hazard analysis. A comparison with traditional hazard analysis using region specific GMPE is also presented. It is observed GMPE based UHRS show a smooth trend compared with site-specific UHRS obtained from broad band models. The PGA values obtained from physics based model are slightly higher than that obtained from GMPE based PSHA.