Stochastic strong motion generation using slip model of 21 and 22 May 1960 mega-thrust earthquakes in the main cities of Central-South Chile

In May 1960 took place the most unusual seismic sequence registered instrumentally. The Mw 8.1, Concepción earthquake occurred May, 21, 1960. The aftershocks of this event apparently migrated to the south-east, and the Mw 9.5, Valdivia mega-earthquake occurred after 33 hours. The structural damage produced by both events is not larger than other earthquakes in Chile and lower than crustal earthquakes of smaller magnitude. The damage was located in the sites with shallow soil layers of low shear wave velocity (Vs). However, no seismological station recorded this sequence. For that reason, we generate synthetic acceleration times histories for strong motion in the main cities affected by these events. We use 155 points of vertical surface displacements recopiled by Plafker and Savage in 1968, and considering the observations of this authors and local residents we separated the uplift and subsidence information associated to the first earthquake Mw 8.1 and the second mega-earthquake Mw 9.5. We consider the elastic deformation propagation, assume realist lithosphere geometry, and compute a Bayesian method that maximizes the probability density a posteriori to obtain the slip distribution. Subsequently, we use a stochastic method of generation of strong motion considering the finite fault model obtained for both earthquakes. We considered the incidence angle of ray to the surface, free surface effect and energy partition for P, SV and SH waves, dynamic corner frequency and the influence of site effect. The results show that the earthquake Mw 8.1 occurred down-dip the slab, the strong motion records are similar to other Chilean earthquake like Tocopilla Mw 7.7 (2007). For the Mw 9.5 earthquake we obtain synthetic acceleration time histories with PGA values around 0.8 g in cities near to the maximum asperity or that have low velocity soil layers. This allows us to conclude that strong motion records have important influence of the shallow soil deposits. These records correlate well with our structural damage observations.