Observations and stochastic modelling of strong ground motions for the 2011 October 23 Mw 7.1 Van, Turkey, earthquake

The 2011 October 23 Van earthquake occurred at 13:41 local time in Eastern Turkey with an epicentre at 43.36oE, 38.76oN (Kandilli Observatory Earthquake Research Institute (KOERI)), 16 km north–northeast of the city of Van, killing around 604 people and leaving thousands homeless. This work presents an overview of the main features of the seismic ground shaking during the Van earthquake. We analyse the ground motion characteristics of the mainshock in terms of peak ground acceleration (PGA), peak ground velocity (PGV) and spectral accelerations (SA, 5 per cent of critical damping). In order to understand the characteristics of the ground motions induced by the mainshock, we also study the site response of the strong motion stations that recorded the seismic sequence. The lack of seismic recordings in this area imposes major constraints on the computation of reliable seismic hazard estimates for sites in this part of the country. Towards this aim, we have used a stochastic method to generate high frequency ground motion synthetics for the Mw 7.1 Van 2011 earthquake. The source mechanism of the Van event and regional wave propagation parameters are constrained from the available and previous studies. The selected model parameters are then validated against recordings. We also computed the residuals for the ground motion parameters in terms of PGA and PGV at each station and the model parameter bias by averaging the residuals over all the stations. The attenuation of the simulated ground motion parameters is compared with recent global and regional ground motion prediction equations. Finally, since it has been debated whether the earthquake of November 9 was an aftershock of the October 23 earthquake, we examine whether static variation of Coulomb stress could contribute to the observed aftershock triggering during the 2011 Van Lake sequence.Comparison with empirical ground motion prediction illustrated that the observed PGA data decay faster than the global empirical ground motion equations, both at moderate and larger distance; this feature is captured by the AC10 model that is derived from Turkish earthquake database. Faster attenuation of ground motion due to the high attenuation of seismic waves in the Eastern Anatolian plateau is in agreement with our stochastic simulations of strong ground motions of the 2011 October 23 Van earthquake. This highlights the importance of retrieving specific regional seismic parameters for the ground motion predictive equations. The stress redistribution computation indicates that the October sequence had an active role in promoting the November aftershocks, being the Coulomb stress variation positive in the entire region of interest. Furthermore, the Van events enhanced the stress level on the Caldiran fault.