The M-w 7.1 Van, Eastern Turkey, earthquake 2011: two-point source modelling by iterative deconvolution and non-negative least squares

The source complexity of the M-w 7.1 (USGS) Van, Eastern Turkey, earthquake of 2011 October 23 is studied using full waveform inversions of seismic records at near-regional distances (120-220 km) and relatively low frequencies (0.05-0.15 Hz). The study relies on iterative deconvolution and on a new method in which pairs of point sources on the fault plane are systematically grid searched, and the moment-rate time functions of the two-point sources are simultaneously calculated by non-negative least-squares inversion. It is demonstrated on synthetic and real data that the wavefield in these ranges is sensitive enough to distinguish two main subevents of the Van earthquake, separated from each other by similar to 10-15 km and similar to 4 s. The double-event character of the Van earthquake is indicated even by a simplified single-point source model, optimally when the trial-point source is near the earthquake centroid. The simple indicators of source complexity developed in this paper are useful in practice in the first hours after an earthquake, when the source position is known only approximately and finite-fault models of slip evolution are not yet available.