High-frequency source imaging of the 2011 October 23 Van (Eastern Turkey) earthquake by backprojection of strong motion waveforms

We backproject strong motion waveforms to image the rupture process of the 2011 October 23 Van (M w 7.1) earthquake. We show that the followed backprojection approach is applicable for studying earthquake sources with a complex and uncertain fault geometry. It is especially suitable for cases where the implementation of conventional source inversion methods is difficult due to incomplete data coverage from local networks. Normalized high frequency (HF, 2-8 Hz) S-waveform envelopes from recordings on local Turkish and Iranian stations are used to scan a pre-defined 3-D source volume over time. The envelopes are forced to focus at the hypocentre by applying the appropriate station corrections, estimated from waveform cross-correlation. We image two distinct sources of HF radiation during the earthquake rupture at depths between 10 and 12 km. The first one, a few seconds after the rupture initiation, is located near the hypocentre. The second, at least 13 km southwestwards, follows 3.5 s later. While a continuous rupture propagation is not clearly imaged, the average rupture speed is 3.5 km s−1. Synthetic tests show that both HF sources could be resolved adequately from this station distribution. Both sources of HF are located at the edge of large slip patches, imaged by seismic and geodetic inversions. The existence of a complex fault geometry, like a pair of en-echelon reverse faults, explains the radiation of HF energy. The first source is attributed to a stopping phase of the eastern subfault or a possible change on a single fault geometry. The second HF source is produced from the abrupt cease of rupture at the shallow part of the upper crust.