Spatiotemporal variations of crustal anisotropy from similar events in aftershocks of the 1999 M7.4 Izmit and M7.1 Duzce, Turkey, earthquake sequences

We analyse spatiotemporal variations of crustal anisotropy along the Karadere-Duzce branch of the North Anatolian Fault from similar earthquakes in the aftershock regions of the 1999 M(W)7.4 Izmit and M(W)7.1 Duzce earthquakes. The similar earthquake clusters are identified by performing cross-correlation on waveforms generated by similar to 18 000 earthquakes. Depending on the applied similarity criterion, about 4-60 per cent of the events belong to similar event clusters. Splitting parameters averaged within each cluster show significant variations for slightly different ray paths, indicating strong spatial variations of crustal anisotropy in this area. We also find clear changes in the spatiotemporal seismicity patterns following the Duzce main shock. Large apparent co-seismic changes (up to 30 per cent) of shear wave splitting delay times are observed across the time of the Duzce main shock at stations near the epicentral region. However, the changes can be mostly explained by the spatial variations of ray paths due to the changing seismicity, rather than changes in the properties of the anisotropic medium. Splitting parameters measured within similar earthquake clusters indicate at most 2 per cent changes in delay times associated with the occurrence of the Duzce main shock. The results do not show systematic precursory changes before the Duzce main shock.