Abstract
We analyse spatiotemporal variations of crustal anisotropy along the Karadere–Düzce branch of the North Anatolian Fault from similar earthquakes in the aftershock regions of the 1999 Mw7.4 İzmit and Mw7.1 Düzce earthquakes. The similar earthquake clusters are identified by performing cross-correlation on waveforms generated by ∼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 Düzce 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 Düzce 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 Düzce main shock. The results do not show systematic precursory changes before the Düzce main shock.
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