Variations of strain-drops of aftershocks of the 1999 Izmit and Duzce earthquakes around the Karadere-Duzce branch of the North Anatolian Fault

We estimate the strain-drops of 7498 aftershocks of the 1999 Izmit and Duzce earthquakes using P waveforms recorded by a local seismic network along the Karadere-Duzce branch of the North Anatolian fault in the 6 months following the Izmit main shock and evaluate the site effects of the various recording stations. The method is associated with separation of source, traveltime and station spectral terms and stacking results at several stages to enhance the signal-to-noise ratio. The strain-drops are obtained by fitting iteratively the separated source spectra of 201 nearest neighbouring events in different amplitude bins to the omega(-2) source spectral model. The obtained strain-drops generally increase with depth between 3 and 10 km and remain approximately constant for the deeper section. A local shallow patch of seismicity north of the Karadere segment has relatively low strain-drop values. Along the relatively straight Karadere segment, the strain-drops are lower than along the geometrically complex bounding regions. In some sections, the range of strain-drop values decay with time from the Izmit and Duzce main shocks, whereas in others the values fluctuate within about constant ranges. The observed spatial variations of strain-drops can be explained by increasing normal stress with depth, along with the degree of geometrical complexity of different fault sections and the expected slip deficit at different depth sections. The seismic energy in the separated station spectra decreases overall with distance from the rupture zone and the spectra of various stations have three peaks at 6, 14 and 25 Hz. The spectral peak at 6 Hz is also observed in trapped waves studies and may be related to the damaged fault zone layer.