Variations of strain-drops of aftershocks of the 1999 İzmit and Düzce earthquakes around the Karadere-Düzce branch of the North Anatolian Fault
Wenzheng Yang, Zhigang Peng, & Yehuda Ben-ZionPublished 2009, SCEC Contribution #1298
We estimate the strain-drops of 7498 aftershocks of the 1999 İzmit and Düzce earthquakes using P waveforms recorded by a local seismic array along the Karadere-Düzce branch of the North Anatolian fault in the 6 months following the İzmit main shock, and evaluate the site effects of the various recording stations. The method is associated with separation of source, travel-time 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 neighboring events in different amplitude bins to the ω-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 İzmit and Düzce mainshocks, while 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 3 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.
Citation
Yang, W., Peng, Z., & Ben-Zion, Y. (2009). Variations of strain-drops of aftershocks of the 1999 İzmit and Düzce earthquakes around the Karadere-Düzce branch of the North Anatolian Fault. Geophysical Journal International, 177, 235-246. doi: 10.1111/j.1365-246X.2009.04108.x.
