Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009geoji.177..235y&link_type=abstract
Geophysical Journal International, Volume 177, Issue 17, pp. 235-246.
Astronomy and Astrophysics
Astronomy
1
Earthquake Dynamics, Earthquake Source Observations, Site Effects, Seismicity And Tectonics, Continental Tectonics: Strike-Slip And Transform
Scientific paper
We estimate the strain-drops of 7498 aftershocks of the 1999 İzmit and Düzce earthquakes using P waveforms recorded by a local seismic network 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, 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 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 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.
Ben-Zion Yehuda
Peng Zhigang
Yang Wenzheng
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