Abstract
<p>In this study, we analyze the ground motion characteristics of October 20, 2006 Manyas (M<sub>L</sub>=5.2) and October 24, 2006 Gemlik (M<sub>L</sub>=5.2) earthquakes. Both earthquakes occurred on the southern branch of the North Anatolian Fault Zone in Marmara region, which has a lower seismic hazard relative to the northern branch. The two events are the largest earthquakes on the southern branch recorded by a modern and vast seismological network; therefore their records are valuable to evaluate seismic risk of the region and the understanding of physics of wave propagation. The analysis show that the attenuation of PGAs is very similar for two earthquakes, but they are not represented by the empirical relation obtained for earthquakes occurred on the northern branch. The waveforms of the Gemlik earthquake recorded by BYTNet array indicate an EW rupture orientation with right-lateral slip which fits to the general character of the southern branch. Ground motions at the stations located within basin are strongly influenced by the presence of locally induced surface waves resulting in lengthening of significant shaking duration with respect to a nearby ridge site. Surface wave characteristics are very similar for the Manyas and Gemlik earthquakes, but variations are observed on components which may be related to 3D basin geometry. Resonance frequencies of the surface waves generated within basin are very close to the 1D site resonances at the stations obtained from H/V ratios of S waves. The resonance frequency is about 0.2 Hz within the large Bursa Plain, whereas it increases to about 0.9 Hz within the smaller Gemlik Plain.</p>
Highlights
The North Anatolian Fault Zone (NAFZ) is a dextral strike-slip fault zone extending more than 1600 kmlong from Karliova in the east to the Aegean Sea in the west, which forms the northern boundary of the westward moving Anatolian Plate
Surface wave characteristics are very similar for the Manyas and Gemlik earthquakes, but variations are observed on components which may be related to 3D basin geometry
It has been determined that the northern NAF (NNAF) accommodates 20 mm/year of the slip rate caused by the plate kinematics, which is about 24 mm/year, with no strike perpendicular motion, while the rest of the slip rate is accommodated along the southern branch with prominent strike perpendicular motion or extension [Flerit et al 2003, Nyst and Thatcher 2004]
Summary
The North Anatolian Fault Zone (NAFZ) is a dextral strike-slip fault zone extending more than 1600 kmlong from Karliova in the east to the Aegean Sea in the west, which forms the northern boundary of the westward moving Anatolian Plate. The AB2010 predictions of the peak accelerations observed during the Manyas and Gemlik earthquakes are better than the OZ2004 predictions This may imply that the attenuation of the PGAs on the southern branch may not be similar to the northern branch. The observed PGAs during the Gemlik and Manyas events are slightly underestimated by AB2010 at closer distances
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