Abstract
The faults’ geometry and their seismic activity beneath the Marmara Sea have been under debate for a couple of decades. We used data recorded by three ocean bottom seismographs (OBSs) over a period of 3 months in 2014 to investigate the relationship of fault geometry to microseismicity under the western Marmara Sea in Turkey. We detected a seismic swarm at 13 to 20 km depth beneath the main Marmara fault (MMF), and the maximum depth of seismogenic zone was 25 km within the OBS observation area. These results provided evidence that the dip of the MMF is almost vertical and that the seismogenic zone in this region extends into the lower crust. Our analysis of past seismicity indicated that the seismic swarm we recorded is the most recent of an episodic series of seismic activity with an average recurrence interval of 2–3 years. The repetitive seismicity indicates that the MMF beneath the western Marmara Sea is coupled and that some of the accumulated strain is released every 2 to 3 years. Our study shows that OBS data can provide useful information about seismicity along the MMF, but more extensive studies using more OBSs deployed over a wider area are needed to fully understand the fault geometry and stick–slip behavior of faults under the Marmara Sea.
Highlights
The North Anatolian fault (NAF) extends 1600 km from its junction with the East Anatolian fault at the Karliova triple junction in eastern Turkey
Our analysis of data recorded by three ocean bottom seismograph (OBS) we deployed for 3 months in the western Marmara Sea showed that most of the microearthquakes we identified occurred along the Marmara fault (MMF), one of them could not be explained by the characteristic right-lateral strikeslip motion of the MMF
We identified a seismogenic zone that extends from 13 to 25 km depth through the upper and lower crust beneath the western Marmara Sea, these results are still preliminary and should be confirm by additional observation
Summary
The North Anatolian fault (NAF) extends 1600 km from its junction with the East Anatolian fault at the Karliova triple junction in eastern Turkey. It extends westward across northern Turkey and into the Aegean Sea, accommodating about 25 mm/year of right-lateral motion between the Anatolia and Eurasia plates (e.g., Reilinger et al 2006) (Fig. 1). After the Northern Aegean earthquake (Mw = 6.9) on 24 May 2014, the only part of the 1600-km-long NAF that has not ruptured since 1939 lies beneath the Marmara Sea, where its geometry is not well understood. Past estimates of the geometry of the NAF beneath the Marmara Sea have been based mainly on bathymetric and shallow structural information. Pinar (1943) is the first study that proposed a single through going strikeslip fault system (the main Marmara fault; MMF) that nearly bisects the Marmara Sea. Le Pichon et al (2001) proposed a more detailed location of MMF based mainly on high-resolution bathymetry and shallow seismic reflection data. To investigate the potential for future earthquakes beneath the Marmara Sea, it is necessary to clarify the fault geometry there and to know which faults are active
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