Abstract
Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain.
Highlights
Geological and geophysical, marine surveys since 2000 have revealed the geometry of the submarine Main Marmara Fault (MMF) system[6,7]
Marmara Fault Valley, nor along the Fault trace, but on top of adjacent structures or at the edge of basins. (d) Depth of selected earthquakes plotted versus longitude, with events from aftershock sequences represented using the same colour code as in panel B
In the Cinarcik Basin, offshore Istanbul, at the western termination of the 1999 Izmit rupture, the installation of seismometer arrays on the Prince Islands at 2–3 km from the main fault led to the identification of a 30 by 8 km aseismic patch that was interpreted as a locked zone on the MMF11
Summary
Building a high-resolution, 3D-velocity model (see Figures in Supp. Mat., Appendix A1). The tomographic model of Bayrakci et al.[31] was used to describe the velocity structure of the pre-kinematic basement and the velocity structure down to 12 km below the Marmara sea-level. This model (see Figure 13a in ref.31) is based on a low-resolution grid of 6 km × 6 km × 2 km. Following [Pribnow et al, 2000]45, we have tested two different approaches to describe the variation for porosity with depth:
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