Investigation of 3-D basin structures in the İzmit Bay area (Turkey) by single-station microtremor and gravimetric methods

SUMMARY We investigate 3-D basin structures and site resonance frequencies in the Izmit Bay area of Turkey by new geophysical surveys based on 239 single station microtremor and 405-point gravity measurements. A fundamental resonance frequency map of the Izmit Bay was produced from the main peak in the horizontal-to-vertical component spectral ratio (HVSR) of microtremors. The HVSR analysis of the microtremor data reveals single, double, broad peaked or no peak type HVSR curves varying in accordance with the surface geology and spatial extent of the three basins present in the Izmit Bay area. In the deepest sections of the Izmit, Golcuk and Derince basins of the Izmit Bay, the fundamental resonance frequencies are dominantly 0.24–0.30 Hz. These resonance frequencies should be taken into consideration along with higher mode frequencies to construct earthquake resistant structures in the Izmit Bay area. The 3-D gravimetric bedrock depth map of the Izmit basin shows that the basin has an asymmetric shape with its deepest section coinciding with the surface trace of the North Anatolian Fault. The northern shoulder of the basin has a gentle dip on the Kocaeli Peneplain side and the southern shoulder is much steeper, and it is bounded by the Samanli Mountains. We derive a power-law relationship that correlates the fundamental site resonance frequencies with the sedimentary cover thickness obtained from the gravity and shear wave velocity data in the Izmit Bay. We use this relationship to estimate bedrock depths beneath Golcuk and Derince basins. Our estimation of maximum basin depths is 1400 m for the Izmit and is 800 m for the Golcuk and Derince basins. Finally, we have analysed a converted Sp phase from a local earthquake recording made at site CMP to calculate and verify the sediment thickness estimations obtained from our gravimetric and microtremor analyses. This calculation shows close agreement with that of the gravimetric and microtremor results. Our results show that the basins in the Izmit Bay area have a very thick sedimentary cover with very low shear velocities underlined by hard bedrock, forming a sharp impedance contrast. We anticipate that these results will be a key contribution to the quantitative assessment of seismic hazard for the Izmit Bay area before the occurrence of strong earthquakes in the Marmara region.