3-D sediment-basement tomography of the Northern Marmara trough by a dense OBS network at the nodes of a grid of controlled source profiles along the North Anatolian fault

A 3-D tomographic inversion of first arrival times of shot profiles recorded by a dense 2-D OBS network provides an unprecedented constraint on the P-wave velocities heterogeneity of the upper-crustal part of the North Marmara Trough (NMT), over a region of 180 km long by 50 km wide. One of the specific aims of this controlled source tomography is to provide a 3-D initial model for the local earthquake tomography (LET). Hence, in an original way, the controlled source inversion has been performed by using a code dedicated to LET. After several tests to check the results trade-off with the inversion parameters, we build up a 3-D a priori velocity model, in which the sea-bottom topography, the acoustic and the crystalline basements and the Moho interfaces have been considered. The reliability of the obtained features has been checked by checkerboard tests and also by their comparison with the deep-penetration multichannel seismic profiles, and with the wide-angle reflection and refraction modelled profiles. This study provides the first 3-D view of the basement topography along the active North Anatolian fault beneath the Marmara Sea, even beneath the deepest part of three sedimentary basins of NMT. Clear basement depressions reaching down 6 km depth below the sea level (bsl) have been found beneath these basins. The North Imralı Basin located on the southern continental shelf is observed with a similar sedimentary thickness as its northern neighbours. Between Central and Çınarcık basins, the Central High rises up to 3 km depth below (bsl). Its crest position is offset by 10 km northwestward relatively to the bathymetric crest. On the contrary, Tekirda and Central basins appear linked, forming a 60-km-long basement depression. Beneath the bathymetric relief of Western High low velocities are observed down to 6 km depth (bsl) and no basement high have been found. The obtained 3-D Vp heterogeneity model allows the consideration of the 3-D supracrustal heterogeneity into the future earthquake relocations in this region. The topographic map of the pre-kinematic basement offers the possibility to take into account the locking depth variations in future geohazard estimations by geomechanical modelling in this region.