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

Abstract

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 Ç ınarcık 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, Tekirdag 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.

The geometry of the North Anatolian transform fault in the Sea of Marmara and its temporal evolution: implications for the development of intracontinental transform faults

“…In fact, the entire geometry of the Western High, with its flanking basins, gives the impression of a shortening structure that originally consisted of a broad synclinal basin, in the middle of which an anticline later rose. This is also supported by the topography of the sediment/acoustic basement interface, which reveals a deep trough which extends from the Central Basin towards the east and to the Tekirdag Basin towards the West, such as Bayrakçı et al (2013) recently depicted. It seems that the throughgoing faulting chose the anticline to follow and in so doing sliced it with small and steep thrusts.…”

Offshore seismicity in the western Marmara Sea, Turkey, revealed by ocean bottom observation

“…Since the obtained value of one KOERI-catalogued earthquake consists with the local magnitude determined by KOERI, we consider the magnitude of newly identified events in this study as reasonable, and these events were too small to detect by using only land seismic stations. The average errors Gurbuz et al (2000) and red line shows our modification of that model based on the model of Bayrakci et al (2013) of calculated hypocenters were about 0.75 km horizontally and 1.25 km vertically. We also recalculated the hypocenters for each of the earthquakes in the KOERI earthquake catalogue during the time of our observations by combining the land data and our OBS data.…”

“…b Location and year of occurrence of past large earthquakes along the NAF We calculated hypocenters by using the HYPOMH program (Hirata and Matsu'ura 1987). In this region, Gurbuz et al (2000) previously developed a minimum 1-D velocity model for land seismic stations by using the VELEST program (Kissling et al 1994), and Bayrakci et al (2013) developed a detailed shallow offshore Pwave structure based on data from an active-source seismic survey. We established our 1-D P-wave velocity model by combining these two previous velocity models (Fig.…”

Investigating P- and S-wave velocity structure beneath the Marmara region (Turkey) and the surrounding area from local earthquake tomography

“…3), which is seen especially well in the S-wave velocity model at shallow depths. We suggest that it might correspond to the thick sedimentary deposits of the Plio-Pleistocene or to the alluvium regions as supported by the low resistivity and gravity values (Kaya et al 2013;Bayrakci et al 2013).…”

“…The projects' profiles give information about depth variations in the basement of up to 7 km between the North Marmara Trough (NMT) and its surroundings. Bayrakci et al (2013) used the 16,000 first arrival times of artificial sources and inverted them using the well-known local earthquake tomography (LET) code Simulps (Thurber 1983(Thurber , 1993 to find out the 3D upper-crustal heterogeneity of the NMT. Although these studies cover a similar area, their results often show considerable discrepancies.…”