Comparison of residual shear strength determined by different methods

The shear stress of stiff or dense soils increases with the displacement and reaches itsmaximum value, and then shear stress decreases and remains a constant value. The minimumand constant shear stress of soils reached at large shear displacements is called as residualshear strength. Residual shear strength generally has a great importance in design ofengineering structures constructed on fissured overconsolidated clays and long-term slopestability analysis in geotechnical engineering. In laboratory testing, modeling the residualconditions of a soil requires large shear displacements attained in drained conditions.Reversal direct shear test (RDS), consolidated-drained triaxial test (CD) and torsional ringshear test (RS) are the widely used testing methods to determine residual shear strengthparameters. These methods have some advantages or limitations when compared with eachother. In this study, residual shear strength parameters of soil samples having different clayfractions were determined by the three different drained tests, the results were compared, andeffect of the testing methods on residual shear strength was investigated. The variation ofresidual shear strength angle versus liquid limit and plasticity index were studied. The resultswere compared with previous studies. As a result, it is found that the residual shear strengthangle determined by the ring shear test is lower than the others, and the residual shear strengthangle decreases with the increasing liquid limit and plasticity index

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, Tekirdağ 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

North Marmara Trough architecture of basin infill, basement and faults, from PSDM reflection and OBS refraction seismics

International audienceAbstract The reflection and refraction seismic data collected during the SEISMARMARA Leg 1 survey in the Sea of Marmara provide detailed imaging of sedimentary record and fault activity with deep penetration into its basement. First, a detailed analysis of pre-stack depth-migrated seismic lines crossing the Central Basin enable us to discuss the space and time relations of the large and smaller nested basins of the inner depression, as well as the diversity of style and rate of activity of motion at the diverse basin border faults. Second, forward modeling of OBS refraction arrival times reveals the effect of compaction on the sedimentary pile whereas its layering imaged by MCS as seismic reflectors rather recorded the tectonic evolution. Another major result of the refraction modeling is the identification of the crystalline basement. The latter is imaged about 1 or 2 km deeper than the base of the layered sedimentary sequence imaged on the coincident MCS profiles. This basement exhibits sharp topography across the Central High, the Kumburgaz Basin and the eastern tip of the Cinarcik–North Imrali Basin in an unexpected way with respect to the sea-bottom depressions. We furthermore imaged several large tilted basement blocks, which separate the deep basins as between the Cinarcik and Imrali basins. Despite the varying width of the NMT and the sizes of the tilted blocks, we propose that the imaged finite deformation results from a similar process of partitioning deformation over more than one or even two faults across the NMT that may have changed activity with time and space