The catastrophic rupture of the North Anatolian Fault east of the Marmara Sea on 17 August 1999 highlighted a need for mapping the underwater extension of that continental transform. A new bathymetric map of Izmit Gulf indicates that the fault follows the axis of the gulf with a few minor bends. Submerged shorelines and shelf breaks that formed during the Last Glacial Maximum provide markers to quantify vertical deformation. Variable tilting of these horizons reveals that vertical deformation is highest just south of the fault. A correlation between vertical deformation of the southern fault block and distance to fault bends can be accounted for by a fault dipping steeply to the south. Hence subsidence (uplift) of the southern, hanging wall block would be expected where the fault strikes at a slightly transtensional (transpressional) orientation to relative plate motion. Subsidence reaches about 8 mm/yr west of the town of Golcuk and might be accommodated in 1–2 m subsidence events during large earthquakes. That scenario is compatible with the tsunami runups and the coseismic subsidence of the southern shore that occurred in 1999. Seafloor morphology also suggests that earthquakes are accompanied by widespread gas and fluid release. The periphery of the deepest basin displays a hummocky texture diagnostic of sediment fluidization, and mud volcanoes occur west of Hersek peninsula that might be activated by earthquakes. Finally, the backscatter imagery reveals a series of lineaments midway through the gulf that are interpreted as products of the 1999 surface rupture. The seafloor is undisturbed farther west, suggesting that surface slip decreased to an insignificant level beyond Hersek. Possibly, the stress shadow from the 10 July 1894 earthquake, which was felt strongly along the western Izmit Gulf, contributed to arrest the 1999 surface rupture.

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[1] How oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the Earth's interior and the oceans. Although considerable work has been done characterizing the evolution of seismic layer 2A, there has been little analysis of the processes that affect layer 2B after formation. Here we present high-resolution 2-D tomographic models of seismic layer 2B along ∼300 km long multichannel seismic transects crossing the Endeavour, Northern Symmetric, and Cleft segments of the Juan de Fuca Ridge. These models show that seismic layer 2B evolves rapidly following a different course than layer 2A. The upper layer 2B velocities increase on average by 0.8 km/s and reach a generally constant velocity of 5.2 ± 0.3 km/s within the first 0.5 Myr after crustal formation. This suggests that the strongest impact on layer 2B evolution may be that of mineral precipitation due to "active" hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber. Variations in upper layer 2B velocity with age at time scales ≥0.5 Ma show correlation about the ridge axis indicating that in the long term, crustal accretion processes affect both sides of the ridge axis in a similar way. Below the 0.5 Ma threshold, differences in 2B velocity are likely imprinted during crustal formation or early crustal evolution. Layer 2B velocities at propagator wakes (5.0 ± 0.2 km/s), where enhanced faulting and cracking are expected, and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2B. Analysis of the layer 2B velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2B velocities, respectively. Additionally, the distance between present up-flow and down-flow zones is related to the amount of sediment cover because, as sediment cover increases and basement outcrops become