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We report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the Mid‐Atlantic Ridge segment at 29°N and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes. The seismic velocity structure is obtained from two‐dimensional (2‐D) and three‐dimensional (3‐D) tomographic inversions of travel times from shots along an axial refraction line. Inversion solutions indicate that the velocity structure in the lower crust is heterogeneous, with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high. The thickness of the lower crust at the segment end is asymmetric across axis, with thinner crust beneath the inside corner. The indicated variations in crustal thickness are consistent with those inferred from mantle Bouguer gravity …