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Data from a microearthquake experiment in the median valley of the Mid‐Atlantic Ridge near 23°N in 1982 are used to measure earthquake source parameters, to determine the laterally heterogeneous seismic velocity structure across the inner floor, and to develop a kinematic tectonic model for this portion of the median valley. Fifty‐three microearthquakes occurred over a 10‐day period beneath the median valley inner floor and eastern rift mountains. Twenty of 23 well‐located inner floor epicenters define a line of activity, about 17 km long, having a strike of N25°E and located near an along‐axis depression some 300–400 m deeper than surrounding regions. Earthquakes with well‐resolved hypocenters generally have focal depths of 4–8 km beneath the seafloor of both the inner floor and the rift mountains; the hypocentral locations are robust with respect to plausible lateral variations in seismic velocity structure …