Results of detailed seismic reflection and refraction measurements over a test area 100X100 km in the central part of the East Mariana Basin (with a floor age of 160 Ma) are presented. The depth of the basement with respect to the surface of the floor (sediment thickness) range from 900 to 1400 m. An averaged gradient-layer model is suggested consisting of layers 2, 3A, and 3B of the earth's crust and layer M of the upper mantle. They are characterized by longitudinal seismic wave velocities of 2.0, 5.7, 6.5, 7.65, and 8.2 km/s, thicknesses of 0.6, 1.5, 1.9, and 2.2 km, and vertical velocity gradients of 0.6, 0.3, 0.15, and 0.02 s (super -1) , respectively. The over-all thickness of the earth's crust comprises 6.2 km. Spatial variations of seismic velocities and thicknesses of the principal seismic layers are estimated. Fine structure of the second crustal layer is resolved; it is subdivided into three sub-layers (2A, 2B, and 2C), characterized by the ranges of changes of velocities and thicknesses of 3.26-3.89, 4.46-5.07, and 5.75-6.33 km/s and 0.28-0.66, 0.55-0.97, and 0.21-0.76 km, respectively. Anisotropy of seismic velocities in the upper part of the crust is revealed: higher velocities are observed in the northwest-southeast direction, i.e., parallel to the extension of the Magellan seamount chain.