During the summers of 1982 and 1983, field work was completed on 10 new seismic refraction profiles in the Long Valley-Mono Craters region located within the tectonically active zone between the eastern Sierra Nevada and the Basin and Range province. The four profiles presented here sample the upper 7--10 km of the crust within the western half of Long Valley caldera, the Mono craters ring fracture system, and the ''normal'' crust north and northeast of Long Valley. The entire area shares a common crystalline basement consisting primarily of Mesozoic granite rocks. At a depth of 2 km beneath the topographic surface, the P wave velocity is uniformly 5.6 km/s beneath which the velocity increases with a gradient of about 0.1 s-1. Clear secondary arrivals observed on a profile crossing the west part of the caldera support evidence from 1972 seismic refraction profiles for a reflecting boundary at a depth of 7--8 km beneath the west margin of the resurgent dome, which may represent the top of a magma chamber. Structural details resolved in the upper 2 km of the crust lead to the following conclusions: (1) P wave velocities in the crystalline basement at depths shallower than 2 km vary from 3.6 to 5.0 km/s. This variation reflects differences in the degree of fracturing of the upper portion of the basement. (2) An apparently homogeneous basement in the upper 7--10 km beneath the Mono Craters ring fracture system indicates that any extensive magma chamber must be as least 10 km deep there. (3) The downdropped crystalline basement beneath the western half of Long Valley caldera dips genty (5¿--10¿) to the northeast. Apparent offsets along steeply dipping caldera-bounding faults are 1.0--1.7 km along the northern, western, and southern sides of the caldera with largest offsets in the north. (4) Rocks forming the caldera fill fall into essentially three P wave velocity groups.

From the surface downward these velocity groups are 100--400 of 1.2--1.8 km/s material corresponding to unconsolidated fluvial or lacustrine deposits or highly fractured rhyolite flows, 200--400 m of 2.8--3.1 km/s material corresponding to sparsely jointed postcaldera rhyolite, rhyodacite and basalt flows, and approximately 1000 m of 3.9--4.4 km/s material corresponding to deposits of Bishop Tuff.