As a tensile fracture was induced in a sand-stone core sample under compressional loading, compressional waves were propagated parallel to the plane of tensile stress. Long before catastrophic failure, when a macroscopic fracture was formed, the energy in the compressional wave showed a dramatic shift in spectral frequency from 0.644 MHz to 1 MHz. This frequency shift is a signature of the partitioning of energy from a compressional body wave into a compressional-mode interface wave. The presence of this signature before failure suggests the presence of an incipient interface wave that is supported by a network of oriented but disconnected microcracks. ¿ 2000 American Geophysical Union