A compressional-mode interface wave propagating along a fracture has been observed in a fracture in limestone. This compressional-mode interface wave co-propagates with the bulk compressional wave, is localized to the region surrounding the fracture, and has particle motions containing both transverse and longitudinal displacements. As stress across the fracture increases, the wave energy shifts to earlier arrival times, and is accompanied by a decrease in the transverse displacement and an increase in the longitudinal displacement of the particle motion.¿ 1997 American Geophysical Union