Previous work has shown that earthquake S-waves can become fault zone guided-waves. We present observational evidence for a second type of earthquake-generated, fault-guided seismic wave, apparently involving coupled P- and S-waves. Our data are from a fault located in igneous rocks in Owens Valley, California. In analogy to standard Love and Rayleigh waves, we suggest that purely S-type modes be indicated on seismograms by ''FL'' and the new coupled P- and S-type by ''FR.'' Since FR waves are sensitive to both the local P- and S-wave velocities, their presence implies unique conditions in and near the fault. Modeling of our seismograms suggests a differential reduction in these velocities, with a larger change in bulk modulus than in shear modulus. We propose that this reduction is due to unsaturated fractures within the core of the fault. We view FR waves as a new tool for detecting and characterizing highly fractured fault zones, providing important information on the relation of earthquakes and subsurface fluid conduits. ¿ American Geophysical Union 1996