F-R-WAVE - A 2ND FAULT-GUIDED MODE WITH IMPLICATIONS FOR FAULT PROPERTY STUDIES

Previous work has shown that earthquake S-waves can become fault zone guided-waves. We present observational evidence for a second type of e arthquake-generated, fault-guided seismic wave, apparently involving c oupled P- and S-waves. Our data are from a fault located in igneous ro cks in Owens Valley, California. In analogy to standard Love and Rayle igh waves, we suggest that purely S-type modes be indicated on seismog rams by ''F-L'' and the new coupled P- and S-type by ''F-R''. Since F- R waves are sensitive to both the local P- and S-wave velocities, thei r presence implies unique conditions in and near the fault. Modeling o f our seismograms suggests a differential reduction in these velocitie s, with a larger change in bulk modulus than in shear modulus. We prop ose that this reduction is due to unsaturated fractures within the cor e of the fault. We view F-R waves as a new tool for detecting and char acterizing highly fractured fault zones, providing important informati on on the relation of earthquakes and subsurface fluid conduits.