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GEOPHYSICAL RESEARCH LETTERS, VOL. 23, NO. 24, PAGES 3547–3550, 1996

F_R Waves: A Second Fault-Guided Mode with Implications for Fault Property Studies

P. E. Malin

Department of Geology, Duke University, Durham, NC

M. Lou

Department of Geology, Duke University, Durham, NC

J. A. Rial

Department of Geology, University of North Carolina at Chapel Hill

Abstract

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 "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, 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 F_R 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.

Received 17 September 1996; accepted 18 October 1996.

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Citation: Malin, P. E., M. Lou, and J. A. Rial (1996), F_R Waves: A Second Fault-Guided Mode with Implications for Fault Property Studies, Geophys. Res. Lett., 23(24), 3547–3550.

Copyright 1996 by the American Geophysical Union.