Numerical Simulation of Fault Zone Guided Waves: Accuracy and 3-D Effects HEINER IGEL, 1 GUNNAR JAHNKE, 1 and YEHUDA BEN-ZION 2 Abstract — Fault zones are thought to consist of regions with reduced seismic velocity. When sources are located in or close to these low-velocity zones, guided seismic head and trapped waves are generated which may be indicative of the structure of fault zones at depth. Observations above several fault zones suggest that they are common features of near fault radiation, yet their interpretation may be highly ambiguous. Analytical methods have been developed to calculate synthetic seismograms for sources in fault zones as well as at the material discontinuities. These solutions can be used for accurate modeling of wave propagation in plane-parallel layered fault zone structures. However, at present it is not clear how modest deviations from such simplified geometries affect the generation efficiency and observations of trapped wave motion. As more complicated models cannot be solved by analytical means, numerical methods must be employed. In this paper we discuss 3-D finite-difference calculations of waves in modestly irregular fault zone structures. We investigate the accuracy of the numerical solutions for sources at material interfaces and discuss some dominant effects of 3-D structures. We also show that simple mathematical operations on 2-D solutions generated with line sources allow accurate modeling of 3-D wave propagation produced by point sources. The discussed simulations indicate that structural discontinuities of the fault zone (e.g., fault offsets) larger than the fault zone width affect significantly the trapping efficiency, while vertical properly gradients, fault zone narrowing with depth, small-scale structures, and moderate geometrical variations do not. The results also show that sources located with appropriate orientations outside and below a shallow fault zone layer can produce considerable guided wave energy in the overlying fault zone layer. Key words: Fault zones, guided waves, finite differences. Introduction Fault zone structures are thought to contain a highly damaged material having lower seismic velocity than the surrounding rocks. If the highly damaged fault zone material is spatially coherent it can act as a waveguide for seismic fault zone head and trapped waves. Fault zone head waves propagate along material discontinuity interfaces in the structure, while trapped waves are critically reflected phases 1 Institut fu¨r Geophysik, Ludwig-Maximilians-Universita¨ t, Theresienstrasse 41, 80333 Munich, Germany. E-mails: igel@geophysik.uni-muenchen.de; jahnke@geophysik.uni-muenchen.de 2 Department of Earth Sciences, University of Sourthern California, Los Angeles, U.S.A. E-mail: benzion@terra.usc.edu Corresponding author: Prof. Dr. Heiner Igel Pure appl. geophys. 159 (2002) 2067–2083 0033 – 4553/02/092067 – 17 $ 1.50 + 0.20/0 Ó Birkha ¨ user Verlag, Basel, 2002 Pure and Applied Geophysics