PROPERTIES OF SEISMIC FAULT ZONE WAVES AND THEIR UTILITY FOR IMAGING LOW-VELOCITY STRUCTURES

A two-dimensional solution for the scalar wave equation in a model of two vertical layers between two quarter spaces is used to study proper ties of seismic waves in a laterally heterogeneous low-velocity struct ure. The waves, referred to as seismic fault zone waves, include head waves, internal fault zone reflections, and trapped waves. The analysi s aims to clarify the dependency of the waves on media velocities, med ia attenuation coefficients, layer widths, and source-receiver geometr y. Additional calculations with extreme low-velocity layers provide ex amples that may be relevant for volcanic and geothermal domains. The i nterference patterns controlling seismic fault zone waves change with the number of internal reflections in the low-velocity structure. This number increases with propagation distance along the structure, decre ases with fault zone width, and increases (for given length scales) wi th the velocity contrast. The relative lateral position of the source within the low-velocity layer modifies the length scales associated wi th internal reflections and influences the resulting interference patt ern. Low values of e affect considerably the dominant period and overa ll duration of the waves. Thus there are significant tradeoffs between propagation distance along the structure, fault zone width, velocity contrast, source location within the fault zone, and e. The lateral an d depth receiver coordinates determine the particular point where the interference pattern is sampled and observed motion is a strong functi on of both coordinates. The zone connecting sources generating fault z one waves and observation points with appreciable wave amplitude can b e over an order of magnitude larger than the fault zone width. Calcula tions for cases with layer P wave velocity of similar to 200 m sl, mod eling a vertical dike or crack with fluid and gas, show conspicuous pe rsisting oscillations. The results resemble aspects of seismic data in volcanic domains. If these waves exist in observed records, their exp licit recognition and modeling will help to separate source and struct ural effects and aid in the interpretation of volcano-seismology signa ls. Although the tradeoffs in parameters governing seismic fault zone waves are significant, each variable has its own signature, and the pa rameters may be constrained by additional geophysical data. Simultaneo us modeling of many waveforms with an appropriate solution can resolve the various parameters and provide a high-resolution structural image .