ANISOTROPIC SEISMIC RESPONSE OF CRUSTAL TECTONIC FEATURES

Tectonic terranes within the crust often possess compositional changes and internal fabrics due to the deformational and metamorphic processes associated with tectonic modification. These material traits (composition, structural geometry, internal fabric) may alter seismic wave propagation as evidenced by anisotropic particle motions, travel time delays, and phase splitting. In addition, internal deformational fabrics may be oriented oblique to geological layer boundaries. All of these effects may produce anisotropic wave propagation.

Observations of seismic anisotropy produced from within the crystalline crust are becoming increasingly common. Studies of crustal anisotropy are more complex because of the presence of pronounced geological heterogeneity and changing raypath orientations due to source-receiver configurations and strong seismic velocity gradients. As a result, calculations of anisotropic effects are not simple in areas of complex crystalline crust and in some cases may only be solved numerically. We have developed computer code to generate synthetic seismograms allowing for full 3D variability in severity and orientation (tilt) of anisotropic material properties as expressed in composition, large-scale features, and internal textural fabrics.

We present the synthetic seismogram response for crustal features of selected geometries, in particular vertical exhumation (Central Ranges, Taiwan) and antiformal doming (Nanga Parbat, Pakistan). We address two primary issues: (1) is the anisotropy observable within the seismograms, and (2) in sufficiently dense "arrays" are there systematics in the P- and/or S-wave phases which can be used to characterize the crustal feature. The selected features are calibrated using petrophysically measured velocities and densities. We also discuss relationships between detectable anisotropy effects and internal configurations of the crustal features.