Coincident normal-incidence and wide-angle reflections from the Moho: Evidence for crustal seismic anisotropy

We explore the consequences of interpreting wide-angle crustal seismic data assuming isotropic models when the real crust may be anisotropic. We have used a simple anisotropic model to generate synthetic travel-times and have inverted these assuming an isotropic layered model for the crust. We show that neglecting the effects of anisotropy can produce estimates of crustal thickness and crustal velocity structure that are significantly in error. For realistic levels of anisotropy (similar to 10%) the error in crustal thickness can be several kilometres, and the error in average velocity can be similar to 0.5 km/s. We show that isotropic inversion of anisotropic data can lead to an apparent mismatch between the position of the Moho inferred from normal-incidence and wide-angle data. As an example of such a mismatch we show data acquired over the continental crust offshore the north of Scotland. The dataset we have modelled includes a wide-angle expanding-spread profile, a conventional ocean-bottom seismometer profile, a high-resolution wide-angle onshore-offshore experiment, a synthetic-aperture 16-km offset CDP profile, and a conventional deep reflection profile. These data show unusually sharp, bright and continuous reflections from the Moho at all offsets. The normal-incidence reflection Moho and the Moho modelled from the wide-angle data both show the same lateral structure, however, they are offset one from the other in normal-incidence two-way travel-time. This mismatch is considerably larger than the maximum error expected in the wide-angle model, and is much greater than the accuracy with which the Moho can be picked on the reflection data. If we assume that this mismatch is caused by crustal-scale seismic anisotropy, then the synthetic results indicate that anisotropy in the crust north of Scotland is about 7%.