Abstract

Seismic reflection records of the 1986 Nevada PASSCAL seismic experiment in the roughly east-west direction (dip line) show a highly reflective crust with a broad (diffuse) Moho which is characterized at normal incidence by strong, subparallel, laterally discontinuous, multicyclic events lacking observable diffractions. Recordings in the roughly north-south direction (strike line) show a transparent crust with short scattered events and a strong continuous Moho. Elastic finite-difference modeling indicates that (1) effects due to the topog-raphy of the reflecting interfaces and/or the existence of heterogeneities increase the traveltimes, thus lowering the velocity estimates, and (2) the seismic response of a layered structure can be disrupted from a strong, laterally continuous event to a series of short multicyclic events by increasing the dimensions of the overlying heterogeneities. Finite-difference modeling demonstrates that large-scale anisotropy, achieved by roughly east-west alignment of strongly elongated lenses (boudins) located at mid- and lower-crustal depths, can explain the observed differences in the seismic response along the perpendicular lines.

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