Abstract
An image of group velocity surfaces within and outside a cuspidal region of a (0001) oriented zinc crystal is obtained with a pointlike scanning laser source and a fixed longitudinal (L) mode piezoelectric detector. Because of axisymmetric excitation and sensing, the group velocity surface of the pure transverse (PT) mode, being shear horizontally (SH) polarized, is absent in the obtained image. In the cuspidal region the quasi transverse (QT) mode has three group velocity branches in a given direction: the fast QT (FQT) branch, the intermediate speed QT (IQT) branch, and the slow QT (SQT) branch. The laser imaging clearly shows the wave fronts of the quasi longitudinal (QL), IQT, and SQT rays in the cuspidal region. Near the epicentral [0001] direction, the arrivals of the head wave (HW) and SQT rays are almost simultaneous and their wave fronts are not distinct. However, farther from the epicentral direction, they separate and outside the cuspidal region the imaging shows the group velocity surfaces of the QL, HW, and SQT rays.
Published Version
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