Early high-frequency precursors, which arrive before the onset of P waves, have been observed at some subducted zones. Seismologists considered this phenomenon to be the result of high-frequency seismic waves propagation in a high-velocity layer (HVL). Although some numerical simulations have been used to explain the existence of early high-frequency precursors, a high frequency 3-D elastic model has not been considered.
In this high frequency 3-D physical modeling study, thin duralumin plates of different thicknesses, embedded in epoxy blocks, were used to study the elastic wave propagation in a thin HVL. The experimental results reveal that high-velocity and high-frequency precursors can be generated using a thin HVL model, and the high-frequency precursors arrive earlier than low-frequency ones. It was also observed that the thinner the HVL, the smaller the amplitudes and higher the frequency of the precursors. Based on the travel times and amplitudes of the precursors, it can be concluded that P-to-P (P converts to P) interactions play a major role in controlling the propagation of precursors. The precursors are the P waves resonance in the HVL; therefore, the curvature and the period of the holes (the local low values) in the dispersion curve may be used to estimate the thickness of the HVL.