The bandwidth and the duration of incident pulsed beam are proved to play important roles in modifying the nonlinear image of amplitude-type scatterer. It is found that the initially positive chirp-type bandwidth can suppress the nonlinear image, while the negative one can enhance it, and that both effects are inversely proportional to the incident pulse duration. Numerical simulations further demonstrate that the location of nonlinear image is at the conjugate plane of the scatterer and that, for negatively pre-chirped pulsed beam, the nonlinear image peak intensity can be higher than that in the corresponding monochromatic case under certain conditions. Moreover the effect of group velocity dispersion on nonlinear image is found to be similar to that of chirp-type bandwidth. 相似文献
The serpentinized peridotites overlying the subducted zones in the Izu-Bonin-Mariana (IBM) arc system have been interpret as the cause of the low-velocity layer identified beneath the IBM froearc, in turn few earthquakes occurred along the plate boundary. Chrysotile, which is a low temperature and highly hydrated phase of serpentine with low frictional strength, has been suggested as the low velocity material in the serpentinized peridotites, besides, brucite is inferred to be likely conducive to stable sliding. However, such idea encounters challenging in our serpentinized peridotites from the southern Mariana forearc, which absent both the above minerals. The presence of talc, which characterized by its weak, low-friction and inherently stable sliding behavior, provides new clue. Here we report the occurrence of talc in serpentinized peridotites collected from the landward trench slope of the southern Mariana forearc. We infer that talc is mainly forming as a result of the reaction of serpentine minerals with silica-saturated fluids released from the subducting slab, and talc also occurs as talc veins sometimes. Due to its unique physical properties, talc may therefore play a significant role in aseismic slip in the IBM subduction zone.
