The Airy phase in oceanic Rayleigh and Scholte waves

Both oceanic Rayleigh waves and seafloor Scholte waves frequently exhibit a prominent Airy phase because the amplitude reduction by geometric spreading is reduced for this arrival. Scholte waves travel as waves trapped by the low shear velocities of the seafloor, and contain detailed information about the mechanical properties of the surficial material. Although it is possible to model Scholte waves by waveform fitting (Nolet and Dorman, GJRAS, 1996) much Scholte wave data is not of such purity to allow fitting by a laterally uniform model, which is at the present limit of what can be easily synthesized. The next best thing to waveform systhesis is matching the observed group velocity to a synthetic one. In the many cases for Scholte waves, the group slowness maximum of the prominent Airy peak is easily interpretable as the slowness of a thin surficial layer of low velocity (the top few meters). In the case of oceanic Rayleigh waves, the Airy phase is caused by a slowness minimum in group slowness which has a particularly simple form, being well-matched to an upward-open parabola in a plot of slowness vs frequency. This can provide a good match over a frequency range of a factor of three (Savage, JGR, 1969). While this representation provides a simple and accurate parameterization, to the observations this seems not to say anything simple about the earth models which produce it. The data kernels for the observations do not have particularly simple forms and extend over the top 50km or so.