Coupled modes at interfaces: A review

Abstract

Wave-to-wave coupling may arise when an acoustic pulse selects a Rayleigh mode of the same speed and both travel together swapping energy across an interface. A distinctive coupled signal called Ti was observed at the Hawaii-2 Observatory from earthquakes on the Blanco and Mendocino Fracture Zones, off the coast of North America. The signal travels along a purely oceanic path; it appears to be a composite of undispersed higher Rayleigh modes propagating along the ocean floor both in the sediments and in the water. Coupled modes may be identified by their frequency composition and by their phase and group velocities. Seismoacoustic coupling at the seafloor is conditioned on (a) the presence of a low-velocity interface at the ocean floor, (b) the wavelength of the Rayleigh mode being shorter than the depth of the water layer, and (c) weak stress-strain nonlinearity at the interface. It is conjectured that coupled interface waves may exist at other interfaces, including the Moho and the core-mantle boundary.