Ray tracing for surface waves

Surface waves (at least the fundamental mode, see below) are the latest arrivals on the seismic record at a given epicentral distance. On classical records, they show up as longperiod oscillations, predominating in the record for shallow earthquakes and distances larger than a thousand kilometers. Their relative importance increases with the distance, which can be explained by their 2-dimensional propagation in a direction parallel to the surface of the Earth. Indeed they are guided waves with standing wave properties in a direction normal to this surface: along this direction the displacements are in phase at all depths. Thus their geometrical spreading, as guided waves, is less than that of body waves which propagate in 3 dimensions.