*4.4. Rayleigh Wave Refraction*

According to the hypothesis and the discussion above, the DF microseisms in the LPDF (DF1) band is generated in the deep ocean close to the continental slope and propagate to stations on land. Depending on the location of the stations, the propagation paths are different, i.e., through sediments and bedrock to stations on inland bedrock, and through sediments on coastal stations on sediments, resulting in different attenuations and energy levels at stations. This difference is observed in this study. In Figure 3, the PSD levels in the DF1 band increase from coast (sediments) to inland (Appalachian Mountain) while the distance to source (ocean) increases. The possible explanation is that the attenuation due to spherical spreading is less effective than energy absorption in sediments. A similar trend is observed by [55] in the same study area. In the DF2 band, the PSD levels at coastal stations are slightly larger than or equal to those at inland stations, which might because the attenuation due to spherical spreading and absorption in sediments are equally effective. In the DF3 band, the PSD levels at coastal stations are obviously larger than those at inland stations, as the attenuation is dominantly spherical spreading and absorption effect is equally effective, which reveals that the DF microseisms in this band are generated in the continental shelf and propagate mainly in sediments.
