The Effects of Local Structure on Seafloor Ambient Noise at the Hawaii-2 Observatory

Analysis of long time series, broadband (0.001-60 Hz) seismometer and hydrophone data from the Hawaii-2 Observatory reveals many time-independent characteristics in power spectral density and coherence that persist regardless of the type or location of the noise sources. These characteristics can be attributed to the water depth, sediment thickness, igneous crustal structure, and other geological features local to the observatory. It is important to recognize these characteristics as due to local structure so that they do not confuse the interpretation of noise generated by storms and earthquakes in terms of other physical processes such as infra-gravity wave excitation and propagation, wave-wave interaction, breaking waves, Rayleigh/Stoneley/Scholte wave effects, and propagation and leakage from the ocean wave guide. Some examples of local, physical mechanisms include: 1) shear wave resonances (modes) in sediments [24], 2) water multiples (organ pipe modes) in the ocean [31], and 3) secondary scattering of Scholte waves from local seafloor heterogeneities [23].