Directionality of ambient noise on the Juan de Fuca plate: implications for source locations of the primary and secondary microseisms

Based on cross-correlations of ambient seismic noise computed using 61 ocean bottom seismometers (OBSs) within the Juan de Fuca (JdF) plate from the Cascadia Initiative experiment and 42 continental stations near the coast of the western United States, we investigate the locations of generation of the primary (11-20 s period) and secondary (5-10 s period) microseisms in the northern Pacific Ocean by analysing the directionality and seasonality of the microseism (Rayleigh wave) signals received in this region. We conclude that (1) the ambient noise observed across the array is much different in the primary and secondary microseism bands, both in its azimuthal content and seasonal variation. (2) The principal secondary microseism signals propagate towards the east, consistent with their generation in deep waters of the North Pacific, perhaps coincident both with the region of observed body wave excitation and the predicted wave-wave interaction region from recent studies. (3) The primary microseism, as indicated by observations of the azimuthal dependence of the fundamental mode Rayleigh wave as well as observations of precursory arrivals, derives significantly from the shallow waters of the eastern Pacific near to the JdF plate but also has a component generated at greater distance of unknown origin. (4) These observations suggest different physical mechanisms for generating the two microseisms: the secondary microseisms are likely to be generated by non-linear wave-wave interaction over the deep Pacific Ocean, while the primary microseism may couple directly into the solid earth locally in shallow waters from ocean gravity waves. (5) Above 5 s period, high quality empirical Green's functions are observed from cross-correlations between deep water OBSs and continental stations, which illustrates that microseisms propagate efficiently from either deep or shallow water source regions onto the continent and are well recorded by continental seismic stations.