Directionality of Ambient Noise on the Juan de Fuca Plate: Implications for Source Locations of the Primary and Secondary Microseism

Based on cross-correlations computed from 61 ocean bottom seismometers (OBSs) within the Juan de Fuca plate from the Cascadia Initiative experiment and 42 continental stations near the western US coast, we investigate the generation locations of the primary (11-20 sec period) and secondary (5-10 sec period) microseisms in the northern Pacific Ocean by analyzing the directionality of the microseism signals received in this region. (1) Ambient noise observed across the array is much different in the primary and secondary microseism bands, both in its azimuthal content and seasonal variation, indicating different source generation locations. (2) The principal secondary microseism signals propagate toward the east, consistent with source generation in deep water of the North Pacific, perhaps coincident with the region of body wave excitation observed by Gerstoft et al. [2008] and Landès et al. [2010]. (3) Local primary microseism sources within and near the Juan de Fuca plate are implied by observations of the azimuthal dependence of the fundamental mode Rayleigh wave amplitudes as well as observations of precursory arrivals in cross-correlations of ambient noise. The strongest local generation region is observed northwest of the Juan de Fuca plate near the coast of British Columbia perhaps near Graham Island. Weaker local sources appear to be oceanward of Vancouver Island and southern Oregon. (4) High quality Green's functions are derived from cross-correlations between deep water OBSs and continental stations proving that deep water generated signals can efficiently propagate onto the continent and are well recorded by continental seismic stations, at least at periods longer than about 5 sec.In conclusion, the primary and secondary microseisms are generated at different locations, with the secondary microseism dominantly coming from deep-water sources and the source of primary microseism having a significant component in the shallow waters of the eastern Pacific. These observations are consistent with different physical mechanisms for the generation of the two microseisms. Both microseisms above 5 sec period propagate efficiently from either deep or shallow water source regions to continental stations.