For most azimuths on both continental and oceanic stations, ambient noise is stronger in the winter than in the spring months. The azimuthal content of ambient noise changes less over time in the secondary microseism than in the primary microseism band Fig. In each of the six diagrams shown in Fig. Note that the yellow and red arrows for the secondary microseisms aim to mark the range over which the signal is the strongest instead of pointing to individual peaks as in the case of the primary microseisms. The back-projected paths and the potential source locations of these peaks will be discussed in Section 4, following.
The red, green and blue arrows in the primary microseism band as well as the red and green arrows in the secondary microseism band mark the azimuths with stronger noise during the winter months, while the yellow peaks in both period bands mark stronger noise during the spring months Fig. For the secondary microseism, the azimuthal distribution of ambient noise is temporally and spatially stable for both the OBS and continental stations.
For the primary microseism, the four peaks marked by arrows are well separated azimuthally. The back-projected great circle paths of these three sets of peaks are plotted in d with the same colours. Possible source regions for the secondary microseisms. The source regions marked here are approximate. Similar to Fig. The azimuth ranges of the SNR peaks are shown, respectively, with solid lines for OBS stations, long dashed lines for continental stations in the north, and short dashed lines for continental stations in the south.
The overlapping regions are coloured dark green and orange for the green and yellow arrows, respectively. Four questions motivate this paper, which we now discuss based on the observations presented in the earlier sections. As shown in Fig. The propagation of the secondary microseism, both on the continent and within the ocean, is principally eastward and displays little seasonal variation. In contrast, the azimuthal content of the primary microseism on the JdF plate and the northern and southern parts of the continent differ from one another.
The azimuthal content of ambient noise varies over time on the continent, with stronger propagation to the southeast and southwest during the winter and to the northeast during the summer. These observations imply well separated locations of generation of the primary and secondary microseisms.
As discussed further below, the secondary microseism appears to be generated far from the observing networks, in the open ocean of the northern Pacific, and the primary microseism appears to be derived both locally, in the shallower waters of the northeastern Pacific, and distantly, possibly from locations over broad regions of the Pacific and northern Atlantic Oceans. The principal direction of ambient noise in the secondary microseism band is generally to the east, as observed both in the ocean and on the continent and for all months considered.
Paths with the same colour are almost parallel to each other in Fig. Peaks with similar azimuthal contents are observed in the primary microseism band Fig. Overall, the strength of the secondary microseism we observe is relatively stable over time with winter months being slightly stronger. Earlier studies have shown similar seasonality for the secondary microseisms but, in some cases, with the strength being less stable over time e. Two factors may have contributed to this difference: 1 geographical factors in different study regions, such as the storm activities and bathymetry distribution, could have affected the observed signal strength, and 2 the strength of the secondary microseisms observed through ambient noise cross-correlations, as in our study, is homogenized by the pre-cross-correlation normalizations and the time averaging processes.
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The azimuthal content of the primary microseism is more complicated and not as easily explained with a small set of deep water source regions as the secondary microseism. In fact, the primary microseism appears to be generated in different areas than the secondary microseism. We discuss the possible source regions of the primary microseism in the next subsection. A more complicated picture emerges than for the secondary microseism, including what we interpret as evidence for both distant and relatively local sources.
Four observations are particularly noteworthy. The green arrows in Fig. This signal is probably caused by a North Atlantic source, as the back-projection in Fig. However, the source region or regions are too distant for us to determine if the primary and secondary microseisms are being generated in the same or different locations. The yellow arrows in Fig. The peak at the OBS stations Fig. Because the yellow arrows in Fig. However, because of the relatively low SNR level, the azimuthal content for these peaks is not precise and, as Fig.
Therefore, we believe that it is likely that this source is distant and in the Southern Hemisphere. We cannot determine if this is a deep water or shallow water source region. The three red paths in Fig. The intersection point is approximate and should not be interpreted to convey the source location accurately, but our estimate lies near the coast of British Columbia and Graham Island. The water depth in much of this area is shallower than 50 m and shallower still near Graham Island. The amplitude of the curves near the blue arrows are weaker compared to near the red arrows, the peak is not observed on the northern continental stations and its directionality is complicated, as seen in Fig.
We believe that this peak also originates locally in shallow waters, probably off the southern Oregon coast. Even though this signal is not obviously evident in the azimuthal plots such as those in Fig. In addition to the SNR of the observed surface wave signals, clear systematic precursory arrivals are observed for many station pairs.
These observations produce complementary evidence about the location of some microseism sources. Strong precursors appear near zero time on both the broadband and the primary microseism signals, but disappear when filtered into the secondary microseism band. Thus, such precursors are nearly entirely in the primary band. The time and duration of the precursor observed on this station pair is consistent with the source location identified by the blue ellipse in Fig.
Systematic observations of precursors from this source and two other source locations are presented in Fig.
Sources and Levels of Ambient Ocean Sound near the Antarctic Peninsula
All presented cross-correlations are between OBS and continental stations and are filtered in the period band 0. Example of precursory signals observed on ambient noise cross-correlations for station pair J42A—I05D. The precursory signal window is marked with a blue ellipse in each diagram and is associated with the blue source region of Fig. Background colour shows the group speed model used in predicting the precursory signals produced by these sources.
The two centre stations B05D and I05D are marked with orange triangles. Paths between station pairs that are shown in b and c are indicated with blue and red lines, respectively. Blue ellipses indicate the predicted precursory arrival times from the blue potential source location shown in a. Red and green ellipses are associated with the red and green potential source locations identified in a. The fact that the precursory signals appear in a systematic way in these record sections indicates that they are generated by localized sources and more than one such source region is needed.
To test this conclusion we assume three potential source regions as marked by red, green and blue ellipses in Fig. We then predict the arrival times of the precursory signals that would be generated on cross-correlations of ambient noise based on a simple group velocity model as indicated by the background colour in Fig.
Sources and Levels of Ambient Ocean Sound near the Antarctic Peninsula
Note that the duration of the predicted precursory arrival window is a function of the geometrical relation between the interstation path and the spatial extent of the source region. A larger source region tends to produce a larger precursory window, on average, but would not cause a wider window if the sources align with a hyperbola whose foci are the two station locations.
The predictions match most of the observed precursory signals regardless of the fact that the group speed model is an over simplification. On cross-correlations centred at station I05D, which is located in Central Oregon, the strongest precursory arrivals match predictions from the blue source region. Cross-correlations centred on station B05D, on the other hand, are more sensitive to energy from the red and green sources due to proximity. This suggests that the signals produced in these source regions are probably scattered and the direct arrival decays quickly with distance.
The locations of the red and blue source regions agree well with the azimuthal content of the fundamental surface waves as shown in Fig. The green source region, however, is not easily identified through its azimuthal dependence as it is possibly merged in azimuth with peaks from other source regions. The observed shallow water source locations and the discontinuity of sources along the coastline are consistent with earlier studies that have argued that the primary microseism sources are limited to certain coastal locations e.
A large number of small earthquakes magnitude 3—4 occurred near Graham Island and on the explorer plate, along the Blanco fracture zone, the Gorda Ridge, and the Mendocino fracture zone in the time period in which our cross-correlations are computed. Therefore, in principal, some of the precursory signals we observe may be earthquake generated. We believe, however, that this is not the case for two principal reasons. This is opposite of what would be expected if small local earthquakes were the source of the precursory arrivals as such earthquakes would generate stronger signals at shorter periods.
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Nonetheless, the amplitudes of the precursory signals, especially those associated with the blue source region, are affected by surrounding small earthquakes. As with the azimuthal content of the Rayleigh waves in the cross-correlations, the use of the arrival times of precursory signals is not a highly accurate means to determine source locations. Therefore, source regions other than the three discussed may and probably do exist.
An example of signals that are not accounted for can be seen in Fig. A formal analysis of the post-cursors, however, is beyond the scope of this paper. Moreover, the Rayleigh wave arrivals could be merged with the precursory signals produced by these localized sources, such as those shown in Fig.
Thus, additional denoising processing may be needed for studies aiming to investigate the shallow velocity structure in this region. In conclusion, local primary microseism sources are indicated by observations from the azimuthal dependence of the fundamental mode Rayleigh wave SNRs as well as observations from precursory signals.
The strongest local generation region is observed to the northwest of the JdF plate near the coast of British Columbia perhaps near Graham Island. Two weaker generation regions are observed in shallow waters near the western United States coastline, one near Vancouver Island and another along the coastline of southern Oregon.