Abstract
AbstractPrimary and secondary microseism originating in the world oceans and peaking at around 14 and 7 s, respectively, characterize the Earth's background noise in that frequency range. Microseism generated in marginal seas with partly shorter periods and higher spatial and temporal variability is less studied and requires stations in immediate proximity to the source to be observed. Such studies can help to elucidate the exact microseism generation areas and mechanisms in a constrained area. We analyze 15 years of broadband data recorded at the seismic station on Helgoland island in the marginal North Sea. In addition to remote primary (RPM) and secondary microseism (RSM) originating in the North Atlantic, we observe strong and dominant local secondary microseism (LSM) with on average higher frequencies above 0.2 Hz, in accordance with shorter wave periods of about 4–8 s in the shallow North Sea. During times with low RSM activity we observe local primary microseism (LPM) at frequencies in agreement with local ocean wave periods. The higher horizontal to vertical (H/V) ratio of LPM with respect to LSM indicates a major non‐Rayleigh wave contribution. LSM and LPM show a strong modulation with local semidiurnal ocean tides and microseism energy maxima preceding the water level maximum by 2.5 and 1.5 hr, respectively. This time shift might be influenced by stronger currents during rising than falling tides. Active sources of tide‐modulated microseism migrate along the North Sea coast in sync with the ocean tidal signal as evidenced by comparison of LSM maxima at stations distributed along the coast.
Highlights
The global ambient noise field in the frequency band below 1 Hz is dominated by two distinct amplitude peaks (McNamara & Buland, 2004; Peterson, 1993)
While local secondary microseism (LSM) has been described as short‐period secondary microseism (SPSM) in former studies (Beucler et al, 2015; Chen et al, 2011), we identify local primary microseism (LPM) in this marginal sea setting
For the island Station HLG in the German Bight region of the southern North Sea remote primary (RPM), RSM, LPM, and LSM contributions to the microseism wavefield were identified by means of their respective frequency content, the correlation with significant wave height (SWH) in the vicinity of Helgoland and the North Atlantic, and their horizontal to vertical (H/V) value
Summary
The global ambient noise field in the frequency band below 1 Hz is dominated by two distinct amplitude peaks (McNamara & Buland, 2004; Peterson, 1993). These are the so called primary (PM) and secondary microseism (SM) peaks (see Table 1 for a list of abbreviations used in the paper). SM with a peak at about 0.15 Hz is the dominant noise source in the frequency range between 0.03 and 1 Hz and can be recorded globally It is caused by the superposition of ocean waves with nearly opposing directions and equal frequencies (Ardhuin et al, 2015; Longuet‐Higgins, 1950) that cause a pressure fluctuation in the water column. Source regions of SM have been observed near the coast due to the reflection of incoming ocean waves at the shore (Bromirski & Duennebier, 2002; Sutton & Barstow, 1996; Traer et al, 2008) and less frequently in open water locations
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