TY - JOUR

T1 - Temporal Modulation of the Local Microseism in the North Sea

AU - Becker, Dirk

AU - Cristiano, Luigia

AU - Peikert, J.

AU - Kruse, Thore

AU - Dethof, F.

AU - Hadziioannou, Celine

AU - Meier, Thomas

N1 - Funding information: We thank the Associate Editor Nori Nakata and two anonymous reviewers for their critical and constructive suggestions that significantly helped to improve the manuscript. Station HLG is part of the GEOFON network (https://doi.org/10.14470/TR560404) of the GeoForschungs-Zentrum Potsdam (GFZ, https://geofon.gfz-potsdam.de). Stations IGAD, BSEG, and SKMB are part of the German Regional Seismic Network (GRSN, https://doi.org/10.25928/mbx6-hr74) of the Bundesanstalt für Geowissenschaften und Rohstoffe (BGR, https://eida.bgr.de). Station G021 is part of the Netherlands Seismic and Acoustic network (https://doi.org/10.21944/e970fd34-23b9-3411-b366-e4f72877d2c5) of the Koninklijk Nederlands Meteorologisch Instituut (KNMI, http://rdsa.knmi.nl).

PY - 2020/9/30

Y1 - 2020/9/30

N2 - Primary 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.

AB - Primary 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.

KW - ocean microseism

KW - ocean tides

KW - seismic noise

KW - seismic noise sources

UR - http://www.scopus.com/inward/record.url?scp=85091484396&partnerID=8YFLogxK

U2 - 10.1029/2020JB019770

DO - 10.1029/2020JB019770

M3 - Article

VL - 125

JO - Journal of Geophysical Research: Solid Earth

JF - Journal of Geophysical Research: Solid Earth

SN - 2169-9313

IS - 10

M1 - e2020JB019770

ER -