On the generation of secondary microseisms observed in northern and central Europe

Abstract

Microseism recordings from four European broadband stations and from three seismic arrays in Scotland, Norway, and Germany are compared with model wave data of the oceanic wave field in the North Atlantic and local ocean wave data from the Norwegian coast at 60°N, both measured during February–March 2000. Two approaches have been tested to locate generation areas of microseismic energy: a new amplitude correlation technique and beam backprojection from the three seismic arrays. Both techniques reveal that the main generation areas are located in specific regions off the coast of Southwest Norway and North Scotland. Seismic stations distant from these generation areas record a superposition of seismic energy from different source regions. Those close to a specific source region also show a high correlation with it. Both techniques give upper limits for the extent of the generation area of the strongest storm on 6/7 March at the southwest Norwegian coast of about 500 km. By using marine X‐band radar measurements of the two‐dimensional wave height spectrum, we estimate that the relative change of the extension of the generation area off the coast of southwest Norway during several storms is less than a factor of 3. This indicates that the size of the generation area is controlled by static features as coastline or bathymetry, and not by the extent of the storms. Microseism energy appears to be mainly controlled by the wave height in distinct and identifiable generation regions, so that the wave climate in these regions can be studied using historical records of microseisms.

Mutual Coupling Between Meteorological Parameters and Secondary Microseisms

“…We stated, after various attempts, that microseisms have an annual drift, which is explained mainly by annual variations of meteorological conditions, especially wind speed over the sea (Bromirski et al 2005), sea wave height (in 2nd power are proportional with microseism amplitude) (Essen et al 2003;Stehly et al 2006), air pressure variations (Peters 2005), and coastal and sea ice conditions (Grob et al 2011). However, this annual drift could be explained by other influences, which have an annual period, such as temperature variations, differences of air pressure or temperature between various parts of the continent, etc.…”

Global trends in extremal microseism intensity

“…This DFM "bright spot" effect has been previously noted for northern European stations (e.g., ESK) by Essen et al [2003] and elsewhere, and may be a ubiquitous feature of source regions featuring steep coastlines in stormy regions (e.g., parts of northern Europe, southern Greenland, northwestern North America, and southwestern South America).…”

“…[10] Especially volatile inter-annual behavior is notable at North Atlantic stations (Figures 2, S2, and S3) [Grevemeyer et al, 2000;Essen et al, 2003]. Station-specific, regional, and global linear trends evaluated via L 2 -norm regression with bootstrap parameter estimates display a bias for positive regression slopes (Figures 2 and S4).…”

“…Recent studies have suggested that climate change may affect the frequency, distribution, and/or intensity of storms and associated ocean wave activity, for example in hurricanes affecting the eastern U.S. and Caribbean [Emanuel, 2005;Federov et al, 2010], in northern Pacific winter cyclones [e.g., Graham and Diaz, 2001;Bromirski et al, 2005a;Lambert and Fyfe, 2006], and in extratropical storms affecting northern Europe [e.g., Grevemeyer et al, 2000;Essen et al, 2003]. In this paper, we concentrate on global winter period storms and demonstrate that microseism data from existing Global Seismographic Network (GSN) [Butler et al, 2004] infrastructure provides a valuable real-time metric for comparing extreme coastal wave events across decadal time spans.…”

Monitoring austral and cyclonic swells in the “Iles Eparses” (Mozambique channel) from microseismic noise

“…These are polarized in the vertical plane with an elliptical retrograde particle motion and can propagate over large distances in the solid earth, with little attenuation. Secondary microseisms can be generated in the deep oceans and at large distances from coastal areas (e.g., Essen et al, 2003;Ardhuin et al, 2011;Obrebski et al, 2012;Davy et al, 2014). For the Indian Ocean, the dominant sources have been located in the southernmost part of the basin, associated with large atmospheric lowpressure systems moving around Antarctica (Reading et al, 2014;Davy et al, 2015) but they can also be generated by major tropical storms (Davy et al, 2014).…”