Source locations of secondary microseisms in western Europe: Evidence for both coastal and pelagic sources

We locate the sources of double-frequency ( or secondary) microseisms in western Europe by frequency slowness analysis of array data as well as polarization and amplitude analysis at individual stations. Array analysis uses data recorded by a temporary array of broadband stations that we deployed in the Quercy region ( southwest of France) and those from the Grafenberg array, from 2 December 2005 to 30 January 2006. We determine attenuation laws for microseisms generated in the Mediterranean Sea and in the Atlantic Ocean, which allow us to use noise amplitudes to estimate distances from the source. We then combine azimuth and amplitude measurements to obtain precise locations of microseisms and estimate their source dimensions. Most of the time, microseismic noise originates in coastal regions where the swell reaches steep rocky coasts with normal incidence, in good agreement with the Longuet-Higgins model for the generation of secondary microseisms. In addition, we find evidence of occasional pelagic sources, which are closely related to moving storms, suggesting that nonlinear interaction between wave components can also generate secondary microseisms.