Ocean-generated microseismic noise located with the Grafenberg array

The main cause for mid-period seismic ground distortions are ocean waves generated by atmospheric disturbances. These act upon the earth through different mechanisms. The microseismic wavefield can be divided into primary (T=12-18 s) and secondary (T=6-9 s) noise. Classical theory tells that the origin of these induced ground distortions depends on the location and the intensity of the low pressure region. A considerable part of the microseismic wave field reaches the GRF-array in southern Germany with high coherency and almost constant amplitudes. Thus it is possible to locate the generating areas using frequency wavenumber analysis. Five discrete generating areas for secondary microseisms and three generating areas fur primary microseisms could be determined in the Atlantic Ocean, the Arctic Sea and the Mediterranean Sea by investigating broadband continuous recordings over four months in winter 1995/96. An essential result is the long-time constancy of the backazimuths of the coherent part of the microseismic wavefield with respect to the origin areas, independent of the location of the moving low pressure zone. Results from a triangulation using additionally broadband data from the NORSAR-array and an independent estimation of the distance of the source region with water wave dispersion data indicate an origin of the secondary microseismic wavefield near the north-Norwegian coast for the strongest source. The array analysis of a temporary network of ten three-component broadband stations in south-east Germany shows that the ratio of energy between coherent Love and Rayleigh waves is much higher for the primary than for the secondary microseismic noise wavefield. This indicates differences in the source mechanisms.