Ocean-generated microseismic noise located with the Grafenberg array

The main cause for mid-period seismic ground distortions are ocean waves ge nerated by atmospheric disturbances. These act upon the earth through diffe rent 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 ori gin of these induced ground distortions depends on the location and the int ensity 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 an alysis. Five discrete generating areas for secondary microseisms and three generati ng areas fur primary microseisms could be determined in the Atlantic Ocean, the Arctic Sea and the Mediterranean Sea by investigating broadband contin uous 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 mic roseismic wavefield with respect to the origin areas, independent of the lo cation of the moving low pressure zone. Results from a triangulation using additionally broadband data from the NORSAR-array and an independent estima tion of the distance of the source region with water wave dispersion data i ndicate an origin of the secondary microseismic wavefield near the north-No rwegian 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 coher ent Love and Rayleigh waves is much higher for the primary than for the sec ondary microseismic noise wavefield. This indicates differences in the sour ce mechanisms.