Physics
Scientific paper
Jul 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991pepi...67..330d&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 67, Issue 3-4, p. 330-347.
Physics
3
Scientific paper
Investigation of microseisms and their origin has been continued. In the previous investigation, a vertical seismograph only was used and so it was not possible to study phase relationships between components. A north-south seismograph has now been used, and it has been possible to relate the phase difference between the north-south and vertical seismograph to the position of the microseism generating areas described in the previous paper. In December 1989, there was a long period of intense coastal activity in the southwest sector (Bristol Channel). During this time the north-south components were consistently 90° behind the vertical. In January, however, the position changed as the maximum activity came from the northwest sector (North Channel), and then the north-south components were 90° ahead of the vertical. These phase relationships became apparent only after prolonged activity in one sector only. The situation is usually very confused when there is much activity in both sectors but occasionally one can observe the phase difference swinging round as a storm goes from south to north. These phase relationships indicate that the microseisms have a strong Rayleigh wave component.
It is usually very difficult to identify microseism caused by storms in Mid-Atlantic, even when there are areas of appreciable wave interference, because they are often overshadowed by those caused by coastal waves. However, during 11-13 November 1989, there were two storms about 1000 and 2000 km to the southwest of the British Isles where wind components in a sector of one storm were directly opposed to those in a sector of the other, giving rise to waves moving in opposite directions in areas between them. The storms were of only medium intensity, giving rise to waves of a 10-12 s period. A corresponding increase in microseism activity of a 5-6 s period was apparent at this time, and as there was very little coastal activity, there could be no ambiguity about the origin of these microseisms.
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