LONG-PERIOD MICROSEISMS AT UMEA, SWEDEN

Microseisms in the period range 14-20 s recorded by the long-period Press-Ewing seismographs at Umeå (63°48.9′N, 20°14.2′E) are investigated with special regard to their properties and their origin. In the 11-year period (1964-1974) under investigation, 28 clear long-period microseismic storms were detected by visual inspection of Umeå records. With an annual average of 2-3 cases, these represent the best developed storms. However, it is very likely that additional weaker storms could be detected by higher instrument magnification in the long-period range or by record filtering. Among the stations in our seismograph network, Uppsala and Kiruna also operate long-period seismographs, but they are of lower magnification than at Umeå. In fact, it is too low to permit visual detection of long-period microseismic storms, except very rarely at Uppsala. © 1990.