Seismic seiches in Norway and England during the Assam earthquake of August 15, 1950*

Abstract

Abstract During the Assam earthquake of August 15, 1950, unusual waves were observed in at least 37 localities in fjords and lakes in Norway. Reports from 29 of these are discussed in this paper. In most places the waves were standing waves, with periods of 1 to 3 minutes and amplitudes of 5 to 100 cm. and began when the acceleration at the seismological observatory in Bergen surpassed 20 milligals in the east-west direction and 40 milligals in the vertical direction. The movements generally ceased when the acceleration decreased to below 10 milligals. The main periods of the long waves declined during this period from about 30 to 15 seconds, but the seismograms also indicate longer periods, of about 1 to 3 minutes, which may have some connection with the seiches. The calculated periods for the basins where waves were observed are, assuming one node, in most places between 1 and 3 minutes. In England, standing oscillations were recorded in water reservoirs at Margate, Chichester, and Portsmouth during the earthquake. The periods cannot be determined from the records, but the calculated periods vary from 15 to 25 seconds, which corresponds fairly closely with the 22 seconds given by Kew Observatory as the period of the long waves of the earthquake. The maximum amplitude in the reservoirs was 2 inches. Seiches in Norway during the Lisbon earthquake of November 1, 1755, and progressive waves during the Kansu earthquake of December 16, 1920, are briefly discussed.