Higher Modes of Seismic Surface Waves: Preliminary Observations

The records from all Swedish stations for the interval 1961 June to 1963 July were examined for the presence of higher modes. Of the 207 earthquakes recorded with appropriate amplitude on long period seismograms, 89 (43 per cent) show evidence of higher modes, usually second Rayleigh modes. The group velocity dispersion curves of the higher modes of 81 of these earthquakes are presented in this paper. The 43 per cent of earthquakes showing higher modes are mainly those whose epicentres are in Eurasia; 70 per cent of the earthquakes with such paths show higher modes.