S-P conversion at the base of the crust

Tlie long-period seismic records at Umeà (Sweden) ofan intermediate-depth earthquake in Burma 011 27 February, 1964, exhibitan exceptionally clear case of 8-P conversion at the Mohorovicié discontinuity.The records are examined from various points of view (arrivaltimes, amplitude ratios, particle motions) and comparisons are made foreach test between observations and theory. General aspects of mode conversionsat the base of the crust are discussed, both their importance andwhy they are so seldom observed