S-P conversion at the base of the crust

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