Properties of the 20 M_⊙ GENEC model before and then during a silicon-oxygen shell merger event occurring very shortly before collapse. The time gap between the two models shown is 15 s. On the left we show the convective Mach number, i.e. the ratio of the velocity predicted by mixing length theory to the sound speed, and also the energy produced by nuclear burning. On the right is shown the density and entropy structure of the shell in question.