With 5x earthquake down you have 41% chance that 1 of the 5 will cause knockdown (probability is (1-(0.9*0.9*0.9*0.9*0.9))*100
When you cast and take damage, it adds a delay to your spells (push back effect). Concentration aura and totem of tranquil mind reduce/negate that effect.
Earthquake knocks down players, so concentration aura should not have an effect on that.
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