That will be at the depth where olivine spontaneously transitions to spinel due to pressure; the earthquakes are due to the volume change. There's an even deeper zone where spinel turns to perovskite (around 600-700 km), but the subducted slab has usually turned ductile before that happens, so I believe you only get those rarely, in very high-speed subduction zones... but I'm getting rusty on this, now. Haven't studied it for a couple of decades!