Horst and graben exist along mid-ocean ridges, where new crust is being made. Explain why.?

A horst is a block of rock which has risen between two faults. A graben is a block of rock that has sunk between two faults. So i need to know why they are found in mid-ocean ridges. This question is from Science Focus 3 by Kerry Whalley, Peter Roberson, Greg Rickard and Isabella Brown.