Complex sentence: Rather, stress builds up in rocks and when it reaches a level that exceeds the strain threshold, the accumulated potential energy is dissipated by the release of strain, which is focused into a plane along which relative motion is accommodated--the fault.
Split into simpler sentences: Rather, stress builds up in rocks and when it cross the limit the strain is wasted, the accumulated potential energy is dissipated by the release. Which is focused into a plane along which relative motion is accommodated--the fault.