Scientists have uncovered a key piece of the puzzle behind the unusual “slow earthquakes” occurring off the east coast of New Zealand’s North Island.

A new international study, published in Science Advances, identifies hidden fault structures called polygonal fault systems (PFSs) as a major influence on the behavior of the northern Hikurangi subduction zone.

These shallow geological features, found in sediments entering the subduction zone, appear to play a critical role in where and how slow slip earthquakes occur.