What is the zones of immobile rock along faults?

The zones of immobile rock along faults, often referred to as "fault gouge" or "fault core," are regions where the rock has been subjected to intense stress and deformation, leading to a reduction in its ability to move. These zones typically contain crushed and fragmented rock, which can inhibit slip during an earthquake. They act as barriers to movement, contributing to the overall stability of the fault until the accumulated stress exceeds the strength of the rock. Understanding these zones is crucial for assessing earthquake risk and fault behavior.