The foci is the focus spot down below the epicenter where the earthquake occurs in the crust and upper mantle. The presence of a subduction zone where the plate collides with one another.
The earthquake range from 0-800 mile of depth the deep foci earthquakes are of range 50 to 300 miles as the stress regimes are deeper downwards like a dip down.
This zone is also called as the dipping seismic or the wadis Benioff zone. As the san Andreas fault has a shallow depth of about 20 m depth which is considered the deepest.
