Here's a breakdown:

* Convergent Plate Boundary: This is where two tectonic plates collide. The collision can result in one plate sliding under the other (subduction) or the plates colliding and pushing upwards (mountain building).

* Reverse Faulting: This occurs when rocks on one side of a fault are pushed upwards relative to the other side. This happens due to compressional forces, which are common at convergent boundaries.

In the case of the San Gabriel Mountains, the North American Plate is colliding with the Pacific Plate. The Pacific Plate is being subducted beneath the North American Plate, causing the land to be pushed upwards and creating the San Gabriel Mountains.

This process is ongoing, which is why the San Gabriel Mountains are still considered geologically active and prone to earthquakes.