This animation shows the process of elastic strain build-up and release by earthquakes on the faults cross-crossing the LA basin
During quiet times in between earthquakes (interseismic periods), the LA basin and the San Gabriel mountains are squeezed ,due to the bend along the San Andreas fault. The convergrence rate between the Catalina Islands, offshore LA, and the San Gqbriel mountains is about 9mm/yr (about 1/3d of inch per year). This process is loading the faults criss-cossing the LA basin. There are places where these faults stick together (locked) and places where they don't (creeping). Stress build up on the locked patches until they fail in earthquakes. This process leads to recurring events similar to the Northridge earthquake of 1994 which reached magnitude 6.8
This "seismic cycle" recurs about every 250 years.