The movement at a strike-slip boundary also called a transform fault, is sideways.  The plates move past each other.  These occur at faults or cracks in the crust and are caused by earthquakes.  In the Coachella Valley, along the San Andreas fault, which is a transform fault.  This is why there are many earthquakes.  We sit right on top of a strike-slip fault that is constantly at work.

See the source image

List of strike slip earthquakes:

The San Francisco Earthquake, 1906 7.9

Fort Tejon Earthquake, 1857, 7.9

Loma Prieta Earthquake, 1989, 6.9

This is the kind of fault that runs through the Coachella Valley.