Robert Pfile Dear Robert, Many large earthquakes produce strange effects at the surface, such as throwing objects in the air or disturbing the line of sight. The violence of the ground motion probably did feel like an explosion. However, this violent motion was not caused by a volcanic explosion. Southern California is along a transform plate boundary, where two plates slide laterally past each other. The plates shift because they are moving in different directions.
Stress builds up until it exceeds the strength along a fault.
No wonder the effects at the surface can be so violent.