When two plates slide up and down past each other, the boundary formed between the two is call a transform fault.
When the plates slide past each other it is often sudden and violent causing an earthquake to occur.
The San Andreas Fault in California is a well known transform boundary and is the boundary between the Pacific and North American plates.
The entire US may not split apart, but billions of years from now California will spilt apart and Los Angeles will move will move Northwest of San Francisco
For more info, please click: http://www.pbs.org/wgbh/aso/tryit/tectonics/transform.html