What happens when plates shift in western united states

Your answer would be an earthquake.

I'm going to assume you're talking about tectonic plates, as this question is under geography.

Tectonic plates make up the surface of Earth. Where two of them meet is where the possibility of an earthquake emerges.

When enough pressure is placed on a fault, the plates can sometimes suddenly shift, causing what we feel as an earthquake. This is what happened at the Western United States, when at the San Andreas Fault, for example, two plates suddenly shifted and caused a massive earthquake.

Hope this helped!