The West coast of the North American continent is tectonically very yactive; Califormia particularly so.
Earthquakes are unlikely to occur "all over" the U.S.A., but seismic recorders all over the country including in stable areas will still detect ones centred considerable distances away.
Their results can then be collated mathematically to determine each earthquake's real location, depth and amplitude.
Yes, it is normal in these events' nature. All part of the planet's inner workings.
They are not predictable but certain regions are far more prone to them than others, and for all the major ones that cause so much death and destruction there are hundreds, even in fairly "solid" regions, detectable only on seismographs..
