as someone who moved east to west years ago its a lot more related to soil composition. Dry, hard, and mostly shallow bedrock soils that dominate around california don't actually transmit eneegy from earthquakes very well and move with the shaking vs jiggling. The east coast has very water logged soils with very deep a and o horizons that jiggle like jello during earthquakes and transmit that energy very well. About ten years ago DC had a low magnitude earth quake that caused a lot of damage and shook buildings, it had more to do with the soils then building code. And like others have said a magnitude 5 earthquake in california hardly feels like anything if I feel it at all, but a magnitude 2 earthquake in swamp land feels 10x worse.