How effective would be retrofitting on the bridges under those circumstances?
Memphis has done major retrofitting of highways and bridges. Many newer buildings in Memphis have been to withstand earthquakes. Google Memphis retrofit earthquakes and you’ll see.
I think it depends on the power of the earthquake. Bridges that might withstand a 7.0 might collapse in an 8.0 that is ten times more powerful. And of course there is distance from the epicenter. But the important thing about a NMF quake is the wide area affected, compared to California.