Coincidentally, I wrote a paper on that in a university disaster management class a long time ago. If I remember right, it’s called the New Madrid Fault (or something to that effect) and is down toward the bootheel part of the state. There was a series of huge earthquakes there a couple of hundred years ago when very few people were around. Today, those same earthquakes would be catastrophic — probably extreme damage all the way from St. Louis down to Memphis.
The earthquake in Salt Lake today was along one of the side faults on the west side of the valley. The big fault — the Wasatch Fault — runs along the east side of the valley parallel with the Wasatch Mountain range and directly through the most heavily populated urban parts of state.
It’s pretty obvious where it is in places due to the sudden couple of hundred feet difference in elevation from one side to the other. When that fault lets loose, the geologists say it will result in upwards of a 7.5 magnitude quake.
The Wasatch Mountains that rise up from the valley by several thousand feet are supposedly the long-term consequences of this fault that lets loose every three hundred years or so. The geologists say it’s probably overdue.