7.0 doesn't sound big, but keep in mind, the epicenter would be quite close to the surface and likely to be right in the middle of a highly populated area on the Wasatch Front (Salt Lake City, Cottonwood Heights, etc). Also, many buildings in the vulnerable areas pre-date construction codes that take seismic risk into account. For example, it wasn't until the late 70's that they started reinforcing masonry construction.
[1] http://www.livescience.com/28782-utah-bigger-earthquakes-was...
Note that it's really not clear to what extent faults rupture periodically; there are researchers who argue that the likelihood of an event is basically independent of the time since the last event (basically a Poisson distribution), those who argue for fairly regular events, and everything in between. It probably varies a lot from fault to fault and is more regular on big plate boundary faults like the San Andreas than on smaller (or slower-moving) intraplate faults like the Wasatch Fault.
The real danger, both here and for the San Andreas system, is that an earthquake will break through the segment boundaries and cause a much, much larger earthquake. It's not been demonstrated that this is has happened on the Wasatch fault (there is one instance about 6000 years ago where this is possible within the data uncertainty) but this has happened in California in the 1906 S.F. and 1857 Fort Tejon earthquakes. Although this might not cause more displacement of the fault in any one location, it creates such a bigger swath of destruction that there are more opportunities for associated disasters (fires, floods from dams breaking, etc.) and make response much more difficult. It's also a bit outside my expertise but I think that for a long rupture, there is enough constructive interference of surface waves from the earthquake that ground shaking is much greater than is predicted from displacement at any point on the fault.
[1]: http://michaelhaskinseportfolio.weebly.com/earthquake-hazard...