I'll make a rechost to this with the facts and link it once I have some!
Also,: if you're unsure whether it's technically earth science, I want to hear it.
Setting the Stage
Off the coast of the American Pacific Northwest is a small remnant of an oceanic plate that is nearly finished subducting from view, called the Juan de Fuca plate. It's responsible for the volcanoes on the west coast of the U.S. (Rainier, Helens, etc.) as hydrated minerals in the descending plate are baked and desiccated, reducing their melting point temperature in a process known as dehydration melting, and that melt trickles up and partially melts/mixes with the silica-rich crust to create the andesitic composition of The Cascades.
The rock that remains continues down towards our planet's core where normally it would disintegrate at the core-mantle boundary, but we believe through random chance it has hit a mantle plume along the way, and thus has been bent upwards and has rammed into the underside of the North American plate, pushing up a section of the Rocky Mountain Range (which has its own interesting history, but that's a story for another day).
The North American Plate's pretty big and heavy, and it's being propelled westward by the expansion of the mid-ocean rift in the Atlantic basin, but there's also an enormous amount of force being exerted in the opposite direction on the subducting Fuca plate. Oceanic crust is primarily basaltic in composition, and thus more dense, so while overall the North American Plate is still being pushed westwards, locally the Fuca plate wins this force war.
The end result is The Great Basin, an area of the Western United States defined by what we call horst and graben geography. As the plate is stretched out, inverted blocks of trapezoidal crust drop down as its surrounding support is pulled apart and lost. This results in literal stretch marks that identify a horst and graben landscape, and you can look on a map for yourself to see the vertical striations of mini-ranges between the Cascades and the Rockies, filling the states of Nevada and Utah.
These blocks can drop down quite a ways, and sediment eroded from the Rocky Mountains fills these valleys and hides the true scale of crustal deformation. If you stand on the western edge of the Rockies along the Wasatch Front looking out over the Salt Lake Valley, the crust beneath that Valley has dropped down approximately 11 kilometres below the surface you see, and the core of Mormon civilization is built atop that relatively loose sediment that has covered the sunken graben block.
The "Promised" Land of Zion
The LDS Church has a really unlucky (and often self-inflicted) calamitous history of westward expansion. Borne out of a Protestant revivalist fervour in the mid-1800s on the American East Coast, members of the LDS faith have been steadily kicked towards the frontier by everyone else who really didn't want them around (can't imagine why). They eventually settled in a place called Nauvoo in the state of Illinois where they decided this was going to be where they would build up the core of their faith.
The Governor of Illinois had other plans and wanted to nip this one in the bud, so facing an "extermination order," Mormons were once again driven westward in one of the most disastrous plains-crossing journeys that honestly they didn't have to make at that time, but lore-wise they were commanded by God to make the crossing, resulting in mass casualties once winter dropped on them mid-crossing.
The survivors broke through the Rockies into what would come to be known as the Salt Lake Valley. Lore-wise, the leader of the LDS church at the time, Brigham Young (Joseph Smith had been assassinated a while back) was prompted by God to settle in that valley--a valley that no other white settlers really saw as prime real estate. The soil sucked, the nearby lake was saturated with pickle-brine levels of salt, and the whole valley smelled like a giant fart.
This would be the new chosen land by divine decree, but the likely decision-making process was that the survivors simply ran out of steam; they knew they wouldn't have the strength or numbers to fend off any hostility they would likely encounter on the west coast from other settlers, and they no longer had the equipment and supplies to keep going, so that was where they were going to stay, like it or not.
It was a rocky start, but this gambit worked out pretty well for them, and they've grown to be one of the largest and most wealthy factions in Christianity. This prosperity has led them to believe even more fervently that they are God's chosen people, and Utah has become a peculiar yet legitimized cultist monoculture in the United States that no longer risks extinction...
The Eventual "...But"
The boundaries between horsts and grabens are fault lines, and thus are sources of earthquakes. The Fuca plate is still subducting, just as the Mid-Atlantic Ridge is still rifting. North America is still being pushed westward. The Great Basin is still stretching. The grabens are still dropping. Eventually, the Salt Lake Valley graben is going to move, and by all accounts it's overdue.
As if this weren't bad enough, Capitalist greed is combined with much of the Utah State Legislature being filled with real estate owners and lobbyists (with significant overlap with upper echelon Church leaders). The State of Utah has really dragged its feet on any kind of earthquake preparation. Typifying the generous and benevolent nature of the LDS Church and Utah body politic, money was spared to upgrade church property and the capitol building itself to be earthquake resistant, but everyone else, as far as the Church and State Legislature is concerned, is on their own.
Residential zoning is extremely generous and effectively unregulated despite complaints from local geologists and civil engineers, and boulders will sometimes tumble down the sides of mountains in areas that were defined as high-risk for landslides, and plow straight through pretty little homes owned by well-to-do Mormon members living on the foothills of the Wasatch Mountains (who, to be fair, were mislead into thinking that these areas were perfectly safe).
This pales in comparison to what is coming, and honestly, even with all the earthquake-proofing upgrades, when "The Big One" hits the Salt Lake Valley, Mormonland is gonna have a really bad time. In earthquake conditions, that valley's sediment infill will undergo liquefaction and start swallowing anything on the surface like Hollywood quicksand. Any construction at the base of the mountains will be wiped out by landslides of hard, metamorphosed quartzite. It will be a truly apocalyptic event for anyone living in that Valley, and even a moderate-energy earthquake would still result in millions, if not billions in damages.
Scientific Value
Of course, Brigham Young and his religious refugees couldn't have had any clue this is the fate of their eventually prosperous valley. It does strike me as a truly unfortunate chain of decisions that are born from that single disastrous decision to cross the American Plains far too late in the season, with repercussions that would not just be felt in their immediate future by many of their caravan freezing to death, but in their distant future when the Earth's crust decides to move again.
In both cases, powerful leaders ignored all the data at hand, and went ahead full-tilt knowing there would be suffering. I cannot for the life of me comprehend how a group can be told even by people who hate them to don't fucking do it, oh my god what the fuck are you doing, listen to us to zero effect. Mother Nature isn't really a bitch; Mother Nature just is. She lives out Her life independent of ours, and some of the most valuable science that benefits humanity comes simply from listening to Her, and respecting reality...
...which is something I guess many religious sects are really, really bad at.
My source for all of this is from a bitter ex-Mormon geology student I smoked weed and maybe did a few more things with.
