Thread by @TheEarthStory

I’m going a little stir crazy, I’ve got class ready for Monday but don’t want to plan ahead until I see how asynchronous teaching goes in a trial run, so let’s start a thread on this week's earthquake and see where it goes.

Why are there #earthquakes in Salt Lake City?

2 days ago, there was a magnitude 5.7 earthquake near Salt Lake City, on a fault related to the Wasatch fault system. So, what is the Wasatch fault, and why are there earthquakes in this area?

geology.utah.gov/map-pub/survey…

The Wasatch Fault is a normal fault, where one side (the hanging wall) of the fault has dropped down relative to the other. Normal faults occur at places where the crust is being pulled apart, and that is happening in the western part of the US. commons.wikimedia.org/wiki/File:Norm…

All the faults in this diagram are normal faults. They show blocks of crust that were pulled apart as a basin formed near Australia. A main fault formed, and blocks stretched on top of it. You can really see how the crust has been stretched out en.wikipedia.org/wiki/Exmouth_P…

The Wasatch fault is a normal fault right along the edge of an area called the Basin and Range province. The crust from Utah in the East to California in the West is being pulled apart.

en.wikipedia.org/wiki/Basin_and…

30 million years ago, Nevada was about ½ as wide as it is today.

Nevada shows a series of high ranges with basins in-between when viewed from space. These are blocks of crust that have been rotated on normal faults.

academic.emporia.edu/aberjame/stude…

The Ranges are the parts that stick up higher, the basins in-between have slowly filled in with sediment.

earthobservatory.nasa.gov/images/46194/i…

The Wasatch is a major normal fault on the East side of the Basin and Range. The crust to the west has dropped down on this normal fault, creating the basin where Salt Lake City sits.

ugspub.nr.utah.gov/publications/p…

Normal faults sometimes leave “Triangular facets” – pieces of triangle-shaped mountain on the side of the fault moving upwards – and the Wasatch Fault has some famous ones, seen here.

commons.wikimedia.org/wiki/Wasatch_R…

Computer crashes as I try to do this so thank you Microsoft for that.

So finally, why is the crust in the western US pulling apart on Normal Faults? This story starts 80 million years ago, as an ocean plate called the Farallon plate was subducting beneath North America.

nps.gov/media/photo/ga…

As that plate subducted, it produced volcanoes that today form the Sierra Nevada mountains in California, loaded with igneous rocks that were once magma bodies

nps.gov/media/photo/ga…

To the East of the Sierra Nevada and the volcanic arc, a high plateau formed. We see similar plateaus today in Tibet or in the Altiplano of South America.

commons.wikimedia.org/wiki/File:Nasa…

That Plateau no longer exists today, but ancient rocks can be used to construct estimates for elevation where they sat. That suggests an ancient plateau, called the Nevadaplano, existed.
researchgate.net/figure/Schemat…

Then something interesting happened. The mountains fell apart.

This process has been nicknamed Orogenic Collapse. There’s a number of reasons it might happen; mountains may just get so big that they form new faults under the force of Gravity.

researchgate.net/publication/24…

Something like this may be happening in Tibet today, as there are normal faults, suggesting extension, found high in Tibet where the rocks should be being squeezed.

speakingofgeoscience.org/2013/08/28/mid…

In Tibet, these normal faults are oriented north-south, suggesting that as India comes pushing in, the rocks are spreading east and west, maybe driven by gravity.

Something like that probably began in Nevada as early at 60 million years ago, with small normal faults forming.

But the real action started 30 million or so years ago, when the oceanic Farallon plate was subducted so much that North America went completely through it.

pubs.usgs.gov/gip/dynamic/Fa…

North America subducted all the way up to the Ridge between the Farallon and the Pacific plates, bringing North America into contact with the Pacific Plate.

Once this happened, it set up a new fault system that eventually grew into the modern-day San Andreas fault system.

The San Andreas system isn’t just a strike-slip fault. It creates forces tugging on the whole of North America, causing the inner part of the continent to rotate.

Also, the Pacific Plate moves slightly away from North America, so something must bend or spread apart, otherwise a hole would form between the plates (arrows show gps measured motion of spots on both plates) science.sciencemag.org/content/287/54…

30 million years ago, this motion started pulling apart the crust of Western North America, spreading out Nevada and creating normal faults.

The Wasatch fault system and small faults that are part of it are all part of this system, driven by motion between North America and the Pacific Plate. This effect can be seen in this incredible video by Dr. Tanya Atwater emvc.geol.ucsb.edu

A month ago we had a really neat talk for our department seminar by Dr. Sean Long of Washington State University suggesting that some faults in Nevada have moved more than others in this step.

Dr. Long used cross sections of multiple mountain ranges across Nevada together to suggest that the faults that moved the most were also in areas where the crust was thicker as the mountains were being built. Each of these frames is a cross section of one major normal fault.

Once the mountains started pulling apart, the normal faults thinned the crust everywhere to just about 30 kilometers thick. The areas with thick crust had more thinning to do, so they moved more than faults in areas with thinner crust.

So today's arrangement of mountains and normal faults in Nevada and Utah is in part controlled by the parts of the crust that were really thick in the Nevadaplano. As they spread out, large normal fault systems like the Wasatch are growing.