Most induced earthquakes are not directly caused by hydraulic fracturing (fracking).
The recent increase in earthquakes in the central United States is primarily caused by disposal of waste fluids that are a byproduct of oil production.
Wastewater disposal wells typically operate for longer durations and inject much more fluid than is injected during the hydraulic fracturing process, making them more likely to induce earthquakes.
In Oklahoma, which has the most induced earthquakes in the United States, 2% of earthquakes can be linked to hydraulic fracturing operations. Given the high rate of seismicity in Oklahoma, this means that there are still many earthquakes induced by hydraulic fracturing. The remaining earthquakes are induced by wastewater disposal. The largest earthquake known to be induced by hydraulic fracturing in the United States was a magnitude 4.0 earthquake that occurred in 2018 in Texas.
Was Fracking discussed on this board already?
Anything that alters crust viscosity is likely a bad thing.
It was assumed, not all that long ago, that the middle of the U.S., essentially everything that lies within the Mississippi watershed, was a geologically stable craton - one big giant piece of crustal block essentially unchanged for 4 billion years.
This has, over the last couple of decades, been proven to be utter nonsense. Where it was previously believed that tectonic plates where solid, it's now understood that the plates are, themselves, amalgamations of previously created and destroyed crustal blocks. Over the course of geological time they break apart and fuse together, constantly.
The reason there's oil in places like Oklahoma is because the middle of the country was a shallow, tropical sea during the early Cretaceous - roughly 150 million years ago. The marine algae deposits were buried by shale and sandstone created by the down-wasting of the Rockies and the oil eventually migrated up through those formations to become, in varying degrees, recoverable.
The point is that no petroleum formation resides in what you would call "really stable rock". Injecting fluids into that rock, either from wastewater disposal or hydraulic fracturing, is going to destabilize it for certain. Destabilized bedrock will exacerbate local faulting - which was previously believed to not be a thing, but which we now know is very, very much a thing.
