is a pretty common way to calculate a factor of safety (and happens to be the one Wikipedia has a formula image for). The numerator is the resisting forces; the denominator is the driving forces. If the top is smaller than the bottom, your factor of safety is less than one and you are experiencing slope failure.

That italic u is water pressure. Notice how it’s being subtracted; more water pressure means fewer resisting forces.

Sort of makes you understand why this earthquake swarm is being blamed on oil and gas wastewater injection (only at the bottom of the article, of course- I wonder if it’s hydrofracking.)