And what causes these massive intraplate rupturings? Something deep within the Earth. More than that we don't know.
By the 1960s scientists had grown sufficiently frustrated by how little they understood of the Earth's interior that they decided to try to do something about it. Specifically, they got the idea to drill through the ocean floor (the continental crust was too thick) to the Moho discontinuity and to extract a piece of the Earth's mantle for examination at leisure. The thinking was that if they could understand the nature of the rocks inside the Earth, they might begin to understand how they interacted, and thus possibly be able to predict earthquakes and other unwelcome events.
The project became known, all but inevitably, as the Mohole and it was pretty well disastrous. The hope was to lower a drill through 14,000 feet of Pacific Ocean water off the coast of Mexico and drill some 17,000 feet through relatively thin crustal rock. Drilling from a ship in open waters is, in the words of one oceanographer, "like trying to drill a hole in the sidewalks of New York from atop the Empire State Building using a strand of spaghetti." Every attempt ended in failure. The deepest they penetrated was only about 600 feet. The Mohole became known as the No Hole. In 1966, exasperated with ever-rising costs and no results, Congress killed the project.