Well, first off, the crust is already cracked open.
The average thickness of the continental crust is about 35 kilometers (22 miles), compared to the oceanic crust thickness of around 9 kilometers (5.6 miles). Fault networks are often shallower than 80 kilometers (50 miles) deep, but they can go as deep as 600 kilometers (375 miles). The crust, the upper mantle beneath it, and the tectonic slabs subducting into it are all cracked in some way.
The majority of the mantle, however, cannot be cracked open. It's solid, sure, but it’s so hot and squashed together that any attempt to split it would be like putting a dent in a highly pressurized tube of toothpaste. Instantly, mantle material would rush in to fill the gap and the shock would be absorbed.
But that’s no fun, is it? Surely we can generate an artificial earthquake and tear the planet apart, you may wonder. Why yes, yes we could. Let’s take a look at the energy that would be required to do so.
Earthquakes create frictional heat, particularly megathrusts. Assuming the crust is generally made of granite, which melts at 1,260°C (2,300°F), we can use the magic of physics to work out how much frictional heat would be required to obliterate the crust during a megaquake: 4.4 x 1023 joules, in fact.
That would require an earthquake 53,000 times more powerful that the 1960 Valdivia earthquake. A 12.8M earthquake, actually.
If a supervillain made this happen, then it would cause Earth’s orbit around the Sun to wobble dramatically, perhaps messing up our seasonal cycles forever more. The molten crust would explode into the ocean, causing a gigantic steam blast that would obliterate whatever country it was near at the time.
(Un)fortunately, this type of megathrust quake could never happen in real life. The forces required to generate it are far beyond the mechanical strength of any rock we know of, which means that the stress of two plates would cause them to jut forwards long before they could build up enough energy.
If you really want to crack open a planet, we’d suggest hitting it with an asteroid. The famous dino-killing monster was enough to crack open the crust as far north as Colorado.