Mine strikes can cut a keel. You should read up on the Samuel B. Roberts. Great example.
Damage effect is primarily driven by weapon placement, be it a mine or torpedo. In the case of the Roberts, the hit was an almost perfect example of as good aplacement as you can get. The weapon detonated just aft of frame 250. On a Perry Class frigate, that places the weapon at the boundary of the two largest (engineering) spaces. Aux 2, and the main Engine Room. It also targets the most sensitive part of the ship, the two Gas Turbines are there, as well as the fuel feeds coming from Aux 2. Roberts keel was cut, the ship almost broke in two (in fact, more or less it did), if not for some emergency welding of braces, stringers, and even some stitching using steel cable, the ship would have physically broken in half. Note that the Perry Class is larger than the south korean corvette, and thus had the ability to absorb damage a little better. In the case of the Princeton, the closer mine went off from more to the side of the ship vice underneath. It is thought a good bit of the damage is the result of a sympathetic detonation off the bow, amplifying severe whipping.
The idea of modern heavy weight torpedoes is to blow the water out from under a combatant, and reverse stress the keel to break, as well as drive whipping for secondary damage effect throughout the ship. They do not actually hit the ship. Interestly enough, the weapons engineers over estimated the size of the charge required to get the effect, and instead, for the average combatant, a heavy wieght has enough pop to cut the ship vertically with proper placement. Since modern weapons place themselves, this is not a large challenge.
NORK uses several types of torpedoes, and depending on the type "used" would drive the terminal placement and corresponding damage. Also, the tactics used by the NORK submarine for the suppossed engagement would drive placement as well. Say for example they engaged with a wake homing weapon, this would put the weapon coming in from astern vice the beam, and place the weapon IVO the running gear upon proximity triggering. The idea of wake homers is a mobility kill at minimum, with total destruction a hoped for event (but not immediately likely). The whipping effect would be enhanced due to placement at one end of the hull, and this would also enhance mission kill probability of the engaged platform vice destruction. The evidence seen so far in photos could be either a mine or a torpedo. Although a beam shot torpedo is fairly archaic in these modern days of acoustic and wake homing torps.
Based on where the hit occured, I would bet on a mine first, torp second, and if it was a torp, a dumb one fired by a lucky and close boat with a contact fuse, or less likely, an acoustic set up for a proximity triggering (the former more likely by the photos I have seen). My money is on a mine, much less political risk, and still makes the point. The trick is, is there more than one mine placed, or was it a one off gamble? And, it also could very well have been a mine placed by a submarine. NORK likes that sort of thing.
It will come out eventually what the likely weapon really was. And yes, I agree, there is a good chance that a NORK combatant will "accidently" have a bad day down the road. Not the first time these kids have played tit for tat. Nor will it be the last.