Changes in the magnetic field are just a small side effect. Bottom line, there isn’t going to be any May 21st cataclysm. Massive geological forces, when they do happen, usually build up for a very long time, and give all sorts of indications.
Something of the scale to cause worldwide earthquakes would have to be immensely larger than the supervolcano that covered India with lava in the distant past. Literally, the majority of the Earth’s mantle would have to be involved.
Not small. I can't find them now but I've seen graphs taken from space of the earth's magnetic field before or during some earthquakes and the whole field is distorted.
Bottom line, there isn’t going to be any May 21st cataclysm. Massive geological forces, when they do happen, usually build up for a very long time, and give all sorts of indications.
I don't know anything about May 21, your post just caught my eye of being way too simplistic.