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.
“The strength of the field at the Earth’s surface ranges from less than 30 microteslas (0.3 gauss) to over 60 microteslas (0.6 gauss). The average magnetic field strength in the Earth’s outer and inner core was measured to be 25 Gauss, 50x stronger than the magnetic field at the surface.”
By comparison:
50 gauss: a typical refrigerator magnet
100 gauss: a small iron magnet
15,000-30,000 gauss: a medical magnetic resonance imaging (MRI) electromagnet.