Would an axis shift affect a compass reading or GPS device?
The answer is, it depends. A GPS device measures your position with respect to a constellation of satellites. The orbits of the satellites is hardly affected at all by the earthquake, at least in the short term. The satellites report their locations in a coordinate frame that is very carefully referred to surveyed points on earth’s surface. If you had a military GPS with a very high resolution in a fixed base configuration on a Japanese island, there is no doubt that your coordinates would change. With respect to GPS coordinates, just about every point in Japan moved about eight feet. For best accuracy, all your GPS maps of Japan need to be updated.
The displacement of the earth’s rotational axis would also effect all users of GPS as well, though the satellites get daily updates from ground, so for users not otherwise affected by the earthquake, the difference will be neglible.
The earth’s magnetic field is tied more strongly to its core than surface, so basically, magnetic headings will not change, except in Japan a compass needle will exhibit the same change in deflection it would if it had been moved eight feet.