Can an earthquake shift the Earth’s axis?
With the earthquake in Chile and the recent earthquake in Japan, some sources report that the earthquakes were so powerful, that they caused a shift or tilt in Earth's axis. But is there any truth to this?
The simple answer is yes. The earthquake that unleashed a devastating tsunami Friday moved the main island of Japan by 8 feet (2.4 meters) and shifted the Earth on its axis by nearly 4 inches (10 centimeters) -- something that experts say will lead to the shortening of the day by 1.6 microseconds, or just over a millionth of a second.
What's the complex answer?
The angular momentum doesn't change, but the angular velocity vector does. This is directly due to a shift in a body's moment of inertia tensor.
Earthquakes can't change the axis of rotation, relative to a given inertial reference frame; that is, the axis of rotation doesn't change relative to the "fixed stars" as a result of an earthquake. An earthquake moves material around within the Earth, so that the position of the rotation axis relative to any given marker on Earth's surface changes.
Rather than saying the earthquake shifted the Earth's axis, it would be more accurate to say that it shifted all of the stuff on the Earth's surface. Because we find it convenient to use points of reference that are fixed to markers on Earth's surface, to us it looks like a shift in the axis.