Ice quakes refer to non-tectonic quakes caused by freezing and defrosting of various parts of the Earth's geology.
An
Icequake (no space) or
ice-quake has been around since Webster1913's time, and has a specific, separate meaning: "The crash or concussion attending the breaking up of masses of ice". I will not be dealing with these here, although these may also be referred to as ice quakes.
Another type of ice quake is caused by the ground freezing quickly; this causes pressures to build up until something moves -- resulting in jerks, tremors, pops, bangs, and sometimes even visible cracks in the surface of the ground. This type of ice quake is technically known as a cryoseism, and is more fully covered in that node.
There is one more type of ice quake; when a glacier starts slipping across the ground on a layer of melted water, it causes a slow, low-level quake. This is new stuff; geophysicist Göran Ekström published this theory in the September 25, 2003 publication of Science. So far no one has shot down this theory, and it seems to be widely accepted. These quakes are sometimes called glacial earthquakes, although 'ice quake' seems to be the more common label at this time.
The theory is that the low-level, low-frequency quakes that seem to be coming from the glaciers are caused by the glaciers building up a layer of meltwater underneath themselves, until they lose traction and scrape their way downhill for up to ten meters. Glaciers can be really, really big -- they can be hundreds of kilometers long, and hundreds of meters deep. When they drag over the rough, rocky ground they can cause quakes up to at least magnitude 5. They also tend to last longer than to tectonic quakes -- thousands of seconds long, rather than hundreds.
http://www.vibrationdata.com/Newsletters/June2006_NL.pdf
http://amos.indiana.edu/library/scripts/icequake.html
http://www.sciencemag.org/cgi/content/abstract/1088057v1?ijkey=WWrGRKBS0YsW2&keytype=ref&siteid=sci