The absence of what seismic wave type provides evidence for migration of magma in volcanoes

As magma moves upward in a volcanic chamber, it causes fracturing of brittle rock which gives rise to short-period earthquakes and the ensuing seismic waves are known as 'A' waves. These type of occurrences are known as volcano-tectonic events. 

this is a good question! i'll do some research and come back when i have the answer!

not too sure but maybe in the hot springs.