Very long period (VLP) signals provide a unique insight into the source process of seismic signals recorded on active or restless volcanoes. This is due to the large wavelengths that may allow the structural heterogeneity to be ignored hence source inversions can be calculated. The source process involved in such events is thought to be related to magma transport, gas slug ascent or dike resonance initiated by a fluid driven process. Numerical modelling of gas slug ascent (constrained by results from published laboratory experiments) shows that this process is capable of generating long period and VLP like signals. Inputting the fluid dynamic derived forces into an elastic medium, with volcano like topography, VLP signals are reproduced. A moment tensor inversion is performed on these synthetic VLP signals retrieving the correct input source mechanism (a pipe structure).