We suggest that our trigger mechanism must be highly repeatable, at very small intervals (< 1s), and that the waveform changes so little that the source region must be VERY small. Hence the trigger maintains its position while the magma is ascending through this region.

Our proposed model has the magma column within the volcanic edifice, with a velocity profile across it. There is a high viscosity region at the edge of the conduit and low viscocity in the centre (fig. 1). Its gradient is controlled by gas and heat loss through the conduit walls into the edifice. Shear fractures occur in the high viscocity boundary layer near the conduit wall when shear stress = viscosity * strain rate exceeds a critical value.

We have a seismically constrained source depth, above which all movement of the magma is friction controlled and aseismic. Below this depth, the magma behaves in a ductile manner (fig 2.) and conduit resonance can take place.