It is due to the presence of frozen magma left in site from the last volcano eruption. This material can be much more dense (and more rigid)  than surrounding looser material (aerial from eruptions, sedimented and compacted) and hence may have higher P- and S-wave velocity.  Intuitively one may think at a lower velocity body beneath a volcano if this body is composed by high temperature rocks (magma or partially melt rocks). The high temperature lower much more the S-wave speed than P-wave speed, and makes the S-wave seismic attenuation much stronger, while P-wave attenuation may be not really lower. Concluding, beneath a volcano (down to 1-2 km beneath the crater) not necessarily Vp and Vs are lower than the surrounding rocks. Sometimes are higher, due to the frozen and rigid lava blocks formed AFTER the last eruptions.

it is possible that high-velocity srrounds the volcanic area in the upper crust. A similar case is found in China around the volcanoes showing low conductivity. They are interpretated as frozon materials after hot materail upwelling. Just for your reference.

P-wave speed is poorly affected by the rock temperature. S-wave velocity on the contrary does depend on temperature. So, P-wave positive anomalies may be measured during the volcano unrest too. 

Thanks for your reply. So it is no way that a molten material at shallower depths gives a higher P-wave anomaly. It is solidified magma intrusion

regards