Is the seismic response of the upper volcanic edifice dominated by the rheological properties of compliant weak rocks?

The upper volcanic edifice is important in influencing the final leg of the migration pathway of magma to the earth’s surface. The expectation is that such migration will have a clear seismic response, which will allow it to be tracked through the shallow subsurface. Consequently, shallow Long Period (LP) seismicity and volcanic tremor are viewed with considerable interest in hazard estimation. However, a detailed analysis of LP seismicity and tremor signals demonstrates that it is possible to generate them in ways that do not require the presence of migrating fluids. Furthermore, it has long been recognised that a short interval of quiescence often precedes eruptions, which is puzzling if seismicity-generating fluids are approaching the surface. Here we look at the role played by compliant and weak rocks, the norm in upper volcanic settings, on the seismic & seismicity response. We find that many of the observed characteristics associated with pre-eruptive seismicity can be explained by considering upper edifice rheology. This analysis also points to exceptionally weak structures, at the scale of the whole edifice.