A constant influx model for dike propagation:Implications for magma reservoir dynamics

摘要

Most observations of seismicity rate during dike propagation on basaltic volcanoesshow (1) rate stationarity despite possible variations of the dike tip velocity, (2) frequent lackof clear and monotonic hypocenter migration following dike propagation, and (3) eventoccurrences located backward with respect to the dike tip position. On these bases, the originof the seismicity contemporary to dike intrusion within basaltic volcanoes cannot be solelyrelated to the crack tip propagation. Seismicity rather appears to be the response of theedifice itself to the volumetric deformation induced by the magma intruding the solid matrix.The volume change induced into the volcano edifice over time by the intruding magmais equal to the magma flux injected into the dike from the reservoir. The consequence of thisis that the stationary seismicity rate observed during the intrusion is a proxy for the magmaflux withdrawn from the reservoir. We consider a two-phase dike propagation model,including a first vertical propagation followed by a lateral migration along a lithologicaldiscontinuity. We explore (1) under which geophysical conditions the vertical dike is fed atconstant flow rate of magma and (2) dike propagation patterns. Implications entailed byconstant volumetric flux on the Piton de la Fournaise volcano case study suggest a minimumsize for the magma reservoir of about 1 km3 and a maximum value for the initial magmareservoir overpressure of about 2.2 MPa. Considering similar magma inflow rates duringvertical and lateral dike propagation phases, we reproduce independent estimates ofpropagation velocities, rise times, and injected volumes when applying the model to theAugust 2003 Piton de la Fournaise eruption.