Fluid ascent during the 2004-2005 unrest at Mt. Spurr inferred from seismic tomography

We present a seismic model of the crust beneath Mt. Spurr Volcano in Alaska based on tomographic inversion of local earthquake data recorded by temporary seismic network during the unrest of Mt. Spurr in 2004-2005. Beneath the northern vent of Mt. Spurr, where the most intensive fumarolic activity was observed during the last unrest, we observe a finger-shaped anomaly with a higher P and lower S velocity, which corresponds to high Vp/Vs ratio. Another anomaly is located to the south of Crater Peak approximately beneath the same location where an explosive eruption occurred in 1992. We propose that these two finger anomalies represent separated conduits of magma and volatiles. The NE conduit extends to the surface and enables free volatile flux. The SE conduit seems to be blocked at a ca. 20 km depth and in case of continuous pressure build up may become a source of an explosive eruption.