The Toba Caldera has been the site of several large explosive eruptions in the recent geological past, including the world's largest Pleistocene eruption 74,000 years ago. The major cause of this particular behaviour may be the subduction of the fluid-rich Investigator Fracture Zone directly beneath the continental crust of Sumatra and possible tear of the slab. Here we show a new seismic tomography model, which clearly reveals a complex multilevel plumbing system beneath Toba. Large amounts of volatiles originate in the subducting slab at a depth of ∼150 km, migrate upward and cause active melting in the mantle wedge. The volatile-rich basic magmas accumulate at the base of the crust in a ∼50,000 km3 reservoir. The overheated volatiles continue ascending through the crust and cause melting of the upper crust rocks. This leads to the formation of a shallow crustal reservoir that is directly responsible for the supereruptions.

We present seismic velocity models of the area beneath Redoubt Volcano (Alaska) corresponding to two time periods, before and after the strong eruption that occurred in March 2009. The calculations were based on tomographic inversion of P and S arrival time data recorded by 19 stations using the local earthquake tomography code LOTOS. We performed thorough analysis of the results based on synthetic tests in each of the time periods that allow real-time variations in structure to be distinguished from artifacts caused by changes in the observation system configurations. In the resulting images corresponding to the period before the eruption, the summit area is characterized by higher values of both P and S velocities and moderate values of the Vp/Vs ratio. This may correspond to the rigid igneous rocks composing the body of the volcano that lack significant liquid content. In the second episode corresponding to the time of the eruption, the P velocity remained high, whereas the S velocity became very low. The anticorrelation of P and S anomalies in the summit area produced a Vp/Vs ratio as high as 2.2, which is seen down to 2-3 km depth. This indicates presence of mobile phase at shallow depth beneath the volcano, which can be either in the form of partial melt or fluid-saturated rocks.

Mt. Spurr is the largest active volcano in Alaska of high explosive potential. The most recent activity, including two recent magmatic eruptions in 1953 and 1992, has occurred via the flanking Crater Peak. From 2004 to 2006, strong seismicity, gas flux, and heating were observed in the summit area, which had remained inactive for more than 5 Ka. To understand the cause of this reactivation, we performed repeated tomography inversions that clearly imaged the magma reservoir beneath Mt. Spurr and showed temporal changes in its shape and intensity. During the two years preceding the unrest, we observed ascension of the upper limit of the reservoir-related anomaly from a depth of 5 to 3 km below the surface, accompanied by strong seismicity. During the following years, the shape of the anomaly remained unchanged, but its intensity weakened. These observations may indicate the release of fluids from the ductile reservoir and fast upward ascent through the brittle cover that caused intensive seismicity and gas flux during the unrest from 2004 to 2006. The origin of this zone will possibly cause a resumption of explosive eruptions in the summit area of Mt. Spurr.