Breathing of the Nevado del Ruiz volcano reservoir, Colombia, inferred from repeated seismic tomography

Abstract

Nevado del Ruiz volcano (NRV), Columbia, is one of the most dangerous volcanoes in the world and caused the death of 25,000 people in 1985. Using a new algorithm for repeated tomography, we have found a prominent seismic anomaly with high values of the Vp/Vs ratio at depths of 2–5 km below the surface, which is associated with a shallow magma reservoir. The amplitude and shape of this anomaly changed during the current phase of unrest which began in 2010. We interpret these changes as due to the ascent of gas bubbles through magma and to degassing of the reservoir. In 2011–2014, most of this gas escaped through permeable roof rocks, feeding surface fumarole activity and leading to a gradual decrease of the Vp/Vs ratio in the reservoir. This trend was reversed in 2015–2016 due to replenishment of the reservoir by a new batch of volatile-rich magma likely to sustain further volcanic activity. It is argued that the recurring “breathing” of the shallow reservoir is the main cause of current eruptions at NRV.

Evolution of the Magma Conduit Beneath the Galeras Volcano Inferred From Repeated Seismic Tomography

“…In both models, anomaly (2) seems to be separated from the shallower anomaly (1), and its top is located at approximately 2 km below the surface. Note that very similar anomalies were observed beneath other volcanoes, such as Nevado del Ruiz (Vargas et al, ), Gorely (Kuznetsov et al, ), and Mount Spurr (Koulakov, West, & Izbekov, ). This anomaly is interpreted as a volatile‐rich magma conduit.…”

“…Observing the results of the P and S wave velocity anomalies in the supporting information shows that this anomaly is associated with high P and low S wave anomalies. Note that the same feature is observed in many other volcanoes throughout the world, such as Nevado del Ruiz (Vargas et al, ), Bezymianny (Ivanov et al, ), and Redoubt (Kasatkina et al, ). In the cases of volcano‐related structures, this pattern can be explained by the fact that the P wave velocity is most sensitive to the composition of the rocks, whereas the S wave velocity is strongly affected by the presence of liquid phases.…”

“…To overcome this problem, Koulakov et al () proposed a selection of identical data sets for repeated inversions that minimizes the artifacts related to different data distributions and implemented this method for studying velocity variations after the Tohoku‐Oki earthquake in Japan in 2011. Later, a similar approach was used for analyzing another Colombian volcano, the Nevado del Ruiz (Vargas et al, ). That study revealed a magma reservoir at a depth of 2–4 km below the surface that gradually deflated during the degassing activity of the volcano during 2011–2015 and then inflated during 2016–2017.…”

“…That study revealed a magma reservoir at a depth of 2–4 km below the surface that gradually deflated during the degassing activity of the volcano during 2011–2015 and then inflated during 2016–2017. In the present study, we use a similar strategy to that used by Vargas et al () for the case of the Galeras volcano.…”

“…The same data set has already been used for tomographic inversion by (Vargas & Torres, ) who constructed the 3‐D model of the P and S velocities beneath Galeras. A fairly large number of events during the whole observation period makes it possible to implement the same approach that Vargas et al () used to study Nevado del Ruiz and to compose data subsets with similar ray distributions for a pair of separate time periods. When defining the time episodes, we considered particular features of the eruption activity (Figure ) and the availability of the seismic data.…”