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.
Highlights
Has been occurring at a rate of ~4 cm per year[8,9] with a deformation center that lies ~10 km to the south of the NRV edifice at a depth of ~14 km[8]
Since the 1980s, NRV has been monitored by a permanent seismic network that has been expanded gradually. Data from this network have been used to determine the locations of seismic events, their spectral characteristics[10], time changes of seismic attenuation beneath the volcano[11] as well as a three-dimensional seismic crustal model[12]
The extensive data set that has been acquired since 2002 allows large improvements in the resolution of tomographic inversions
Summary
Several authors have investigated time variations of seismic structure beneath active areas using repeated tomographic inversions[13,14,15]. For each event in the first dataset, we look for a “paired” event in the second dataset with a maximum number of common phases recorded at the same stations This yields two datasets with similar ray paths, which effectively minimizes data coverage variations. We were not able to achieve totally identical distributions but checked that the impact on inversions results was small This was achieved by comparing the three different velocity models obtained for the reference time-interval. The ability of the inversion algorithm to resolve time changes of seismic structure was assessed using a series of synthetic tests (Supplementary materials). Starting from the same synthetic model (e.g., a checkerboard in Extended Data Figure S3), we found that changes of seismic ray coverage lead to differences of velocity values that are much smaller than those obtained from the experimental data. More details on the synthetic modeling are given in Supplementary Materials
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