The changes in physical properties leading up to a volcanic eruption provide clues to processes occurring within volcanoes and may reveal premonitory signals. The Sierra Negra shield volcano, located in the Galápagos Islands, erupted on 26 June 2018 after months of continued inflation and escalating earthquake activity. We applied ambient noise interferometry to five months of continuous seismic data from 12 broadband stations to calculate crustal shear‐wave velocity changes before and during the eruption. Using the Moving Window Cross‐Spectral technique and a pre‐eruption stack of ambient seismic data as reference, we found a −0.27% decrease in velocity 17 days before the eruption in station‐pairs that pass beneath the caldera's north‐eastern sector. Sensitivity kernels suggest that the velocity changes of this precursory signal are greatest at depths corresponding to the shallow sill (∼2 km) beneath the wide caldera. Our results, considered in light of geodetic, seismicity, and petrological results, suggest that this velocity decrease is in part caused by dilatation from a ML 4.8 earthquake, and degassing after a possible magma intrusion below or at the base of the sill. The precursory velocity decrease within the sill region offers an important tool for forecasting future eruptions at Sierra Negra.

<p>The Galápagos Archipelago originates from a plume-like structure that rises from the mantle about 250 km south of the islands. The Isabela Island, located on the western part of the Archipelago, contains several of the most active volcanoes in Galápagos, among them Alcedo, Cerro Azul, and Sierra Negra, whose last eruptions occurred in 1953, 2008, and 2018, respectively.</p><p>Several studies from different disciplines have been performed to image the subsurface structures at the volcanoes on Isabela. They report a melt-rich sill located at 2 km depth, a crystal-mush zone below Sierra Negra located at depths approximately between 8 to 15 km, and a magma intrusion for depths between the sill and the crystal mush before the 2010 eruption of Sierra Negra. However, the resolution of these studies is limited along many areas and depths because of multiple reasons, like non-ideal station distribution, limitations on the selected methodologies, or sparse earthquake locations.</p><p>Using seismic data recorded by two temporal seismic networks deployed in the Archipelago, we used the ambient seismic noise to obtain a 3D S-wave velocity model; we used this information to improve the understanding of the structure of the subsurface in the area. One of the networks -XE array- was composed of 18 stations deployed between July 2009 and June 2011; the second network -YH array, composed of 10 stations, was deployed between August 1999 and March 2003. Provided the distribution of the seismic stations, a higher resolution was obtained on Isabella Island. Therefore, we focused our analysis on the regional-scale feeding systems of the volcanoes in Isabela, in particular, Alcedo, Sierra Negra, and Cerro Azul volcanoes.</p><p>Through an iterative linear-least-squares inversion methodology, we obtained Rayleigh phase-velocity maps for periods in the range 2.5-25 s. Subsequently, we inverted the obtained tomographic maps for retrieving the S-wave velocity distribution as a function of depth. Our results indicate two main discontinuities, located at 3 and 11 km depth, agreeing with the expected depth for the discontinuity between old and new oceanic crust. The first layer presents an average S-wave velocity of 2.4 km/s, while the second and third layers - 3.0 km/s and 3.4 km/s, respectively. Our results show two relevant low-velocity zones in the subsurface: one is located between Sierra Negra and Alcedo volcanoes centered at 20 km depth, the second one is below Sierra Negra at 8 km depth, which we interpret as magma accumulation zones. In addition, our results show a high-velocity zone at 3 km depth, coincident with the previously reported melt-rich sill.</p><p>This work not only validates the results obtained by previous works but provides information with higher resolution for certain depths of the subsurface of hazardous volcanoes on Galápagos.</p>

<p>The general objectives of the “Seismic Ambient Noise Imaging and Monitoring of Shallow Structures” (SANIMS) project, funded by the Spanish Ministry of Science, Research and Innovation (Ref.: RTI2018-095594-B-I00), are focused into the application and development of methods based on ambient noise seismic data recorded by dense networks to image and monitor natural and human-altered environments. To achieve this objective, temporal seismic networks have been installed since late 2019 in two very different settings; the Cerdanya Basin, a sedimentary basin located in the eastern Pyrenees and the city of Barcelona.</p><p>Regarding the Cerdanya Basin, a relatively unaltered setting, a network of up to 25 broad-band stations has been installed for a period of one year. Additionally, a high resolution grid of seismic nodes will be deployed for 2 months in the central part of the basin, with interstation distances of 1.5 km. In order to constraint the uppermost crustal structure using ambient noise, vertical component recordings will be processed using the phase cross-correlation and time-frequency domain phase-weighted stacking to extract fundamental mode Rayleigh waves. The surface waves will then be used to measure inter-station group and phase velocity dispersion curves that will be inverted using the Fast Marching Surface Tomography method. Depending on data quality, we will also process the horizontal components to extract Love waves for joint inversions with Rayleigh waves to constrain radial anisotropy and/or the application of new strategies to perform attenuation tomography.</p><p>Regarding areas strongly altered by human activity, we have deployed a network of 15 short-period stations within the city of Barcelona, in most of the cases installed in the basement of secondary schools, for a duration of 9-12 months. The objective of this deployment is twofold; acquire new valuable scientific data and introduce the students in an Earth Science research project. Although the Barcelona area has been investigated using MHVSR methods by different authors, the new data acquired by the SANIMS project will expand the available data and will allow to analyze the time variability of the measurements. This new dataset will also be used to analyze the applicability of the methods based on Rayleigh wave ellipticity inversion of ambient noise and earthquake data to provide S-velocity depth profiles. Under the assumption of an isotropic horizontally layered medium, the ellipticity inversion is not affected by the directivity of the diffusive noise wave field and seems therefore to be a good option to determine local S-velocity depth profiles in areas with little lateral inhomogeneities and uneven distribution of noise sources.</p><p>We expect that the use of ambient noise methods will allow to map the basement and to obtain new higher resolution ambient noise tomographic images of the upper crust in the Cerdanya Basin and to better constrain the subsoil properties of Barcelona, hence improving the existing seismic hazard maps. Besides, comparing the results in both areas will allow to compare the performance of the different methods based on ambient noise in quiet and noisy areas.</p>

<p align="justify"><span>Changes in external stress state and fluid content alter the mechanical properties of an geological media. </span><span>Variations in seismic wave velocity can be used as proxies for changes in stress the onset of mechanical demage and/or possible fluid ingression. Temporal variations in seismic wave velocity have previously been monitored and observed prior to volcanic eruptions. In the absence of additional constraints related to stress or fluid changes on the volcano, these pre-eruptive changes are difficult to interpret and hence the causes of them are often not well understood. </span><span>In this study, Coda Wave Interferometry (CWI) is used to measure time-lapse changes in seismic velocity on seismic multiplets (repeating similar earthquakes). In particular, we focus our analysis on using this technique to calculate the velocity changes on the data recorded prior to the 2018 eruption of Sierra Negra volcano, Galapagos Island.</span> <span>On 26th June 2018 at 09:15 UTC, a magnitude 5.3 earthquake occurred near the south-west caldera rim and an intense seismic swarm started around 17:15 UTC. Seismic tremor dominated at about 19:45 UTC, which marked the onset of the eruption. </span><span>A very large seismicity sequence preceded the eruption. The pricise relationship between the magnitude 5.3 event and the eruption is not fully constraind. Here we search for multiplets in order to achieve high time resolution velocity change information in the hours between the large earthquake and the eruption. </span><span>Our aim is to understand whether changes in seismic velocity measured with CWI on multiplets method provide new insight into the physical processes related to the eruption.</span></p><p align="justify"><br><br></p>