The seismic crisis that began in May, 2018 off the coast of Mayotte announced the onset of a volcanic eruption that started two months later 50 km southeast of the island. This seismicity has since ...been taken as an indicator of the volcanic and tectonic activity in the area. In response to this activity, a network of stations was deployed on Mayotte over the past three years. We used the machine learning-based method PhaseNet to re-analyze the seismicity recorded on land since March 2019. We detect 50,512 events compared to around 6508 manually picked events between March 2019 and March 2021. We locate them with NonLinLoc and a locally developed 1-D velocity model. While eruptions are often monitored through the analysis of Volcano-Tectonic (VT) seismicity (2–40 Hz), we focus on the lower frequency, Long Period (LP) earthquakes (0.5–5 Hz), which are thought to be more directly related to fluid movement at depth. In Mayotte, the VT events are spread between two clusters, whereas the LP events are all located in a single cluster in the bigger proximal VT cluster, at depths ranging from 25 to 40 km. Moreover, while the VT earthquakes of the proximal cluster occur continuously with no apparent pattern, LP events occur in swarms that last for tens of minutes. We show that during the swarms, LP events generally migrate downward at a speed of 5 m/s. While these events do not appear directly linked to upward fluid migration, their waveform signature could result from propagation through a fluid-rich medium. They occur at a different location than VT earthquakes, also suggesting a different origin which could be linked to the Very Long Period events (VLP) observed above the LP earthquakes in Mayotte.
The heat flux of an active volcano provides crucial information on volcanic unrest. The hydrothermal activity often responsible for volcanic unrest can be accompanied by an increase in the extent and ...intensity of hydrothermal alteration, which could influence the thermal properties of the volcanic edifice. Therefore, an understanding of the influence of alteration on the thermal properties of rocks is required to better interpret volcano heat flux data. We provide laboratory measurements of thermal conductivity, thermal diffusivity, and specific heat capacity for variably altered (intermediate to advanced argillic alteration) andesites from La Soufrière de Guadeloupe (Eastern Caribbean). We complement these data with previously published data for altered basaltic-andesites from Merapi (Indonesia) and new data for altered rhyodacites from Chaos Crags (USA). Our data show that thermal conductivity and thermal diffusivity decrease as a function of increasing porosity, whereas the specific heat capacity does not change systematically. Thermal conductivity decreases as a function of alteration (the percentage of secondary minerals) for the rocks from La Soufrière and Merapi (from ~1.6 to ~0.6 W·m−1·K−1 as alteration increases from ~1.5 to >75 wt%), but increases for the rocks from Chaos Crags (from ~1.1 to ~1.5 W·m−1·K−1 as alteration increases from ~6 to ~15 wt%). Although the thermal diffusivity of the rocks from Chaos Crags increases from ~0.65 to ~0.75–0.95 mm2·s−1 as alteration increases from ~6 to ~15 wt%, the thermal diffusivity of the rocks from La Soufrière and Merapi does not appear to be greatly influenced by alteration. The specific heat capacity is not significantly affected by alteration, although there is a slight trend of increasing specific heat capacity with alteration for the rocks from La Soufrière. We conclude that the decrease in thermal conductivity as a function of alteration in the rocks from La Soufrière and Merapi is the result of the low conductivity of the secondary mineral assemblage, and that a combination of the high thermal conductivity of cristobalite and the reduction in porosity as a result of the void-filling mineral precipitation can explain the increase in thermal conductivity in the rocks from Chaos Crags. Calculations show that an increase in alteration of a dome or edifice can result in decreases and increases in heat flow density, depending on the type of alteration. Therefore, alteration-induced changes in the thermal properties of dome or edifice rocks should be considered when interpreting volcano heat flux data. We conclude that it is important not only to monitor the extent and evolution of alteration at active volcanoes, but also the spatial distribution of alteration type.
•Hydrothermal alteration changes the thermal properties of volcanic rocks.•Alteration can increase or decrease thermal properties, depending on alteration type.•Alteration can change the conductive heat flow density at a volcano.•Alteration should be considered when interpreting volcano heat flux data.
Abstract
Nanoscale liquid immiscibility is observed in the 2018–2021 Fani Maoré submarine lavas (Comoros archipelago). Heat transfer calculations, Raman spectroscopy, scanning and transmission ...electron microscopy reveal that in contrast to thin (500 µm) outer rims of homogeneous glassy lava (rapidly quenched upon eruption, >1000 °C s
−1
), widespread liquid immiscibility is observed in thick (1 cm) inner lava rims (moderately quenched, 1–1000 °C s
−1
), which exhibit a nanoscale coexistence of Si- and Al-rich vs. Ca-, Fe-, and Ti-rich melt phases. In this zone, rapid nanolite crystallization contrasts with the classical crystallization process inferred for the slower cooled ( < 1 °C s
−1
) lava interiors. The occurrence of such metastable liquid immiscibility at eruptive conditions controls physicochemical characteristics of nanolites and residual melt compositions. This mechanism represents a common yet frequently unobserved feature in volcanic products, with the potential for major impacts on syn-eruptive magma degassing and rheology, and thus on eruptive dynamics.
The electrical resistivity distribution at the base of La Soufrière of Guadeloupe lava dome is reconstructed by using transmission electrical resistivity data obtained by injecting an electrical ...current between two electrodes located on opposite sides of the volcano. Several pairs of injection electrodes are used in order to constitute a data set spanning the whole range of azimuths, and the electrical potential is measured along a cable covering an angular sector of ≈120° along the basis of the dome. The data are inverted to perform a slice electrical resistivity tomography (SERT) with specific functions implemented in the EIDORS open source package dedicated to electrical impedance tomography applied to medicine and geophysics. The resulting image shows the presence of highly conductive regions separated by resistive ridges. The conductive regions correspond to unconsolidated material saturated by hydrothermal fluids. Two of them are associated with partial flank collapses and may represent large reservoirs that could have played an important role during past eruptive events. The resistive ridges may represent massive andesite and are expected to constitute hydraulic barriers.
This article provides a spatial and comparative approach to evaluate the territorial accessibility in the event of a volcanic crisis in the French West Indies. A spatial assessment of resources and ...populations exposed to volcanic hazards is performed, followed by an assessment of the risk of territorial isolation due to lahars. Modelling of the risk of terrestrial isolation builds upon graph-based computations and indices that take into account the specific vulnerability of river-crossing structures and the knowledge of historical lahars. Another application of the graphs concerns scenarios for the evacuation of population, the sole efficient response to an eruption. This results in an optimised division of areas to evacuate in order to assess the potential reduction of the load on the road network. These different results are integrated into a prototype for evacuation maps intended for local authorities. The situation of Guadeloupe is of greater concern than that in Martinique, given the level of exposure, the potential losses of accessibility in case of lahars, and the greater and on-going volcanic unrest of La Soufrière volcano in Guadeloupe.
A new depth‐averaged model has been developed for the simulation of both concentrated and dilute pyroclastic currents and their interactions. The capability of the model to reproduce a real event is ...tested for the first time with two well‐studied eruptive phases of the 2010 eruption of Merapi volcano (Indonesia). We show that the model is able to reproduce quite accurately the dynamics of the currents and the characteristics of the deposits: thickness, extent, volume, and trajectory. The model needs to be tested on other well‐studied eruptions and the equations could be refined, but this new approach is a promising tool for the understanding of pyroclastic currents and for a better prediction of volcanic hazards.
Plain language Summary
Pyroclastic currents are composed of hot gas and rock fragments. They are very dangerous, and their complex behavior makes the related hazards difficult to predict. They are generally formed of two distinct parts: (1) a basal flow that carries ashes and large blocks (up to cubic meters) that is very destructive but follows existing valleys and (2) a dilute part, called pyroclastic surge, that carries ashes in hot turbulent gases. This part is less destructive for infrastructures, but it is less confined by the topography, escapes easily from the valleys, and is very dangerous for the inhabitants. A new numerical model has been developed to simulate their emplacement. It is tested here with the eruption of Merapi volcano in 2010. We show that the model reproduces the main characteristics of the real phenomenon. This new model gives promising perspectives for the understanding of pyroclastic current emplacements and for future estimation of related hazards and impacts on the population, the infrastructure, and the environment.
Key Points
A new depth‐averaged model of pyroclastic currents coupling the concentrated and the dilute parts
The model is tested with the 2010 eruption of Merapi volcano and reproduces the main characteristics observed in the field
This is a new version of the code VolcFlow developed for the modeling of volcanic flows
Over the past two decades, La Soufrière volcano in Guadeloupe has displayed a growing degassing unrest whose actual source mechanism still remains unclear. Based on new measurements of the chemistry ...and mass flux of fumarolic gas emissions from the volcano, here we reveal spatio-temporal variations in the degassing features that closely relate to the 3D underground circulation of fumarolic fluids, as imaged by electrical resistivity tomography, and to geodetic-seismic signals recorded over the past two decades. Discrete monthly surveys of gas plumes from the various vents on La Soufrière lava dome, performed with portable MultiGAS analyzers, reveal important differences in the chemical proportions and fluxes of H2O, CO2, H2S, SO2 and H2, which depend on the vent location with respect to the underground circulation of fluids. In particular, the main central vents, though directly connected to the volcano conduit and preferentially surveyed in past decades, display much higher CO2/SO2 and H2S/SO2 ratios than peripheral gas emissions, reflecting greater SO2 scrubbing in the boiling hydrothermal water at 80–100 m depth. Gas fluxes demonstrate an increased bulk degassing of the volcano over the past 10 years, but also a recent spatial shift in fumarolic degassing intensity from the center of the lava dome towards its SE–NE sector and the Breislack fracture. Such a spatial shift is in agreement with both extensometric and seismic evidence of fault widening in this sector due to slow gravitational sliding of the southern dome sector. Our study thus provides an improved framework to monitor and interpret the evolution of gas emissions from La Soufrière in the future and to better forecast hazards from this dangerous andesitic volcano.
Indonesia is exposed to earthquakes, volcanic activities, and associated tsunamis. This is particularly the case for Lombok and Sumbawa Islands in West Nusa Tenggara, where evidence of tsunamis is ...frequently observed in its coastal sedimentary record. If the 1815 CE Tambora eruption on Sumbawa Island generated a tsunami with well-identified traces on the surrounding islands, little is known about the consequences of the 1257 CE tremendous eruption of Samalas on the neighboring islands, and especially about the possible tsunamis generated in reason of a paucity of research on coastal sedimentary records in this area. However, on Lombok Island, the eruption of the Samalas volcano produced significant volumes of pyroclastic flows that entered the sea in the North and East of the island. These phenomena must have produced a tsunami that left their traces, especially on Sumbawa Island, whose western coastline is only 14 km away from Lombok’s eastern shore. Therefore, the main goal of this study is to investigate, find evidence, and determine the age of marine-origin sediments along the shore of the Alas Strait, Indonesia. We collected and analyzed samples of coral and seashells from marine deposits identified along the west coast of Sumbawa, i.e., in Belang Island and abandoned fishponds in Kiantar Village, in order to identify the sources and the occurrence period of these deposits events. Based on the radiocarbon dating of coral and seashell samples, we concluded that none of the identified marine deposits along the western coast of Sumbawa could be related chronologically to the 1257 CE eruption of Samalas. However, possible tsunami deposits located in Belang Island and abandoned fishponds in Kiantar Village yielded 4th century CE, 9th century CE, and 17th century CE. We also conclude that past large earthquakes triggered these tsunamis since no known volcanic eruption occurred near the Alas Strait at that time that may have triggered a tsunami.
One of the most powerful eruptions of the Holocene, known as the Samalas eruption, had a VEI of 7 and took place on Lombok, eastern Indonesia in 1257 CE. Thick tephra fall covered the entire island, ...and pyroclastic density currents (PDCs) up to 50 m thick buried almost half of Lombok. Until now, there has been no detailed study of the geomorphological evolution of Lombok related to this eruption, especially for the eastern part of Lombok. The main goal of this paper is therefore to reconstruct the paleo-topography on the eastern part of Lombok before the eruption of the Samalas volcano in 1257 CE, and to analyse the subsequent landscape evolution following this eruption. Data were collected from more than 1300 points in order to gather geomorphological, geological, and geochemical information for the eastern part of Lombok over the last millennium. Data were obtained from wells (which are widespread in the studied area), natural outcrops (mainly cliffs along the shoreline and the rivers), and human-made outcrops in quarries, as well as from the two-dimensional resistivity profiling (Dipole-dipole array) carried out on eastern Lombok in 2016 and 2017. We estimate that 4435 ± 5.5 × 106 m3 of pumice-rich PDC, over a surface of 171 km2, buried the eastern part of Lombok in the aftermath of the 1257 CE eruption, and as a result of this rapid pumice-rich PDC deposition there has been some aggradation of the shoreline at some points, resulting in most of the pre-1257 coral reef being buried. After several intense rainfall events, the deposited volcanic material was probably eroded during the first decades following the 1257 CE eruption by mountain streams, producing lahars. We estimate that the remaining volume of pumice-rich PDC deposits following this erosion is 625 ± 5.5 × 106 m3 over a surface of 89 km2, which is equivalent to 14% of the initial volume.
•Paleo-topography reconstruction based on stratigraphic data and resistivity profile.•Landscapes on the eastern part of Lombok significantly evolved since the 13th century.•4435 ± 5.5 × 106 m3 of pumice-rich PDC buried the eastern part of Lombok in 1257.•The remaining volume in 2017 is 625 ± 5.5 × 106 m3 (14% of the initial volume).
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