Magma pressure discharge induces very long period seismicity

Volcano seismicity is one of the key parameters to understand magma dynamics of erupting volcanoes. However, the physical process at the origin of the resulting complex and broadband seismic signals remains unclear. Syn-eruptive very long period (VLP) seismic signals have been explained in terms of the sudden expansion of gas pockets rising in the liquid melt. Their origin is linked to a magma dynamics which triggers the explosive process occurring before the explosive onset. We provide evidence based on acoustic, thermal, and ground deformation data to demonstrate that VLP signals at Stromboli are generated at the top of the magma column mainly after the explosion onset. We show that VLP amplitude and duration scale with the eruptive flux which induces a decompression of 103–104 Pa involving the uppermost ~ 250 m of the feeding conduit. The seismic VLP source represents the final stage of a ~ 200 s long charge and discharge mechanism the magma column has to release excess gas accumulated at the base of a denser and degassed magma mush. The position of the VLP seismic source coincides with the centroid of the shallow mush plug and tracks elevation changes of the magma free surface.

Influence of permeability on the hydrothermal system at Vulcano Island (Italy): inferences from numerical simulations

Volcano-hydrothermal systems are governed by complex interactions between fluid transport, and geochemical and mechanical processes. Evidence of this close interplay has been testified by distinct spatial and temporal correlations in geochemical and geophysical observations at Vulcano Island (Italy). To understand the interaction between fluid circulation and the geochemical and geophysical manifestations, we perform a parametric study to explore different scenarios by implementing a hydro-geophysical model based on the equations for heat and mass transfer in a porous medium and thermo-poroelastic theory. Numerical simulations allow us to define the controlling role of permeability distribution on the different modeled parameters as well as on the geophysical observables. Changes in the permeability within the highly fractured crater area could be responsible for the fluctuations in gas emission and temperature recorded during the crisis periods, which are accompanied by shallow volcano-seismicity in the absence of significant deformation and gravity variations. Despite the general medium permeability of the volcanic edifice, the presence of more highly permeable pathways, which allow the gas to rapidly escape, as testified by the presence of a well-developed fumarolic field, prevents the pressure buildup at shallow depths. Graphic abstract

Pressure-Driven Opening and Filling of a Volcanic Hydrofracture Recorded by Tuffisite at Húsafell, Iceland: A Potential Seismic Source

The opening of magmatic hydraulic fractures is an integral part of magma ascent, the triggering of volcano seismicity, and defusing the explosivity of ongoing eruptions via outgassing magmatic volatiles. If filled with pyroclastic particles, these fractures can be recorded as tuffisites. Tuffisites are therefore thought to play a key role in both initiating eruptions and controlling their dynamics, and yet their genesis remains poorly understood. Here we characterise the processes, pressures and timescales involved in tuffisite evolution within the country rock through analysis of the sedimentary facies and structures of a large sub-horizontal tuffisite vein, 0.9 m thick and minimum 40 m in length, at the dissected Húsafell volcano, western Iceland. The vein occurs where a propagating rhyolitic sheet intrusion stalled at a depth of ∼500 m beneath a relatively strong layer of welded ignimbrite. Laminations, cross-stratification, channels, and internal injections indicate erosion and deposition in multiple fluid pulses, controlled by fluctuations in local fluid pressure and changes in fluid-particle concentration. The field evidence suggests that this tuffisite was emplaced by as many as twenty pulses, depositing sedimentary units with varying characteristics. Assuming that each sedimentary unit (∼0.1 m thick and minimum 40 m in length) is emplaced by a single fluid pulse, we estimate fluid overpressures of ∼1.9–3.3 MPa would be required to emplace each unit. The Húsafell tuffisite records the repeated injection of an ash-laden fluid within an extensive subhorizontal fracture, and may therefore represent the fossil record of a low-frequency seismic swarm associated with fracture propagation and reactivation. The particles within the tuffisite cool and compact through time, causing the rheology of the tuffisite fill to evolve and influencing the nature of the structures being formed as new material is injected during subsequent fluid pulses. As this new material is emplaced, the deformation style of the surrounding tuffisite is strongly dependent on its evolving rheology, which will also control the evolution of pressure and the system permeability. Interpreting tuffisites as the fossil record of fluid-driven hydrofracture opening and evolution can place new constraints on the cycles of pressurisation and outgassing that accompany the opening of magmatic pathways, key to improving interpretations of volcanic unrest and hazard forecasting.

Machine learning based detection of volcano seismicity using the spatial pattern of amplitudes

Summary We propose a new algorithm, focusing on spatial amplitude patterns, to automatically detect volcano seismic events from continuous waveforms. Candidate seismic events are detected based on signal-to-noise ratios. The algorithm then utilizes supervised machine learning to classify the existing candidate events into true and false categories. The input learning data are the ratios of the number of time samples with amplitudes greater than the background noise level at 1 s intervals (large amplitude ratios) given at every station site, and a manual classification table in which ‘true'' or ‘false'' flags are assigned to candidate events. A two-step approach is implemented in our procedure. First, using the large amplitude ratios at all stations, a neural network model representing a continuous spatial distribution of large amplitude probabilities is investigated at 1 s intervals. Second, several features are extracted from these spatial distributions, and a relation between the features and classification to true and false events is learned by a support vector machine. This two-step approach is essential to account for temporal loss of data, or station installation, movement, or removal. We evaluated the algorithm using data from Mt. Ontake, Japan, during the first ten days of a dense observation trial in the summit region (November 1–10, 2017). Results showed a classification accuracy of more than 97 per cent.

An Experimental Study of Volcanic Tremor Driven by Magma Wagging

<p> </p><p><br>Volcanic tremor is a feature of most explosive eruptions. Pre-eruptive tremors can be characterized by monotonic increases in the maximum frequency, frequency bandwidth and amplitude that are correlated with increases in gas flux from a volcanic vent. An enigmatic feature of this behavior is that is observed at volcanoes with widely ranging conduit geometries and structures. Accordingly, the ``magma wagging'' model introduced by [1] and extended by [2] hypothesizes an underlying mechanism that is only weakly-sensitive to volcano architecture: Within active volcanic conduits, the flow of gas through a permeable foamy annulus of gas bubbles excites and maintains an oscillation of a central magma column through a well-known Bernoulli effect. Furthermore, this oscillation has spectral properties that evolve depending on annulus thickness and permeability and the total flow of gas. </p><p>In this thesis, we carry out a critical experimental test of the underlying mechanism for excitation. We explore the response of columns with prescribed elastic and linear damping properties to forced air annular airflows. From high-speed video measurements of linear and orbital displacements and time series of accelerometer measurements we characterize and understand the excitation, evolution, and steady-state oscillating behaviors of analog magma columns over a broad range of conditions. Where the time scale for damping is much longer than the natural period of free oscillation, column oscillation is continuously excited by relatively short period Bernoulli modes through a reverse energy cascade. We also identify three distinct classes of wagging: i. rotational modes that confirm predictions for whirling modes by [3]; as well as ii. mixed-mode; and iii. chaotic modes that are extensions to previous studies[1,2]. Our results show that rotational modes are favored for symmetric, and high-intensity forcing. Mixed-mode responses are favored for a symmetric and intermediate intensity forcing. Chaotic modes occur in asymmetric or low intensity forcing. To confirm and better understand our laboratory results and also extend them to conditions beyond what is possible in the laboratory we carry out two-dimensional numerical simulations of our analog experiments.</p><p>Taken together, results from our experimental and numerical studies can be extended to a natural system to make qualitative predictions testable in future studies of pre- and syn-eruptive volcano seismicity. Far before an eruption, the total gas flux is low and magma wags in a chaotic mode no matter what is the spatial distribution of the gas flux. At a pre-eruptive state, as gas flux increases, if the distribution of gas flux is approximately symmetric, we expect a transition to mixed and possibly rotational modes. During an eruption, fragmentation and explosions within the foamy annulus can cause spatial heterogeneity in permeability resulting in non-uniform gas flux that favors chaotic wagging behavior. </p><p>[1] A. M. Jellinek and D. Bercovici. Seismic tremors and magma wagging during explosive volcanism. Nature, 470(7335):522-525, 2011</p><p>[2] D. Bercovici, A. M.  Jellinek, C. Michaut, and D. C. Roman. Volcanic tremors and magma wagging: gas flux interactions and forcing mechanism. Geophys. J.Int., 195(2):1001-1022, 2013</p><p>[3] Y. Liao and D. Bercovici. Magma wagging and whirling: excitation by gas flux. Geophys. J.Int., 215(1):713-735, 2018</p>

The 2011 unrest at Katla volcano: seismicity and geological context

Katla is one of the most active volcanoes in Iceland and is characterised by persistent seismicity. It is partly covered by the Mýrdalsjökull glacier and its historic activity is dominated by phreatomagmatic eruptions within the caldera associated with catastrophic glacial floods. In July 2011 a sudden jökulhlaup was released from the glacier, associated with tremor, elevated seismicity inside the caldera and a new cluster of seismicity on the south flank. This was likely caused by a hydrothermal or magmatic event, possibly a small subglacial eruption. Similar unrests occurred in 1955 and 1999. We have identified changes of the seismicity pattern coinciding with the 2011 unrest, suggesting a modification in the volcanic system. It may be speculated that if the persistent seismicity at Katla is an indication of a pressurized magma system ready to erupt, small events like those of 1955, 1999 and 2011 may trigger larger eruptions in the future. We have also conducted a pilot study of the geology of the southern flank, where the new seismicity is recorded, and identified sources for flank eruptions in the recent eruptive history of Katla. These include rhyolitic domes and surtseyan craters. Therefore, a wide range of volcanic processes have to be taken into account as possible source for the new seismicity and volcanic hazard.

Análisis de la actividad volcánica tipo VT y LP del volcán Nevado del Ruiz entre 1985-2012 [Analysis of volcanic activity type VT and LP of the Nevado del Ruiz volcano between 1985-2012]

En la Cordillera de los Andes existen varios complejos volcánicos, siendo el Complejo volcánico Cerro Bravo – Cerro Machín (CVCBCM), el más septentrional de ellos. El Volcán Nevado del Ruiz (VNR), ubicado en dicho complejo, está activo y su evento más devastador fue el ocurrido en noviembre 13 de 1985, cuando el flujo de lodo originado tras el deshielo parcial del casquete glaciar, producto de una erupción freato-magmática, arrasó la población de Armero (Tolima), con un saldo de 23000 personas muertas. Como respuesta a tal catástrofe, se creó el Observatorio Vulcanológico y Sismológico de Manizales, que lleva ya 27 años de constante monitoreo de la actividad de los volcanes del CVCBCM, para lo cual utiliza varias técnicas geofísicas, geodésicas y geoquímicas. Este trabajo tiene como objetivo mostrar la actividad del volcán a partir de la sismicidad, considerando el conteo de eventos sísmicos y la energía liberada por los mismos, diferenciando los sismos fractura y los de movimiento de fluidos. La labor de clasificación y lectura de los sismos, cuenta con la participación activa de estudiantes de pregrado, fungiendo como asistentes de investigación, lo que les permite colaborar en la vigilancia volcánica y adquirir conocimientos dirigidos a su futuro desempeño como vulcanólogos.Palabras claves: Monitoreo volcánico, sismicidad, sismos de fractura, sismos de movimiento de fluidosIn the Andes there are several volcanic complexes, with the Volcanic Complex Cerro Bravo - Cerro Machin (CVCBCM), the northernmost of them. The Nevado del Ruiz volcano (VNR) located in said complex, is active and was the most devastating event occurred in November 13, 1985, when the mudflow caused after partial melting of the ice cap, the result of an eruption freato- magmatic, razed the town of Armero (Tolima), leaving 23,000 people dead. In response to this catastrophe, the Volcanological and Seismological Observatory of Manizales, which has been 27 years of constant monitoring of the activity of the volcanoes of CVCBCM, for which utilizes various geophysical, geodetic and geochemical techniques was created. This work aims to show the volcano seismicity from considering counting seismic events and the energy released by them, differentiating the fracture earthquakes and fluid motion. The work of sorting and reading earthquakes, with the active participation of undergraduates, serving as research assistants, allowing them to collaborate in volcano monitoring and acquire knowledge aimed at their future roles as volcanologists.Keywords: Volcanic Monitoring, seismicity, seismic fracture fluid motion earthquakes

KARAKTERISTIK FLUIDA DAN GAS PANAS BUMI SEBAGAI PENGARUH PENINGKATAN AKTIVITAS VULKANIK GUNUNGAPI SLAMET (Characteristic Of Geothermal Fluid And Gas Induced By Increased Volcanic Activity Of Slamet Volcano)

AbstrakSejak awal Maret 2014 status aktifitas Gunungapi Slamet di Jawa Tengah dinaikan dari Normal menjadi Waspada seiring dengan peningkatan jumlah gempa vulkanik. Seismisitas Gunungapi Slamet memperlihatkan gempa letusan 1106 kejadian dan gempa hembusan 6857 per hari. Sementara itu, gempa vulkanik dalam hanya terekam 2 kali selama periode Maret – Agustus 2014. Sumber gempa berada pada kedalaman antara 1 - 2 km di bawah kawah Gunungapi Slamet sebagai indikasi gempa permukaan. Peningkatan aktifitas vulkanik Slamet menghasilkan pelepasan gas CO2 yang berpengaruh terhadap fluida panas bumi yang ditunjukan dengan terjadinya perubahan keasaman air dari normal menjadi alkalin, pembentukan bualan gas CO2 pada air panas Pancuran 3 di Baturraden, dan peningkatan saturasi kalsit. Bualan gas CO2 menjadi indikasi terjadinya proses pendidihan pada temperatur 273 C pada kedalaman 454 m di bawah permukaan laut. Kondisi ini menjadikan temperatur reservoir lebih tinggi sebagai indikasi sistem panas bumi Gunungapi Slamet merupakan sistem panas bumi aktif bertemperatur tinggi. Kata kunci : gempa permukaan, fluida panas bumi, bualan gas, saturasi kalsitAbstractSince the beginning of March 2014 the status of activities Slamet volcano in Central Java has been declared from Normal becomes Alert due to significant increase in the number of volcanic earthquakes. Slamet volcano seismicity shows eruption earthquakes as many as 1106, and gas emission earthquakes as many as 6857 events per day. Meanwhile, the deep volcanic earthquake recorded only 2 times during the period March to August 2014. The hypocentre of these earthquake was at a depth of 1-2 km below Slamet volcano crater as an indication of the surface earthquakes.Increased magmatic activity resulted in the release of CO2 gas effect on the geothermal fluid which is indicated by changes in water acidity from normal to alkaline, formation of CO2 bubble gas on Pancuran 3 hot spring at Baturraden area, and calcite saturation enhancement. The presence of CO2 bubble gas is indication of boiling at temperatures 273 C at a depth of 454 m below sea level. This condition makes the reservoir temperature becomes higher as indication that the geothermal system of Slamet volcano is active geothermal systems with high temperature (high enthalpy).Keywords: surface earthquake, geothermal fluid, bubble gas.