Hydroacoustic Measurements of the 2014 Eruption at Ahyi Volcano, 20.4°N Mariana Arc

Ahyi is a fully submerged arc volcano in the Northern Mariana Islands, northwestern Pacific Ocean. In April and May 2014, the volcano erupted over a period of 15 days. Results from direction-of-arrival calculations show that underwater sound phases associated with the episode were recorded as far as Wake Island, where a hydrophone triplet array is operated as part of the International Monitoring System. After a 3.5-hr-long sequence of hydroacoustic precursory events, explosive volcanic activity occurred in two distinct, several-days-long bursts, accompanied by a notable decrease in low-frequency arrivals that may indicate a shift in signal source parameters. Acoustic resolution of the hydrophone data supersedes broadband networks by almost 1 order of magnitude, successfully identifying seismic events at Ahyi as low as 2.5 mb. Total radiated acoustic energy of the eruption is estimated at 9.7 1013 J, which suggests that submarine volcanic activity contributed significantly to the ocean soundscape

The Longevity of Lava Dome Eruptions

Understanding the duration of past, on-going and future volcanic eruptions is an important scientific goal and a key societal need. We present a new methodology for forecasting the duration of on-going and future lava dome eruptions based on a database (DomeHaz) recently compiled by the authors. The database includes duration and composition for 177 such eruptions, with "eruption" defined as the period encompassing individual episodes of dome growth along with associated quiescent periods during which extrusion pauses but unrest continues. In a key finding we show that probability distributions for dome eruption durations are both heavy-tailed and composition-dependent. We construct Objective Bayes statistical models featuring heavy-tailed Generalized Pareto distributions with composition-specific parameters to make forecasts about the durations of new and on-going eruptions that depend on both eruption duration-to-date and composition. Our Bayesian predictive distributions reflect both uncertainty about model parameter values (epistemic uncertainty) and the natural variability of the geologic processes (aleatoric uncertainty). The results are illustrated by presenting likely trajectories for fourteen dome-building eruptions on-going in 2015. Full representation of the uncertainty is presented for two key eruptions, Soufri{\'{e}}re Hills Volcano in Montserrat (10--139 years, median 35yr) and Sinabung, Indonesia (1--17 years, median 4yr). Uncertainties are high, but, importantly, quantifiable. This work provides for the first time a quantitative and transferable method and rationale on which to base long-term planning decisions for lava dome forming volcanoes, with wide potential use and transferability to forecasts of other types of eruptions and other adverse events across the geohazard spectrum.Comment: 17 pages, 4 figures, 3 table

The 2008 Eruptive Unrest at Cerro Azul Volcano (Galápagos) Revealed by InSAR Data and a Novel Method for Geodetic Modelling

Cerro Azul is one of the most active volcanoes in the western Galápagos Islands, but its unrest episodes are poorly studied. Unrest, which started in 2007, culminated in two eruptive phases from 29 May to 11 June 2008. We investigate this unrest and the associated eruptions using interferometric synthetic aperture radar (InSAR) data and geodetic modelling. To overcome the unwrapping errors affecting some of our InSAR data, we invert the wrapped phase directly by estimating the integer ambiguities simultaneously with the geophysical parameters. Our results highlight how the eruption was preceded by long‐term pre‐eruptive inflation (October 2007–April 2008). During the first eruptive phase, most of the magma responsible for the inflation fed the lateral propagation of a radial dike, which caused a first deflation of the magmatic reservoir. During the second eruptive phase, the further lateral propagation of the dike fed a radial eruptive fissure at the base of the edifice, causing further deflation of the magmatic reservoir. From the first to the second eruptive phase, the radial dike changed its strike propagating toward a topographic low between Cerro Azul and Sierra Negra

Challenges of Volcanic Crises on Small Islands States

International audienc

Distribution of Temperature and Strength in the Central Andean Lithosphere and Its Relationship to Seismicity and Active Deformation

We present three-dimensional (3D) models of the present-day steady-state conductive thermal field and strength distribution in the lithosphere beneath the Central Andes. Our primary objective was to investigate the influence that the structure of the Central Andean lithosphere has on its thermal and rheological state, and the relationship between the latter and the active deformation in the region. We used our previous data-driven and gravity-constrained 3D density model as starting point for the calculations. We first assigned lithology-derived thermal and rheological properties to the different divisions of the density model and defined temperature boundary conditions. We then calculated the 3D steady-state conductive thermal field and the maximum differential stresses for both brittle and ductile behaviors. We find that the thickness and composition of the crust are the main factors affecting the modeled thermal field, and consequently also the strength distribution. The orogen is characterized by a thick felsic crust with elevated temperatures and a low integrated strength, whereas the foreland and forearc are underlain by a more mafic and thinner crust with lower temperatures and a higher integrated strength. We find that most of the intraplate deformation coincides spatially with the steepest strength gradients and suggest that the high potential energy of the orogen together with the presence of rheological lateral heterogeneities produce high compressional stresses and strong strain localization along the margins of the orogen. We interpret earthquakes within the modeled ductile field to be related to the weakening effect of long-lived faults and/or the presence of seismic asperities.Fil: Ibarra, Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Prezzi, Claudia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Geociencias Básicas, Aplicadas y Ambientales de Buenos Aires; ArgentinaFil: Bott, Judith. German Research Centre for Geosciences; AlemaniaFil: Scheck Wenderoth, Magdalena. German Research Centre for Geosciences; AlemaniaFil: Strecker, Manfred. Universitat Potsdam; Alemani

Tracking submarine volcanic activity at Monowai: Constraints from long-range hydroacoustic measurements

Monowai is a submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. In the past, activity at the volcano had been intermittently observed in the form of fallout at the sea surface, discolored water, changes in seafloor topography, and T phase seismicity, but there is no continuous record for more recent years. In this study, we investigated 3.5 years of recordings at a hydrophone array of the International Monitoring System (IMS), located near Juan Fernández Islands for long‐range underwater sound waves from Monowai. Results from direction‐of‐arrival calculations and density‐based spatial clustering indicate that 82 discrete episodes of activity occurred between July 2003 and March 2004, and from April 2014 to January 2017. Volcanic episodes are typically spaced days to weeks apart, range from hours to days in length, and amount to a cumulative sum of 137 days of arrivals in total, making Monowai one of the most active submarine arc volcanoes on Earth. The resolution of the hydrophone recordings surpasses broadband network data by at least one order of magnitude, identifying seismic events as low as 2.2 mb in the Kermadec Arc region. Further observations suggest volcanic activity at a location approximately 400 km north of Monowai in the Tonga Arc, and at Healy or Brothers volcano in the southern Kermadec Arc. Our findings are consistent with previous studies and highlight the exceptional capabilities of the IMS network for the scientific study of active volcanism in the global ocean. Supporting Informatio

Do peatlands or lakes provide the most comprehensive distal tephra records?

Despite the widespread application of tephra studies for dating and correlation of stratigraphic sequences (‘tephrochronology’), questions remain over the reliability and replicability of tephra records from lake sediments and peats, particularly in sites >1000 km from source volcanoes. To address this, we examine the tephrostratigraphy of four pairs of lake and peatland sites in close proximity to one another (<10 km), and evaluate the extent to which the microscopic (crypto-) tephra records in lakes and peatlands differ. The peatlands typically record more cryptotephra layers than nearby lakes, but cryptotephra records from high-latitude peatlands can be incomplete, possibly due to tephra fallout onto snow and subsequent redistribution across the peatland surface by wind and during snowmelt. We find no evidence for chemical alteration of glass shards in peatland or lake environments over the time scale of this study (mid- to late- Holocene). Instead, the low number of basaltic cryptotephra layers identified in distal peatlands reflects the capture of only primary tephra-fall, whereas lakes concentrate tephra falling across their catchments which subsequently washes into the lake, adding to the primary tephra fallout received in the lake. A combination of records from both lakes and peatlands must be used to establish the most comprehensive and complete regional tephrostratigraphies. We also describe two previously unreported late Holocene cryptotephras and demonstrate, for the first time, that Holocene Icelandic ash clouds frequently reached Arctic Sweden

The Role of Pore Fluid Pressure on the Failure of Magma Reservoirs:Insights From Indonesian and Aleutian Arc Volcanoes

We use numerical models to study the mechanical stability of magma reservoirs embedded in elastic host rock. We quantify the overpressure required to open tensile fractures (the failure overpressure), as a function of the depth and the size of the reservoir, the loading by the volcanic edifice, and the pore fluid pressure in the crust. We show that the pore fluid pressure is the most important parameter controlling the magnitude of the failure overpressure rather than the reservoir depth and the edifice load. Under lithostatic pore fluid pressure conditions, the failure overpressure is on the order of the rock tensile strength (a few tens of megapascals). Under zero pore fluid pressure conditions, the failure overpressure increases linearly with depth (a few hundreds of megapascals at 5 km depth). We use our models to forecast the failure displacement (the cumulative surface displacement just before an eruption) on volcanoes showing unrest: Sinabung and Agung (Indonesia) and Okmok and Westdahl (Aleutian). By comparison between our forecast and the observation, we provide valuable constraint on the pore fluid pressure conditions on the volcanic system. At Okmok, the occurrence of the 2008 eruption can be explained with a 1,000 m reservoir embedded in high pore fluid pressure, whereas the absence of eruption at Westdahl better suggests that the pore fluid pressure is much lower than lithostatic. Our finding suggests that the pore fluid pressure conditions around the reservoir may play an important role in the triggering of an eruption by encouraging or discouraging the failure of the reservoir. Key Points Numerical calculation of the failure overpressure required to cause magma intrusion is dependent on pore fluid pressure High pore fluid pressure encourages eruptions by reducing the failure overpressure Different pore fluid pressure conditions can explain the difference of eruptive behavior between volcanoe