Chasing Magma Around Iceland’s Reykjanes Peninsula

The Icelandic Meteorological Office has been tracking unrest near erupting Fagradalsfjall since December 2019, while researchers elsewhere explore new methods to see Iceland’s seismic swarms.

Hydroacoustic Observations of Two Contrasted Seismic Swarms along the Southwest Indian Ridge in 2018

In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a readjustment of stresses in the surrounding crust.

Swell-Triggered Seismicity at the Near-Front Damage Zone of the Ross Ice Shelf

Ocean swell interacting with Antarctic ice shelves produces sustained (approximately, 2×106 cycles per year) gravity-elastic perturbations with deformation amplitudes near the ice front as large as tens to hundreds of nanostrain. This process is the most energetically excited during the austral summer, when sea ice-induced swell attenuation is at a minimum. A 2014–2017 deployment of broadband seismographs on the Ross Ice shelf, which included three stations sited, approximately, 2 km from the ice front, reveals prolific swell-associated triggering of discrete near-ice-front (magnitude≲0) seismic subevents, for which we identify three generic types. During some strong swell episodes, subevent timing becomes sufficiently phase-locked with swell excitation, to create prominent harmonic features in spectra calculated across sufficiently lengthy time windows via a Dirac comb effect, for which we articulate a theoretical development for randomized interevent times. These events are observable at near-front stations, have dominant frequency content between 0.5 and 20 Hz, and, in many cases, show highly repetitive waveforms. Matched filtering detection and analysis shows that events occur at a low-background rate during all swell states, but become particularly strongly excited during large amplitude swell at rates of up to many thousands per day. The superimposed elastic energy from swell-triggered sources illuminates the shelf interior as extensional (elastic plate) Lamb waves that are observable more than 100 km from the ice edge. Seismic swarms show threshold excitation and hysteresis with respect to rising and falling swell excitation. This behavior is consistent with repeated seismogenic fracture excitation and growth within a near-ice-front damage zone, encompassing fracture features seen in satellite imagery. A much smaller population of distinctly larger near-front seismic events, previously noted to be weakly associated with extended periods of swell perturbation, likely indicate calving or other larger-scale ice failures near the shelf front.

The 2011–2014 Pollino Seismic Swarm: Complex Fault Systems Imaged by 1D Refined Location and Shear Wave Splitting Analysis at the Apennines–Calabrian Arc Boundary

In the years between 2011 and 2014, at the edge between the Apennines collapsing chain and the subducting Calabrian arc, intense seismic swarms occurred in the Pollino mountain belt. In this key region, <2.5 mm/yr of NE-trending extension is accommodated on an intricate network of normal faults, having almost the same direction as the mountain belt. The long-lasting seismic release consisted of different swarm episodes, where the strongest event coinciding with a ML 5.0 shock occurred in October 2012. This latter comes after a ML four nucleated in May 2012 and followed by aseismic slip episodes. In this study, we present accurate relocations for ∼6,000 earthquakes and shear-wave splitting analysis for ∼22,600 event-station pairs. The seismicity distribution delineates two main clusters around the major shocks: in the north-western area, where the ML 5.0 occurred, the hypocenters are localized in a ball-shaped volume of seismicity without defining any planar distribution, whilst in the eastern area, where the ML 4.3 nucleates, the hypocenters define several faults of a complex system of thrusts and back-thrusts. This different behavior is also imaged by the anisotropic parameters results: a strong variability of fast directions is observed in the western sector, while stable orientations are visible in the eastern cluster. This tectonic system possibly formed as a positive flower structure but as of today, it accommodates stress on normal faults. The deep structure imaged by refined locations is overall consistent with the complex fault system recently mapped at the surface and with patterns of crustal anisotropy depicting fractures alignment at depth. The possible reactivation of inherited structures supports the important role of the Pollino fault as a composite wrench fault system along which, in the lower Pleistocene, the southward retreat of the ionian slab was accommodated; in this contest, the inversion of the faults kinematics indicates a probable southward shift of the slab edge. This interpretation may help to comprehend the physical mechanisms behind the seismic swarms of the region and defining the seismic hazard of the Pollino range: nowadays a region of high seismic hazard although no strong earthquakes are present in the historical record.

Volcanic unrest at Hakone volcano after the 2015 phreatic eruption: reactivation of a ruptured hydrothermal system?

Since the beginning of the twenty-first century, volcanic unrest has occurred every 2–5 years at Hakone volcano. After the 2015 eruption, unrest activity changed significantly in terms of seismicity and geochemistry. Like the pre- and co-eruptive unrest, each post-eruptive unrest episode was detected by deep inflation below the volcano (~ 10 km) and deep low frequency events, which can be interpreted as reflecting supply of magma or magmatic fluid from depth. The seismic activity during the post-eruptive unrest episodes also increased; however, seismic activity beneath the eruption center during the unrest episodes was significantly lower, especially in the shallow region (~ 2 km), while sporadic seismic swarms were observed beneath the caldera rim, ~ 3 km away from the center. This observation and a recent InSAR analysis imply that the hydrothermal system of the volcano could be composed of multiple sub-systems, each of which can host earthquake swarms and show independent volume changes. The 2015 eruption established routes for steam from the hydrothermal sub-system beneath the eruption center (≥ 150 m deep) to the surface through the cap-rock, allowing emission of super-heated steam (~ 160 ºC). This steam showed an increase in magmatic/hydrothermal gas ratios (SO2/H2S and HCl/H2S) in the 2019 unrest episode; however, no magma supply was indicated by seismic and geodetic observations. Net SO2 emission during the post-eruptive unrest episodes, which remained within the usual range of the post-eruptive period, is also inconsistent with shallow intrusion. We consider that the post-eruptive unrest episodes were also triggered by newly derived magma or magmatic fluid from depth; however, the breached cap-rock was unable to allow subsequent pressurization and intensive seismic activity within the hydrothermal sub-system beneath the eruption center. The heat released from the newly derived magma or fluid dried the vapor-dominated portion of the hydrothermal system and inhibited scrubbing of SO2 and HCl to allow a higher magmatic/hydrothermal gas ratio. The 2015 eruption could have also breached the sealing zone near the brittle–ductile transition and the subsequent self-sealing process seems not to have completed based on the observations during the post-eruptive unrest episodes.

Intrusive Seismic Swarms as Possible Precursors of Destructive Earthquakes on Mt. Etna’s Eastern Flank

The Timpe Fault System (TFS) represents the source of shallow earthquakes that strike numerous towns and villages on Mt. Etna eastern flank. In the last 40 years, three destructive seismic events reached I0 = VIII EMS (heavily damaging) - in 1984 (October 25), 2002 (October 29) and 2018 (December 26). These events followed a few days after the occurrence of strong seismic swarms and the sudden acceleration of the eastern flank seaward. However, if the 2002 and 2018 events were caused by stress induced by eruptive dike propagation, in October 1984 no eruption occurred. In this work, parameters such as localization, cumulative seismic moment and hourly occurrence frequency of the 1984 seismic swarm, have been analyzed and shown to have typical values of Mt. Etna intrusive seismic swarms. This suggests that the 1984 episode may have been an aborted intrusive magma episode that triggered similar processes (long and powerful intrusions with acceleration of the eastern flank movement and destructive earthquakes), as in 2002 and 2018. These three episodes suggest that an evaluation of some seismic parameters during future intrusive swarms may furnish indications of a possible re-activation of the TFS.

Temporal evolution of relative seismic velocity in the Golf of Corinth - Greece.

<p>Our aim is to monitor the temporal evolution of the crust in Greece, with a particular focus on the Gulf of Corinth.  Indeed, Greece is one of the most exposed country to earthquakes in Europe. The Gulf of Corinth,  is known for its fast extension rate of about 15 mm/yr in the western part and 10mm/yr in the eastern part. This fast extension is associated with recurrent seismic swarms and by a few destructive earthquakes. This seismicity is likely the result of a combination of multiple driving processes including fluid migration at depth.</p><p>In the present work, we use seismic noise recorded from 2010 to 2020 by all seismic stations deployed in Greece, and in particular by the dense Corinth Rift Laboratory network, to compute the seismic velocity variation (dv/v) in several subregions. By comparing the result obtained at different periods, we are able to distinguish the temporal evolution of the upper, mid and lower crust. This temporal evolution is compared to the seismicity of the Gulf of Corinth.</p>

Trade-offs between deep (magmatic) and shallow (hydrological) forcings on volcanic unrest at La Soufrière de Guadeloupe (Lesser Antilles)

<p>Fumarolic gas composition and temperature record deep processes that generate and transfer heat and mass towards the surface.  These processes are a result of the emplacement, degassing and cooling of magma and the overturning of the above hydrothermal system.  A reasonable expectation, and too often an unproved assumption, is that fumarole temperatures and the deep heat sources vary on similar timescales.  Yet signals from deep and shallow processes have vastly different temporal variations.  This indicates that signals arising from deep activity may be masked or modified by intervening hydrothermal processes, such as fluid-groundrock reactions in which secondary minerals play a major role.  Clearly, this complicates the interpretation of the signals such as the joint variation of fumarole vent temperature and geochemical ratios in terms of what is occurring at depth.  So what do the differences between the timescales governing deep and shallow processes tell us about the intervening transport mechanisms?</p><p>At the volcanic dome of La Soufrière de Guadeloupe, the Observatoire Volcanologique et Sismologique de la Guadeloupe has performed weekly-to-monthly in-situ vent gas sampling over many years.  These analyses reliably track several geochemical species ratios over time, which provide important information about the evolution of deep processes.  Vent temperature is measured as part of the in-situ sampling, giving a long time series of these measurements.  Here, we look to exploit the temporal variations in these data to establish the common processes, and also to determine why these signals differ.  By fitting sinusoids to the gas-ratio time series we find that several of the deep signals are strongly sinusoidal.  For example, the He/CH<sub>4</sub> and CO<sub>2</sub>/CH<sub>4</sub> ratios, which involve conservative components and mark the injection of deep and hot magmatic fluids, oscillate on a timescale close to 3 years. We also analyse the frequency content of the temperature measurements since 2011 and find that such long signals are not seen.  This may be due to internal buffering by the hydrothermal system, but other external forcings are also present.  From these data we build up a more informed model of the heat-and-mass supply chain from depth to the surface.  This will potentially allow us to predict future unrest (e.g. thermal crises, seismic swarms), and distinguish between sources of unrest.</p>

Thermal Imaging of the Lithosphere beneath Arabian Shield and Implications for "Harrats" Volcanic Field

<p><span>Widespread Cenozoic volcanisms in the Arabian shield including “Harrats” have been referring to lithospheric thinning and/or mantle plume activity as a result of Red Sea rift-related extension.</span></p><p><span>A fundamental key in understanding the deriving mechanism of these volcanic activities and its relationship to 2007-2009 seismic swarms required a reliable model of the present-day lithospheric thermo-chemical structure.</span></p><p><span>In this work, we modeled crustal and lithospheric thickness variation as well as the variations in thermal, composition, seismic velocity, and density of the lithosphere beneath the Arabian shield within a thermodynamically self - consistent framework.</span></p><p><span>The resulting thermal and density structures show large variations, revealing strong asymmetry between the Arabian shield and Arabian platform within the Arabian Plate.</span></p><p><span>We model negative density anomalies associated with the hot mantle beneath Harrats, which coincides with the modelled lithosphere thinned (~ 65 km) as a result of the second stage of lithospheric thinning following the initial Red Sea extension.</span></p>