Vulcano Fossa's fumarole field (Italy) has been active for more than a century and has become a well‐studied benchmark for fumarolic degassing, often being considered the “model” hydrothermal system. Satellite thermal monitoring is increasingly being used to monitor such systems, so we here use Vulcano to test a new method for assessing heat flux at such systems. Our methodology involves converting ground‐based vent temperature measurements to heat fluxes emitted by the fumaroles, with the diffuse heat flux obtained from satellite‐sensor (in our case Advanced Spaceborne Thermal Emission and Reflection Radiometer) data. While diffuse heat losses were typically 9 MW, vent heat losses were 1 MW. The average total flux of 10 MW over the 19‐year period of study places Vulcano in the top 20 most active hydrothermal systems globally. This work highlights the value of high spatial resolution infrared satellite data in building thermal inventories for persistently active hydrothermal systems.

Understanding the thermo-rheological regime and physical character of lava while it is flowing is crucial if we are to adequately model lava flow emplacement dynamics. We present measurements from simultaneous sampling and thermal imaging across the full width of an active channel at Piton de la Fournaise (La Réunion, France). Our data set involves measurements of flow dynamics at three sites down-channel from the vent. Quantification of flow velocities, cooling rates, sample texture, and rheology allows all thermo-rheological parameters to be linked, and down- as well as cross-channel variations to be examined. Within 150 m from the vent, we recorded an unexpected velocity increase (from 0.07 to 0.1 m/s), in spite of cooling rates of 0.19–0.29 °C/m and constant slope. This change requires a switch from a Newtonian-dominated regime to a Bingham plug–dominated regime. Sample analysis revealed that the plug consists of foam-like lava, and the shear zones involve vesicle-poor (low-viscosity) lava. With distance from the vent, shear zones develop, carrying the vesicular plug between them. This causes flow to initially accelerate, helped by bubble shearing in narrow lateral shear zones, until cooling takes over as the main driver for viscosity increase and, hence, velocity decrease.

Formalised elicitation of expert judgements has been used to help tackle several problematic societal issues, including volcanic crises and pandemic threats. We present an expert elicitation exercise for Piton de la Fournaise volcano, La Réunion island, held remotely in April 2021. This involved 28 experts from nine countries who considered a hypothetical effusive eruption crisis involving a new vent opening in a high-risk area. The tele-elicitation presented several challenges, but is a promising and workable option for application to future volcanic crises. Our exercise considered an “uncommon” eruptive scenario with a vent outside the present caldera and within inhabited areas, and provided uncertainty ranges for several hazard-related questions for such a scenario (e.g. probability of eruption within a defined timeframe; elapsed time until lava flow reaches a critical location, and other hazard management issues). Our exercise indicated that such a scenario would probably present very different characteristics compared to recent eruptions, and that it is fundamental to include well-prepared expert elicitations in updated civil protection evacuation plans to improve disaster response procedures.

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<p>Volcanic islands are often subject to flank instability, being a combination of magma intrusions along rift zones, gravitational spreading and extensional faulting observable at the surface. The Kilauea is one of the most active volcano on Earth and its south flank show recurrent flank acceleration related to large earthquakes and magmatic intrusions. <br>Here we focus on the M 7.7 Kalapana earthquake that occurred on 29 November 1975. It triggered ground displacement of several meters all over the south flank of the Kilauea volcano. The identification and quantification of the co-seismic rupture aim to better understand the overall flank motion and its connection to key structural components, such as between the southwest and east rift zones and the deep basal detachment where large earthquakes episodically nucleate.<br>Using optical imagery correlation technique, we analyzed the displacement that occurred during the 1975 earthquake. We used 26 and 22 historical air photos as pre-event (October 1974 and July 1975, respectively) and 7 and 44 for the post-event time period (December 1976 and March 1977, respectively).  Results show metrical horizontal displacement (north-south direction) along a 25 km long East West sector of the Kilauea south flank. We show that the ground rupture is continuous with most portions of faults that have been reactivated. Locally, the displacement values we found are in good agreement with punctual EDM measurements. Several fault segments have been activated close to the shore and their extension were previously unnoticed. Interestingly, we observe a constant increase of the offset away from the epicenter in the West direction, from a few meters up to ~12 meters, west of the Hilina Pali road. The deformation turns out to be higher where the faults are oriented NE-SW (western sector) compared to E-W oriented structures. It also shows that the flank is strongly influenced by gravitational effect, typical from large landslide processes. This observation provides additional information to better understand the connection between the Hilina fault system and the basal detachment.  Episodic flank motions on volcanic islands are rare events and this work contributes to the overall comprehension of volcano flank instability elsewhere.</p>

<p>The dynamics of fault slip in the upper hundreds of meters of Earth’s crust has long been an open question, as their behavior differs from classical elastic dislocation models and their observation still raises challenges. Here, we analyze centimeter-scale ground resolution aerial optical images of the surface ruptures associated with the 8 Mw ≥ 5.0 sub-surface earthquakes that stroke during the Reykjanes seismo-tectonic unrest, starting on February 24, 2021, and ending with the start of an eruption at Fagradasfjall on March 19, 2021. For four major earthquakes, we apply a sub-pixel correlation technique of pre-, syn- and post-crisis aerial and drone orthomosaics to describe the displacement field on surface blocks. We find that surface offsets reached up to 50 cm, with almost pure dextral strike-slip in a NS direction. These orientations contrast with the overall NE-SW-oriented extensional structures originating from magmatic intrusions and appear as a bookshelf faulting system conjugated to the left-lateral strike-slip plate boundary, oriented ~N070.</p><p>On hard grounds (e.g.: lava flows), shallow ruptures reached the surface, reactivating pre-existing structures and displaying an en-échelon succession of hectometric-sized fractures. We believe these ruptures are representative of medium-sized faults behavior in the last few hundred meters of the crust. On soft grounds, however, the rupture was only betrayed by meter-sized en-échelon systems, evidenced by thousands of discrete sub-metric surface fractures we were able to observe in the field and map from the orthomosaics. The sharp deformation gradient we imaged indicates that the dislocation drastically decreased above ten to a few tens of meters below the surface. In this layer, diffuse deformation takes on most of the slip deficit, mainly through inelastic processes. As a result, evidence of the February 2021 earthquake did not endure erosion for more than a few months. Except for an isolated sinkhole which allowed us to assume that one fault pre-existed, there were no markers of its presence before the earthquake. We emphasize that this issue must frequently lead to an underestimation of the seismic hazard when performed from surface traces.</p>

<p>The Reykjanes Peninsula has recently been subject to a seismo-tectonic unrest triggering widespread ground cracks. This started with a strong seismic swarm from 24 February to 17 March 2021 and culminated in a volcanic eruption on March 19, terminating an 800 years quiescence period in the region. The Peninsula hosts four overlapping and highly oblique rift zones. The structural relations between the plate boundary (N070), the rift zones (N030 to N040) and the barely visible fault zones oriented N175 are challenging to assess, as most structures, beside the rifts, are poorly preserved or absent in the landscape. </p><p>To get the full picture of the fracture field generated by the 2021 Reykjanes rifting event, we collected an unprecedented amount of structural data, mapping almost the entire fresh fracture field. Field observations show widespread ground cracks in up to ~7 km distance from the intrusion area with en-echelon metrical segments with a right-lateral sense of shear. Most of these structures are not visible anymore, either covered by lava flows or eroded due to weathering. They are unique testimony of the strong seismicity preceding the eruption and would have remained unnoticed if not caught up by our fixed-wing drone, surveying an area of ~30 km<sup>2</sup>. We used the resulting high-resolution (<5 cm) orthomosaics and DEMs to study three main NS-oriented fracture zones of 3 to 4 kilometers long, mostly generated by ten earthquakes ranging from M5 to M5.6. Results show metric to decametric en-echelon structures with cracks of very limited extension, even in the vicinity of the eruption site. Two of the three main fracture zones clearly show fault reactivation, suggesting episodicity in the rifting processes. Apart from local sinkholes, the third area has probably also been reactivated, but the loose ground composition did not preserve previous structures.</p><p>We further used high-resolution optical image correlation technique to analyze aerial photos and drone imagery acquired before and after the large earthquakes sequence in the three fracture zones. Results show clear NS-oriented shear structures with a right-lateral sense of motion of up to 50 cm. This is in good agreement with moment tensors we computed from waveform data at seismic stations up to 1000 km distance. We observe consistent non-double-couple mechanisms, with tension-crack components oriented northwest-southeast. The orientations suggest strike-slip faulting with nodal planes oriented in the same direction as the main fault traces. We also found that the three fracture zones have sigmoid shapes and their overall extension bounds the near-field deformation of the plate boundary. These sigmoids may suggest a local high geothermal gradient and elasto-plastic deformation affecting the Reykjanes Peninsula, that further decreases toward the South Icelandic Seismic Zone.</p>