Tracking dynamics of magma migration in open-conduit systems

Open-conduit volcanic systems are typically characterized by unsealed volcanic conduits feeding permanent or quasi-permanent volcanic activity. This persistent activity limits our ability to read changes in the monitored parameters, making the assessment of possible eruptive crises more difficult. We show how an integrated approach to monitoring can solve this problem, opening a new way to data interpretation. The increasing rate of explosive transients, tremor amplitude, thermal emissions of ejected tephra, and rise of the very-long- period (VLP) seismic source towards the surface are interpreted as indicating an upward migration of the magma column in response to an increased magma input rate. During the 2014 flank eruption of Stromboli, this magma input pre- ceded the effusive eruption by several months. When the new lateral effusive vent opened on the Sciara del Fuoco slope, the effusion was accompanied by a large ground deflation, a deepening of the VLP seismic source, and the cessation of summit explosive activity. Such observations suggest the drainage of a superficial magma reservoir confined between the crater terrace and the effusive vent. We show how this model successfully reproduces the measured rate of effusion, the observed rate of ground deflation, and the deepening of the VLP seismic source. This study also demonstrates the ability of the geophysical network to detect superficial magma recharge within an open-conduit system and to track magma drainage during the effusive crisis, with a great impact on hazard assessment

Resonant band hybridization in alloyed transition metal dichalcogenide heterobilayers

Bandstructure engineering using alloying is widely utilised for achieving optimised performance in modern semiconductor devices. While alloying has been studied in monolayer transition metal dichalcogenides, its application in van der Waals heterostructures built from atomically thin layers is largely unexplored. Here, we fabricate heterobilayers made from monolayers of WSe$_2$ (or MoSe$_2$) and Mo$_x$W$_{1-x}$Se$_2$ alloy and observe nontrivial tuning of the resultant bandstructure as a function of concentration $x$. We monitor this evolution by measuring the energy of photoluminescence (PL) of the interlayer exciton (IX) composed of an electron and hole residing in different monolayers. In Mo$_x$W$_{1-x}$Se$_2$/WSe$_2$, we observe a strong IX energy shift of $\approx$100 meV for $x$ varied from 1 to 0.6. However, for $x<0.6$ this shift saturates and the IX PL energy asymptotically approaches that of the indirect bandgap in bilayer WSe$_2$. We theoretically interpret this observation as the strong variation of the conduction band K valley for $x>0.6$, with IX PL arising from the K-K transition, while for $x<0.6$, the bandstructure hybridization becomes prevalent leading to the dominating momentum-indirect K-Q transition. This bandstructure hybridization is accompanied with strong modification of IX PL dynamics and nonlinear exciton properties. Our work provides foundation for bandstructure engineering in van der Waals heterostructures highlighting the importance of hybridization effects and opening a way to devices with accurately tailored electronic properties.Comment: Supporting Information can be found downloading and extracting the gzipped tar source file listed under "Other formats

Ice mass loss in northwestern Greenland ―Results of the GRENE Greenland project and overview of the ArCS Greenland project―

第6回極域科学シンポジウム分野横断セッション：[IA] 急変する北極気候システム及びその全球的な影響の総合的解明―GRENE北極気候変動研究事業研究成果報告2015―11月19日（木）　国立極地研究所 2階 大会議

Seismic and infrasound monitoring of Bowdoin Glacier, Greenland

Outlet glaciers in Greenland have retreated and lost mass over the past decade. Understanding the dynamics of tidewater glaciers is crucial for forecasting sea-level rise and for understanding the future of the Greenland Ice Sheet, given the buttressing support that tidewater glaciers provide to inland ice. However, the mechanisms controlling glacier-front location and the role played by external forcings (e.g., meltwater input and tidal oscillation) in basal motion and fracture formation leading to iceberg calving are poorly understood. Today it is known that glaciers generate seismic and infrasound signals that are detectable at local and teleseismic distances and can be used to monitor glacier dynamics. Here, we present examples of data recorded by a temporary network of seismic and infrasound instruments deployed at a tidewater glacier (Bowdoin Glacier, Greenland) in July 2015. Some stations were installed on ice at distances as close as ～ 250 m from the calving front, representing the closest deployments to the calving front that have been made to date. Multiple seismic and infrasound events were recorded by five seismic and six infrasound sensors, and linked to surface crevassing, calving, and ice-cliff collapses, and presumably also hydrofracturing, iceberg rotations, teleseismic earthquakes, and helicopter-induced tremors. Using classic seismological and array analysis approaches (e.g., “short-term averaging/long-term averaging” and “f-k” analysis), as well as image processing techniques, we explore this unique dataset to understand the glacial response to external forcings. Our observations, supported by GPS measurements of ice velocity, local weatherstation records, and time-lapse photography, provide a valuable resource for studying seismogenic glacial processes and their dependence on ocean tides and other environmental factors

Gas flux cyclic regime at an open vent magmatic column inferred from seismic and acoustic records

On August 7, 2014, a new effusive vent opened on the northern flank of Stromboli. A characteristic pattern was observed in both seismic and infrasonic signal amplitudes prior to this effusive eruption. The pattern consisted of the repeating cycle: (1) quiet phase, (2) puffing phase, and (3) explosion phase. Correlation between seismic and infrasound signal suggests that pulses in the puffing phase were caused by repetitive bursts of small gas pockets at the central crater, while the explosion phase coincided with an explosion at the central crater. We show that degassing of the magma column occurred in cycles of increasing gas flux, which controlled the transition from a bubbly flow (puffing phase), to a slug flow (explosion phase) gas regime. The quiet phase was characterized by a constant time length of 150 s, indicating that the gas rose in the magma column as well-organized waves of gas layers. These cycles represent cyclic changes of the gas flux regime in the shallow magma column, associated with increases in the magma-gas supply input rate before the effusive eruption

Recent ice mass loss in northwestern Greenland : Results of the GRENE Greenland project and overview of the ArCS project

The Greenland ice sheet and peripheral ice caps are rapidly losing mass. This mass change has been captured by satellite remote sensing, but more detailed investigations are necessary to understand the spatiotemporal variations and mechanism of the ice loss. It has increased particularly in northwestern Greenland, but in-situ data for northern Greenland are generally sparse. To better understand the ice mass loss in northwestern Greenland, we studied the ice sheet, ice caps and calving glaciers in the Qaanaaq region, as a part of the Green Network of Excellence (GRENE) Arctic Climate Change Research Project. Field and satellite observations were performed to measure the mass loss of the ice caps and calving glaciers in the region. Detailed processes were investigated based on field measurements to understand mechanisms driving the ice loss. The field activities include mass balance monitoring on Qaanaaq Ice Cap since 2012, integrated field observations near the front of Bowdoin Glacier since 2013 and ocean measurements near the calving glaciers. In this contribution, we summarize the results of the GRENE Greenland project, and introduce an overview of the next project to be carried out under the framework of the Arctic Challenge for Sustainability Project (ArCS)