La Palma island is one of the highest potential risks in the volcanic archipelago of the Canaries and therefore it is important to carry out an in-depth study to define its state of unrest. This has been accomplished through the use of satellite radar observations and an original state-of-the-art interpretation technique. Here we show the detection of the onset of volcanic unrest on La Palma island, most likely decades before a potential eruption. We study its current evolution seeing the spatial and temporal changing nature of activity at this potentially dangerous volcano at unprecedented spatial resolutions and long time scales, providing insights into the dynamic nature of the associated volcanic hazard. The geodetic techniques employed here allow tracking of the fluid migration induced by magma injection at depth and identifying the existence of dislocation sources below Cumbre Vieja volcano which could be associated with a future flank failure. Therefore they should continue being monitored using these and other techniques. The results have implications for the monitoring of steep-sided volcanoes at oceanic islands.

Since its last effusive eruption in 2002, Nyiragongo has been an open-vent volcano characterized by the world's largest persistent lava lake. This lava lake provides a unique opportunity to detect pressure change in the magmatic system by analyzing its level fluctuations. We demonstrate that this information is contained in the seismic and infrasound signals generated by the lava lake's activity. The continuous seismo-acoustic monitoring permits quantification of lava lake dynamics, which is analyzed retrospectively to identify periods of volcanic unrest. Synchronous, high-resolution satellite SAR (Synthetic Aperture Radar) images are used to constrain lava lake level by measuring the length of the SAR shadow cast by the rim of the pit crater where the lava lake is located. Seventy-two estimations of the lava lake level were obtained with this technique between August 2016 and November 2017. These sporadic measurements allow for a better interpretation of the continuous infrasound and seismic data recorded at the closest station (∼6 km from the crater). Jointly analyzed seismo-acoustic and SAR data reveal that slight changes in the spectral properties of the continuous cross-correlated low-frequency seismo-acoustic records (and not solely single events) can be used to track fluctuations of the lava lake level on a daily and hourly basis. We observe that drops of the lava lake and the appearance of significant long period (LP) "lava lake" events are a consequence of a probable deep lateral magma intrusion beneath Nyiragongo, which induces changes in its shallow plumbing system. In addition to contributing to understanding lava lake dynamics, this study highlights the potential to continuously monitor pressure fluctuations within the magmatic system using a single seismo-acoustic station located several kilometers from the vent.

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La Palma, Canary Islands, underwent volcanic unrest which culminated in its largest historical eruption. We study this unrest along 2021 using Interferometric Synthetic Aperture Radar (InSAR) and a new improved interpretation methodology, comparing achieved results with the crustal structure. We reproduce the final phase of La Palma volcanic unrest, highligthing a shallow magma accumulation which begins about 3.5 months before the eruption in a crustal volume charactherized by low density and fractured rocks. Our modeling, together with our improved pictures of the crustal structure, allows us to explain the location and characteristics of the eruption and to detect failed eruption paths. These can be used to explain post-eruptive phenomena and hazards to the local population, such as detected gases anomalies in La Bombilla and Puerto Naos. Our results have implications for understanding volcanic activity in the Canaries and volcano monitoring elsewhere, helping to support decision-making and providing significant insights into urban and infrastructure planning in volcanic areas.

A new method to generate ultrahigh-power microwave pulses compatible with mildly relativistic electron sources is proposed. This method involves a novel microwave compressor in the form of a metal helically corrugated waveguide, which can enhance the power of frequency-modulated nanosecond pulses up to the multigigawatt level. The results of the proof-of-principle experiments at kilowatt power levels are in good agreement with theory. DOI: 10.1103/PhysRevLett.92.118301 PACS numbers: 84.40.Ik, 41.20.Jb, 41.60.Cr, 84.40.Fe Multigigawatt pulsed rf power, which has recently become available due to the development of relativistic electronics, opens up many new opportunities both for fundamental studies and future applications [1]. For example, it is important to recall that the power P 0 m 2 c 5 =e 2 8:7 GW (m, e are electron mass and charge, respectively, and c is speed of light) focused on an area of the order of the wavelength squared imparts a relativistic oscillatory velocity to electrons. Power at the multi-GW level was obtained at relatively long wavelengths of 3-10 cm when using electron beams with particle energies higher than 1 MeV and currents of tens of kA that were available at a small number of unique high-current accelerators. In this Letter, an alternative method of producing multi-GW pulses is proposed. It uses the amplification of significantly lower power radiation, which is generated with a certain frequency modulation during the pulse. Compression of this pulse is achieved after propagation in a waveguide with the proper frequency dispersion. If a compression factor K p 10-100 is possible, then the input pulse power required will be of the order of hundreds of MW. Such pulses are produced routinely in Cherenkov relativistic traveling wave amplifiers (TWTs) or backward-wave oscillators (BWOs), driven with modest energy 0.5-1.0 MeV electron beams [2,3]. However, compression of power from these devices requires additionally a sufficiently broadband frequency modulation within the pulse. A novel rf compressor in the form of a helically corrugated cylindrical waveguide can have the necessary dispersion while simultaneously it can sustain high strength rf fields as well as possessing a low level of reflections. Similar helically corrugated waveguides were intensively studied for gyro-TWTs [4].Consider compression of a quasimonochromatic pulse with frequency, monotonously varying in time from ! 1 to ! 2 . If the wave group velocity in the dispersive medium is an increasing function of frequency, v gr ! 2 > v gr ! 1 , then the tail of the pulse will overtake its leading edge, resulting in pulse shortening and a corresponding growth in amplitude if the losses are sufficiently low. To produce ultrahigh powers, the compressor characteristics should be optimized in order to achieve a maximum power compression factor at reasonable energy losses. Among the various methods of compression (see, e.g., [5]), the use of a metal waveguide is attractive because of its capability of handling high power, as well as...