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This work analyses seismic signals at the Irazú volcano (83°51′09″ W, 9°58′45″ N; active stratovolcano in Central America) in a continuous 34-day record starting on 13 April 2013. Since December 2012, the activity of the volcano was increasing and along with it the frequency of occurrence of certain long-period (LP) signals. In this work, 6 families of LP signals have been identified, with 83 total events. These events show energy concentrated mainly in the frequency range between 1 and 3 Hz, significant variations in amplitude, and some locations on the SW flank from the volcano at 1900 m.a.s.l. (corrected for site effects). The LP signals are interpreted as linked to the interaction between the fluids of the active hydrothermal system and a fault system (NE-SW direction) in the west slope of the Irazú volcano, but without being associated with magma ascent. This contribution provides useful information for the evaluation of the volcanic hazard of the Irazú volcano, in terms of the analysis of possible precursors. [ABSTRACT FROM AUTHOR]

Seismic data have been used to catalog the location and volume of most of the rockfalls that occurred at the Piton de la Fournaise volcano from May 2007, just after the major collapse of the Dolomieu summit crater floor, to May 2011. This catalog made it possible to compare the evolution of the number and volume of rockfalls at a high temporal resolution and to investigate their links with eruptions, seismicity, deformations and rainfalls affecting the Piton de la Fournaise volcano. Results show that the purge of unstable areas created by the Dolomieu crater floor collapse occurred in two phases: a first phase, lasting three months, during which the intense rockfall activity immediately following the collapse decreased abruptly and a second phase, lasting more than two years, during which the daily volume of the rockfalls slowly decreased before reaching a steady state. A detailed study of 4 time periods, including 3 eruptive cycles, indicates that strong seismicity can increase the number of rockfalls. Furthermore, when a dike reaches the surface at the summit of the central cone, giving birth to an eruption, the associated local forcing can in some cases increase the volume of rockfalls, possibly by creating or expanding weak zones. In most cases, this is not observed for dike intrusions that do not reach the surface. The newly created weak zones are most often far from the eruptive fissures of the ongoing eruption, but close to the location of the next eruption. This suggests that these distant zones are also weakened at depth, thus creating preferred paths for future dikes propagating toward the surface. The impact of rainfall on rockfall activity was also studied, given that La Réunion Island is subject to very intense rainfall events. Our results show that rainfall can in some cases trigger rockfalls rapidly, with response times of less than a day, but not systematically, and no triggering rainfall threshold was found. [ABSTRACT FROM AUTHOR]

Prior to many volcanic eruptions, an acceleration in seismicity has been observed, suggesting the potential for this as a forecasting tool. The Failure Forecast Method (FFM) relates an accelerating precursor to the timing of failure by an empirical power law, with failure being defined in this context as the onset of an eruption. Previous applications of the FFM have used a wide variety of accelerating time series, often generating questionable forecasts with large misfits between data and the forecast, as well as the generation of a number of different forecasts from the same data series. Here, we show an alternative approach applying the FFM in combination with a cross correlation technique which identifies seismicity from a single active source mechanism and location at depth. Isolating a single system at depth avoids additional uncertainties introduced by averaging data over a number of different accelerating phenomena, and consequently reduces the misfit between the data and the forecast. Similar seismic waveforms were identified in the precursory accelerating seismicity to dome collapses at Soufrière Hills volcano, Montserrat in June 1997, July 2003 and February 2010. These events were specifically chosen since they represent a spectrum of collapse scenarios at this volcano. The cross correlation technique generates a five-fold increase in the number of seismic events which could be identified from continuous seismic data rather than using triggered data, thus providing a more holistic understanding of the ongoing seismicity at the time. The use of similar seismicity as a forecasting tool for collapses in 1997 and 2003 greatly improved the forecasted timing of the dome collapse, as well as improving the confidence in the forecast, thereby outperforming the classical application of the FFM. We suggest that focusing on a single active seismic system at depth allows a more accurate forecast of some of the major dome collapses from the ongoing eruption at Soufrière Hills volcano, and provides a simple addition to the well-used methodology of the FFM. [ABSTRACT FROM AUTHOR]

Identifying the source mechanisms of low-frequency earthquakes at ice-covered volcanoes can be challenging due to overlapping characteristics of glacially and magmatically derived seismicity. Here we present an analysis of two months of seismic data from Llaima volcano, Chile, recorded by the permanent monitoring network in 2019. We find over 2000 repeating low-frequency events split across 82 families, the largest of which contains over 200 events. Estimated locations for the largest families indicate shallow sources directly beneath or near the edge of glaciers around the summit vent. These low-frequency earthquakes are part of an annual cycle in activity at the volcano that is strongly correlated with variations in atmospheric temperature, leading us to conclude that meltwater from ice and snow strongly affects the seismic source mechanisms related to glacier dynamics and shallow volcanic processes. The results presented here should inform future assessments of eruptive potential at Llaima volcano, as well as other ice-covered volcanoes in Chile and worldwide. • We investigate micro-seismic activity at Llaima volcano in early 2019. • We observe dozens of families of persistent repeating earthquakes. • Estimated locations suggest sources at shallow depths near or beneath glaciers. • Long-term activity suggests strong relationship with snow and ice meltwater. [ABSTRACT FROM AUTHOR]

Applying seismic interferometry and network covariance matrix-based analyses to detect and locate the source of volcanic tremor during the 2010 Eyjafjallajökull effusive flank and explosive-effusive summit eruptions has provided new insights into this iconic event. The tremor source locations derived from the network covariance matrix approach were spatially distinct during the two eruptions. The tremor was radiated between the surface and 5–6 km depth during the effusive flank eruption, including an apparently progressive upward migration in early April 2010, but was strictly confined to the surface during the summit eruption. Each phase of the summit eruption left a distinct fingerprint in the seismic records. Effusive phases radiated continuous tremor between 0.6 and 5 Hz, whereas explosive phases produced tremor in a more pulsating fashion over a wider frequency band (0.2–10 Hz). A period of intermittent tremor bursts (called banded tremor) on 15 April, associated with formation of a new vent at the summit, was most likely generated by magma-gas-meltwater interaction within a subglacial enclosure. The banded tremor ceased following an abrupt draining of the newly formed subglacial cauldron, resulting in a large slurry glacial meltwater flood (jökulhlaup). This study highlights the importance of new data processing methodologies for future monitoring of volcanic tremor in real-time. • New insights into the dynamics of the 2010 Eyjafjallajökull eruption. • Distinct tremor fingerprint for each phase of the eruption. • Volcanic tremor location in 3D. • Subglacial magma-gas-meltwater interactions can generate banded tremor similarly to volcano-hydrothermal systems and geysers. • Tremor location can be implemented for real-time monitoring. [ABSTRACT FROM AUTHOR]

Abstract: Magma is fed to a volcano through a complex “plumbing” system that involves not only shallow structures beneath the volcano edifice, but also deep structures and processes within the underlying crust and upper mantle. This paper summarizes seismic experiments carried out over many years at Gunung Merapi in Central Java. These have resolved the 3D seismic velocity structure of the Merapi edifice, and provided a 3D structural image of the lithosphere and subduction zone beneath Central Java. Earthquake locations reveal that with distance from the trench, the dip of the subducting slab steepens from nearly horizontal (0–150km), through 45° (150–250km), to 70° (>250km). The slab appears as a 30km thick double layer of seismicity in a depth range of 80km to 150km, and it can be identified seismically to a depth of more than 600km. The active volcanoes of Merapi, Sumbing, and Lawu are located at the edge of a large low velocity body that extends from the upper crust to the upper mantle beneath Central Java. Shear wave signals recorded above this anomaly are strongly attenuated compared to neighboring areas. The anomalous body has a detected volume of >50,000km3 and a decrease in P and S velocities relative to adjacent regions of up to 30%. The resulting Vp/Vs ratio of up to 1.9 is unusually high for lower crust. Additionally, the anomaly extends along a 45 degree-slope downward from beneath the volcanic arc and meets the slab at 100km depth. We interpret this sloping anomaly as a pathway for fluids and partial melts. Increased seismicity is observed at depths of ~100km, possibly as a result of dehydration of the subducting slab with related fluid releases causing partial melting of overlying mantle material. The large velocity reduction and high Vp/Vs ratio in the region are consistent with an increase in temperature, a reduction of shear strength, and the presence of fluids or melts of 13 to 25vol.%. The detected strong anomaly beneath Central Java is unique in size and amplitude compared to other subduction zones. The geophysical evidence suggests that this segment of the arc has a high magma flux and is thus capable of developing even larger shallow crustal reservoirs and more voluminous explosive eruptions in the future. [Copyright &y& Elsevier]

Abstract: The link between seismicity and degassing is investigated during the VEI 4 eruptions of Merapi volcano (Indonesia) in October and in early November 2010. Seismicity comprised a large number and variety of earthquakes including Volcano-Tectonic events, a sustained period of Long Period Seismicity (LPS), i.e., Long-Period events (LP), Very Long Period events (VLP) and tremor. LPS seismicity is ascribed to the excitation of fluid-filled cavity resonance and inertial displacement of fluids and magma. We investigate here LPS that occurred between 17 October and 4 November 2010 to obtain insights into the volcano eruption processes which preceded the paroxysmal phase of the eruption on 4–5 November. We proceed to the moment tensor inversion of a well-recorded large VLP event during the intrusion phase on 17 October 2010, i.e., before the first explosion on 26 October. By using two simplified models (crack and pipe), we find a shallow source for this VLP event at about 1km to the south of the summit and less than 1km below the surface. We analyze more than 90 LP events that occurred during the multi-phase eruption (29 October–4 November). We show that most of them have a dominant frequency in the range 0.2–4Hz. We could locate 48 of those LP events; at least 3 clusters of LP events occurred successively. We interpret these observations as generated by different fluid-filled containers in the summit area that were excited while magma rose. We also observe significant variations of the complex frequency during the course of the eruption. We discuss these changes in terms of a variable ratio of fluid to solid densities and/or by possible conduit geometry change and/or permeability of the conduit or the edifice and/or by resonance of different fluid-containers during the release of more than 0.4Tg of SO2 and large but unknown masses of other volcanic gases. Finally, we also discuss how the major explosions of the eruption were possibly triggered by passing waves resulting from regional tectonic earthquakes on 3 and 4 November. [Copyright &y& Elsevier]

Abstract: Merapi volcano (Indonesia) is one of the most active and hazardous volcanoes in the world. It is known for frequent small to moderate eruptions, pyroclastic flows produced by lava dome collapse, and the large population settled on and around the flanks of the volcano that is at risk. Its usual behavior for the last decades abruptly changed in late October and early November 2010, when the volcano produced its largest and most explosive eruptions in more than a century, displacing at least a third of a million people, and claiming nearly 400 lives. Despite the challenges involved in forecasting this ‘hundred year eruption’, we show that the magnitude of precursory signals (seismicity, ground deformation, gas emissions) was proportional to the large size and intensity of the eruption. In addition and for the first time, near-real-time satellite radar imagery played an equal role with seismic, geodetic, and gas observations in monitoring eruptive activity during a major volcanic crisis. The Indonesian Center of Volcanology and Geological Hazard Mitigation (CVGHM) issued timely forecasts of the magnitude of the eruption phases, saving 10,000–20,000 lives. In addition to reporting on aspects of the crisis management, we report the first synthesis of scientific observations of the eruption. Our monitoring and petrologic data show that the 2010 eruption was fed by rapid ascent of magma from depths ranging from 5 to 30km. Magma reached the surface with variable gas content resulting in alternating explosive and rapid effusive eruptions, and released a total of ~0.44Tg of SO2. The eruptive behavior seems also related to the seismicity along a tectonic fault more than 40km from the volcano, highlighting both the complex stress pattern of the Merapi region of Java and the role of magmatic pressurization in activating regional faults. We suggest a dynamic triggering of the main explosions on 3 and 4 November by the passing seismic waves generated by regional earthquakes on these days. [Copyright &y& Elsevier]

Abstract: The aim of this work is to apply the Hidden Markov Model (HMM) method to recognise seismic signals belonging to different active volcanoes. We use data obtained from two field surveys carried out in 1997 and 1999 at Stromboli and Etna, respectively. For Stromboli we used two types of seismic signals for recognition purposes: Strombolian explosions and background seismic noise whilst for Etna we used volcanic tremors and tremor bursts. We initially proceeded to visually identify the signals, and to segment the data to obtain a model for each event class. We then applied these models separately to each volcano dataset, finally combining both datasets as a test of the portability of the system. The method analyses the seismograms and compares the characteristics of the data to a number of pre-defined event classes. If a signal is present, the method detects its occurrence and produces a classification. We observed that, to obtain reliable results, a careful and adequate segmentation process is crucial and that each signal type must be represented by its own specific model. Once we had built this model, the success level of the system was high. The success rates obtained indicated that the method was fully able to detect, isolate, and identify signals from raw seismic data. These results imply that, once an adequate training process has been used, this method is particularly appropriate for work in real time, as well as concurrently with the data acquisition system. [Copyright &y& Elsevier]

Long-term instrumental monitoring of open-vent volcanoes provides the necessary datasets to characterize volcanic activity and unravel its temporal changes. This is particularly important for active lava domes, which can undergo rapid transitions in behavior over the course of their eruption. Here, we analyzed seismic, acoustic infrasound and thermographic data collected between January 2018 and September 2020 to resolve volcanic processes taking place at the Santiaguito lava dome complex in Guatemala. During this period lava effusion filled the crater of the active Caliente lava dome. The extrusive activity was accompanied by small-to-moderate explosions, prolonged episodes of gas emissions, and occasional rockfalls. Automated algorithms were applied to identify seismic signals associated with different processes and to characterize the temporal evolution of activity. We identified ~70–250 tectonic events per week and detected signals associated with gas-and ash explosions occurring at a rate of ~70–100 events/week. Lava dome growth activity was accompanied by the emplacement of a lava flow along the eastern upper flank of Caliente and seismicity possibly due to the occurrence of rockfalls. We observed episodes of harmonic tremor in seismic and acoustic data associated with sustained gas emissions, estimated to originate at shallow depths of about 500–750 m below the crater. Data indicated that both the recurrence rate of tremor (~10–50 events/week) and its duration (~40–130 min/week) were slightly lower and shorter between January 2019 and March 2020 than in rest of the study period, despite minor variations in explosive activity. Finally, within a period of 11 weeks, between 18 January and 4 April 2018, we found 129 volcano tectonic earthquakes; we were able to locate 10 of them at depths between 1.3 and 2.3 km, ~1.5 km southwest of Caliente. This multi-parametric study provides valuable insights into geophysical signals and associated processes at Santiaguito, helping to resolve temporal occurrence of each event type during protracted effusive-explosive activity. • Analysis of seismic and infrasound data at Santiaguito revealed a variety of signals. • The data were classified using automated search algorithms. • Signals associated with gas-and-ash explosions showed an impulsive onset and a rapid amplitude decay. • Gliding harmonic tremor signals were observed in the seismic and acoustic data. [ABSTRACT FROM AUTHOR]