Active Crater Research Articles

Branched Conduit Structure Beneath the Active Craters of Sakurajima Volcano Inferred From Muography

AbstractObserving in‐conduit physical mechanism underlying the switching of eruptive activity between the craters of multi‐vent volcanoes could contribute to hazard assessment at these complex volcanic edifices. We performed monitoring of mass density changes beneath two active craters of Sakurajima volcano in Kyushu, Japan between September 2018 and July 2023 with cosmic‐ray muography. From December 2022, the mass density decreased beneath Minamidake crater and increased beneath Showa crater in accordance with a switch of eruptive activity from Minamidake crater to Showa crater. Plug formation and convective magma degassing processes have been observed with muography in accordance with the deformation of the surface of volcanic edifice measured by Interferometric Synthetic Aperture Radar. We found a moderate inverse correlation with a Pearson’s coefficient of −0.52 between the mass densities observed beneath the two active craters throughout the entire period. The inverse correlation of mass densities indicates a magmatic gas flux dynamics which characterizes a branched conduit structure. Muography thus reveals the shallow magmatic processes during switching of eruptive activity between the craters of multi‐vent volcanoes and allows to infer to the conduit structure.

Thermal Emissions of Active Craters at Stromboli Volcano: Spatio‐Temporal Insights From 10 Years of Satellite Observations

AbstractOpen‐vent volcanoes continuously emit magmatic products and frequently feature multiple adjacent craters. Temporal shifts of thermal emissions between craters are especially detectable by InfraRed satellites. Here, SENTINEL‐2 and LANDSAT‐8/9 Short Wave InfraRed (SWIR) high‐spatial resolution satellite data, are combined to investigate 10 years (2013–2023) of thermal activity at Stromboli volcano (Italy). The correlation between Volcanic Radiative Power (VRP, in Watts) and Volcanic Radiative Energy (VRE, in Joules), retrieved by moderate MODIS and VIIRS Middle InfraRed (MIR) data, with the Thermal Index SWIR (TISWIR) data, allows us to quantify long‐term series of heat fluxes (VRPSWIR) and energy (VRESWIR). Combining moderate and higher spatial resolution data and fitting cumulative trends of TISWIR with VREMIR allows to measure thermal activity sourced by single craters during Strombolian activity. Long‐term results highlight that thermal emissions are clustered in the northern and southern parts of the crater terrace, with total energy emitted (∼12 × 1014 J) equally distributed. The thermal increase since April 2017 marked a reactivation of shallow magma transportation and an intensification of the activity after the 2014 eruption. Distinct thermal behaviors are shown by the NE, C, and SW craters, related to mechanisms of explosions. We found that short‐term thermal variations match well those resolved by ground‐based signals, and the NE crater as the most sensitive to the transition to higher‐intensity activity. Our multispatial/multisensory investigation allows, for the first time, the long‐term quantification of heat flux from Stromboli's craters, with an improved understanding of open‐vent dynamics and a new approach to monitor multiple active craters.

Magnitude-corner frequency distribution of explosion earthquakes: a case study of Anak Krakatau Volcano, Sunda Strait

Abstract We investigated the distribution between the magnitude and corner frequency of explosion earthquakes at the Anak Krakatau volcano during June - July 2014. We analyzed 100 explosion waveforms recorded at the KRA3 and KRA4 seismic stations located approximately 0.55 km and 3.84 km away from the active crater, respectively. We observed a relationship between the moment magnitude and the corner frequency of the explosion earthquake distribution. The result shows that the moment magnitude is inversely proportional to the corner frequency in a logarithmic scale using the full waveform of explosion earthquakes. The distribution equation of full waveform (0-20s) at KRA3 and KRA4 as follows; fc = −2.72145 log(Mw ) + 7.99523, and fc = −1.13971 log(Mw ) + 5.97855 respectively. The magnitude range is approximately 1.5–3 Mw. Meanwhile, the corner frequency range is about 4-8 Hz. Our result shows a similar distribution to those in the tectonic earthquake and volcano studies at different locations which is the magnitude is inversely proportional to the corner frequency. We also observed the relation between seismic moment and corner frequency which shows that the seismic moment is inversely proportional to the corner frequency with the range of seismic moment is about 1013-1015 N.m. Meanwhile, the stress drop is proportional to the corner frequency with the range of stress drop is about 100 and 103 bar. Our results are similar to those of other studies. These parameters show the basic statistical properties of volcanic eruptions. This study is important for understanding the dynamics of volcanic eruptions at the Anak Krakatau volcano.

Investigation of the Extent of Hydrothermal Activities of the Sikidang Crater, Dieng Volcanic Complex (Central Java), Using Geomagnetic Method

Abstract The Sikidang Crater (± 2000 m asl) is one of the active craters in the Dieng Volcanic Complex (DVC). It is situated on the southeastern flank of Mt. Pangonan. The crater is significant for development of both tourism and energy sectors in Dieng. Thermal manifestations inside the crater which consist of boiling mud pools and pots, hot springs, fumaroles, gas vents, steaming and altered ground make the place a popular tourist attraction. They also serve as windows to the shallow subsurface processes and help better understand the dynamics of the Dieng geothermal system. However, the land use of the crater and its vicinity disregards the fact that the actual extent of the hydrothermal activity of the crater has not been figured out, and that the foci of the thermal fluid discharges in the crater often shift. Some agriculture plots, housing areas, as well as tourism and geothermal infrastructures are located within or near the morphological boundary of the crater. The inherent hydrothermal hazard of the crater therefore, must be well mitigated to minimize the negative impacts to the people, as well as their assets. Our research aims to delineate the lateral and vertical extent of the hydrothermal activity of the crater by geomagnetic method. Geomagnetic anomaly in geothermal field occurs due to the difference in the intensity of hydrothermal alteration of the rocks. The equipment used was a pair of Proton Precision Magnetometers GEOTRON G5. Our result shows that low geomagnetic anomaly (-2200 to -400 nT) correlated with demagnetized, hydrothermally altered andesitic rocks covers an area of ≥ 1.15 km2, i.e., more than seven times larger than the area outlined by morphological boundary of the crater. The anomalous zone extends N/NE - S/SW from beyond the areas of inter-volcanic plain between Pangonan and Igir Binem in the north, to the areas beyond Sikunang and Kendil cones in the south. Below the Sikidang mud pool the demagnetized rock mass reaches a maximum thickness of 2000 m. The supporting data, i.e., the fluctuations of the gas discharge, and the seismicity of the DVC indicate that the recent dynamics of the Sikidang Crater is mostly controlled by the internal processes within the crater.

Vulcanian eruptions at Sakurajima Volcano: geophysical data, numerical modelling, and petrological evidence

We investigated the magma conduit system beneath three active craters that have repeatedly generated Vulcanian eruptions at Sakurajima Volcano, Japan, by analysing seismic, infrasound, and tilt data. The hypocentres of the earthquakes associated with the Vulcanian eruptions are distributed separately at shallow depths of approximately 0.5 km beneath the craters. Infrasound indicated that the swelling of the crater floor starts approximately 0.2 s after the occurrence of earthquakes and that the eruption starts about 0.3 to 0.5 s later. During an eruption, tilt vectors at a station approximately 2.7 km far from the active craters indicated a deflationary trend directed toward one of the three active craters. A 1-D conduit flow simulation indicated pressure build-up at a depth of 0.4–1.0 km beneath the crater, consistent with previously reported pressure sources during eruptions detected by geodesy. Volcanic lapilli emitted from the three craters have the same chemical composition; hence, Vulcanian eruptions of all three studied craters originate from a common magma source.

Array analysis of seismo-volcanic activity with distributed acoustic sensing

SUMMARY Continuous seismic monitoring of volcanoes is challenging due to harsh environments and associated hazards. However, the investigation of volcanic phenomena is essential for eruption forecasting. In seismo-volcanic applications, distributed acoustic sensing (DAS) offers new possibilities for long-duration surveys. We analyse DAS strain rate signals generated by volcanic explosions and tremor at Stromboli volcano (Italy) recorded along 1 km of dedicated fibre-optic cable. We validate DAS recordings with colocated nodal seismometers. Converting node measurements to strain rate, we observe a perfect match in phase between DAS and node waveforms. However, DAS amplitudes appear to be around 2.7 times smaller than those of node records, which we explain as due to the inefficient ground-to-fibre strain transfer in the loose cable. We invert time delays between strain rate waveforms and confirm that the DAS enables us to retrieve a dominant and persistent seismic source in the proximity of active craters. This stable source location is confirmed by node array analyses. Despite an observed high noise level of strain rate signals outside a range of 2–15 Hz, our results demonstrate the potential of this new technology in monitoring volcanic areas.

Statistical Insights on the Eruptive Activity at Stromboli Volcano (Italy) Recorded from 1879 to 2023

Stromboli is an open-conduit active volcano located in the southern Tyrrhenian Sea and is the easternmost island of the Aeolian Archipelago. It is known as “the lighthouse of the Mediterranean” for its continuous and mild Strombolian-type explosive activity, occurring at the summit craters. Sometimes the volcano undergoes more intense explosions, called “major explosions” if they affect just the summit above 500 m a.s.l. or “paroxysms” if the whole island is threatened. Effusive eruptions are less frequent, normally occurring every 3–5 years, and may be accompanied or preceded by landslides, crater collapses and tsunamis. Given the small size of the island (maximum diameter of 5 km, NE–SW) and the consequent proximity of the inhabited areas to the active craters (maximum distance 2.5 km), it is of paramount importance to use all available information to forecast the volcano’s eruptive activity. The availability of a detailed record of the volcano’s eruptive activity spanning some centuries has prompted evaluations on its possible short-term evolution. The aim of this paper is to present some statistical insights on the eruptive activity at Stromboli using a catalogue dating back to 1879 and reviewed for the events during the last two decades. Our results confirm the recent trend of a significant increase in major explosions, small lava flows and summit crater collapses at the volcano, and might help monitoring research institutions and stakeholders to evaluate volcanic hazards from eruptive activity at this and possibly other open-vent active basaltic volcanoes.

Using Template Matching to Detect Hidden Fluid Release Episodes Beneath Crater Lakes in Ruapehu, Copahue, and Kawah Ijen Volcanoes

AbstractVolcano crater lakes, while picturesque, can sometimes mask the occurrence of small eruptions or hydrothermal fluid release events. However, these seemingly hidden events hold a wealth of valuable information about the underlying volcanic conduit and may pose risks to those near the volcano’s summit. This study presents a novel method for identifying these hard‐to‐detect fluid release events by examining seismic data from Ruapehu volcano in New Zealand. We undertake a multi‐timescale template‐matching analysis that uses a newly discovered seismic eruption precursor, to identify patterns related to the rapid consolidation of hydrothermal seals, pressurization processes, and the subsequent release of hydrothermal fluids. As a result, we identified a potential instance of sudden fluid‐release events that were previously unnoticed due to the presence of the crater lake. Our findings support a conceptual model of cyclic pressure variation within the conduit beneath an active crater lake. This model involves the formation and disruption of seals, followed by depressurization through hydrothermal fluid release events. Fluid discharge recession recorded as seismic amplitude decay, provides information about the properties of the reservoir, conduit, and the fluid being discharged. We also applied this technique to Kawah‐Ijen (Indonesia) and Copahue (Chile‐Argentina), identifying multiple potential events at these volcanoes. These findings enhance our understanding of the conditions leading to explosive eruptions, including those that could breach the crater lake.

Remote volcano monitoring using crowd-sourced imagery and Structure-from-Motion photogrammetry: A case study of Oldoinyo Lengai's active pit crater since the 2007–08 paroxysm

Active volcanic craters are highly dynamic geological features that undergo morphological changes on a broad range of spatial and temporal scales. Such changes have implications for the stability of the edifice, the eruptive style and the associated hazards. However, monitoring the morphological evolution of active craters at high spatial resolution and over long periods of time can be challenging, especially at remote volcanoes. In this study, we demonstrate the potential of Structure-from-Motion Multi-View Stereo photogrammetry technique based on crowd-sourced data, applied to the case study of Oldoinyo Lengai (OL) volcano in northern Tanzania. Following the 2007–08 paroxysm, OL volcano resumed its characteristic effusive activity and started to fill in with lava the newly-formed 300 m wide and 130 m deep pit crater. Monitoring capability is limited at OL due to its location in a remote non-urbanized area, therefore, the eruptive and morphological evolution is poorly constrained (e.g., lava emission rates, number of vents, location of unstable areas), with hazard implications for tourists visiting the summit area. Here we use crowd-sourced images, including Unoccupied Aircraft System (UAS) images, ground-based videos and pictures collected between October 2014 and June 2022, to reconstruct high-resolution topographic time-series of OL's summit crater. With these data, we have generated 7 Digital Elevation Models (DEMs) of OL's pit crater spanning the past 8 years, and estimated the emitted volume of lava and the corresponding time averaged discharge rates (TADR). From this we characterize the geomorphological evolution of OL pit crater since the 2007–08 paroxysm and perform a preliminary hazard assessment of the crater area. InSAR COSMO-SkyMed and Sentinel-1 data covering the periods 2013–2014 and 2018–2019 were also used in this study to complement our observations. Our results indicate that the main location of lava emission within the crater floor has repeatedly shifted over the years and that the 2008 cone has experienced a subsidence over time. OL's TADR has increased over the years, reaching values one order of magnitude higher in the period 2021–2022 compared to 2014–2018. Assuming similar TADR in the coming years, the crater could be filled in by lava within the next decade, leading to new lava overflows on the flanks of the volcano.

The influence of volcano topographic changes on infrasound amplitude: lava fountains at Mt. Etna in 2021

Infrasound signals are used to investigate and monitor active volcanoes during eruptive and degassing activity. Infrasound amplitude information has been used to estimate eruptive parameters such as plume height, magma discharge rate, and lava fountain height. Active volcanoes are characterized by pronounced topography and, during eruptive activity, the topography can change rapidly, affecting the observed infrasound amplitudes. While the interaction of infrasonic signals with topography has been widely investigated over the past decade, there has been limited work on the impact of changing topography on the infrasonic amplitudes. In this work, the infrasonic signals accompanying 57 lava fountain paroxysms at Mt. Etna (Italy) during 2021 were analyzed. In particular, the temporal and spatial variations of the infrasound amplitudes were investigated. During 2021, significant changes in the topography around the most active crater (the South East Crater) took place and were reconstructed in detail using high resolution imagery from unoccupied aerial system surveys. Through analysis of the observed infrasound signals and numerical simulations of the acoustic wavefield, we demonstrate that the observed spatial and temporal variation in the infrasound signal amplitudes can largely be explained by the combined effects of changes in the location of the acoustic source and changes in the near-vent topography, together with source acoustic amplitude variations. This work demonstrates the importance of accurate source locations and high-resolution topographic information, particularly in the near-vent region where the topography is most likely to change rapidly and illustrates that changing topography should be considered when interpreting local infrasound observations over long time scales.

Vulcanian eruption processes inferred from volcanic glow analysis at Sakurajima volcano, Japan

Volcanic glow, which is often observed at active craters during nighttime, contains information on high-temperature zones deep in the crater that are hidden from view. We analyzed 90 eruption videos capturing volcanic glow before the onset of Vulcanian eruptions at the Showa crater of Sakurajima volcano (Japan), and found that they show clear temporal changes. The red (R)-value and the green-to-red ratio (GR ratio) are used to track glow intensity and temperature change, respectively. We found two types of temporal change in glow intensity: a short-term change approximately 1 s before the onset of explosions, and a long-term change 3 to 330 s before an explosion. Short-term changes were observed for 28 of 90 eruptions, and coincided with an increase in the GR ratio and with a modest precursory infrasound pressure increase. Since an increasing GR ratio means a temperature increase, we consider that the change was associated with the opening of tensile cracks on the crater floor before an explosion. However, the other 62 eruptions did not show short-term changes, suggesting that the opening of cracks is sporadic. Long-term changes were observed for 73 of 90 eruptions, the GR ratio did not exhibit a significant increase. We suggest that long-term changes are caused by gradual gas leakage, which precedes initiation of Vulcanian eruptions. We hypothesize that the long-term gas leakage causes gradual decompression of the conduit, which induces supersaturation of volatile-rich magma beneath the lava plug causing explosive expansion, thereby triggering an eruption.

The Thermal Plumbing System of Stromboli Volcano, Aeolian Islands (Italy) Inferred From Electrical Conductivity and Induced Polarization Tomography

AbstractWe performed the first 3D island‐scale tomography of the electrical conductivity of Stromboli volcano (Aeolian Islands, Italy) using 2D acquisition lines (37.2 km) and a total of 18,880 measurements and 2,402 unique electrode locations. This 3D data set was inverted using a Gauss‐Newton algorithm, parallel‐processing on an unstructured tetrahedral mesh containing 678,420 finite‐element nodes and 3,580,145 elements to account for the topography of the volcanic island. The tomogram exhibits a conductive body (10−2–1.0 S m−1) consistent with the location of CO2 and temperature anomalies observed at the ground surface. It corresponds to the hydrothermal system with high electrical conductivity associated with alteration. In order to confirm this interpretation, a 2.5D large‐scale induced polarization tomography was performed crossing the volcano. The joint interpretation of the conductivity and normalized chargeability is done with a petrophysical model previously tested and verified at both shield‐ and strato‐volcanoes. This model implies that alteration (through the effect of the cation exchange capacity associated with clay minerals and zeolites) plays a strong role in both controlling the electrical conductivity and normalized chargeability at Stromboli volcano. A temperature tomogram, derived from the geoelectrical measurements, is consistent with surface temperature anomalies and the Very Long Period (VLP) seismicity related to the mild‐explosive activity. This survey displays at 600 m a.s.l. a lateral shift in the highest temperature location, also corresponding to the source of VLP seismicity. Structural boundaries have a major role in the hottest hydrothermal fluids rising below the active crater terrace of Stromboli volcano.

Shallow resistivity structure around the 2018 craters of Mt. Motoshirane of Kusatsu-Shirane Volcano, Japan, revealed by audio-frequency magnetotellurics

In 2018, a phreatic eruption occurred at Mt. Motoshirane, a pyroclastic cone group of the Kusatsu-Shirane Volcano in central Japan. The eruption was abrupt, and no signs of volcanic activity have been observed in recent years, in contrast to the other major pyroclastic cone group, Mt. Shirane, which hosts the active crater lake and has endured repeated phreatic eruptions. To understand the mechanism of the eruption at Mt. Motoshirane, information on the shallow hydrothermal system, which is thought to be the source region of phreatic eruptions, is required; however, few studies have been conducted on this particular cone group. In this study, we conducted an audio-frequency magnetotelluric survey in 2020 to reveal the shallow resistivity structure around the 2018 craters. A three-dimensional resistivity structure model showed generally two layers, with high resistivities at shallow depths overlain by low resistivities underneath. The boundary between the layers corresponded to the top boundary of the Neogene basement rocks. These low resistivities were not found beneath the 2018 craters; therefore, part of the Neogene basement rocks could have been lost by the eruption beneath the 2018 craters. This is consistent with the geochemical study on the mineral assemblage of the erupted deposits, which suggested that the explosions reached the depth of the basement.Graphical

Detecting Volcano‐Related Underground Mass Changes With a Quantum Gravimeter

AbstractWe present the world's first time series acquired in the summit area of an active volcano with an absolute atom interferometry gravimeter. The device was installed ∼2.5 km from the active craters of Mt. Etna volcano and produced a continuous high–quality gravity time series, despite the unfavorable environmental conditions at the installation site and the occurrence of phases of high volcanic tremor during the acquisition interval. Comparison with data from superconducting gravimeters installed elsewhere on Mt. Etna highlights correlated anomalies, demonstrating that the quantum device measured gravity variations driven by bulk mass changes. The latter are reflective of volcanic processes, involving the dynamics of magma and exsolved gas in the upper part of Mt. Etna's plumbing system. Our results confirm the operational possibilities of quantum gravimetry and open new horizons for the application of the gravity method in geophysics.

Classification of volcano-seismic events using waveforms in the method of k-means clustering and dynamic time warping

In most volcano observatories classification of seismic events is made as a routine work to get some information on eruptive activities. Automating the classification with artificial intelligence (AI) techniques is a current issue but many AI techniques are not directly applicable to waveforms because they consist of individually different data lengths. This study develops a simple method in which the distance between waveforms with different lengths is evaluated by dynamic time warping and waveforms are classified by k-means clustering. In this method each of the classified groups is specified by a prototype, one of the waveforms that may represent the group best and good classifications are selected considering high generality and good fitting to the members. The seismic events of Sakurajima volcano are classified in this method and waveforms, spectra, time sequences and source locations are compared among groups for some good classifications. It is revealed that one of the groups consisting of tremor-like waveforms has an intimate connection with a magma ascent event beneath the active crater of the volcano.

Volcanic unrest at Nevados de Chillán (Southern Andean Volcanic Zone) from January 2019 to November 2020, imaged by DInSAR

The volcanic complex of Nevados de Chillán, located in the Southern Volcanic Zone (SVZ) of the Andes, has been active for the past 640 ± 20 ka. Its volcanic activity includes dome forming eruptions, explosive events, and lava flows. The most recent eruption cycle started in January 2016. We employ DInSAR time-series from Sentinel-1 data to investigate the unrest episode from January 2019 to November 2020. Two distinct periods of unrest are recognized in the time series. The first period (from January to October 2019) coincides with explosive events, dome growth inside the active crater, and a decrease in seismic activity but does not present a significant deformation. The second period (October 2019 to November 2020) is characterized by a displacement towards the sensor's line-of-sight of 100–120 mm. The observed surface deformation is compatible with an inflation source approximately 1.5 km south-southwest of the present active vent, at 5.5 ± 0.5 km depth from the surface, and with a volume change of 0.044 ± 0.014 km3. The most likely explanation for the observed inflation of Nevados de Chillan is the intrusion of magma in a reservoir feeding the current eruption cycle.

The Phoenix Complex

The Phoenix Complex

Quantitative Assessment of Temporal Changes in Subaqueous Hydrothermal Activity in Active Crater Lakes During Unrest Based on a Time-Series of Lake Water Chemistry

Regular sampling of lake water has been performed at many volcanoes to assess the state of volcanic activity. However, it is not clear whether the absolute concentrations or, instead, rate of changes in concentrations are more suitable for such assessments. In this study, we show that temporal changes in concentrations of an element in lake water are described by a simple differential equation, assuming changes in lake volume and chemical processes are negligible. The time constants (63% response time for changes in the chemical concentration in lake water) have a wide range varying between 20 and 1,000 days for the studied volcanoes in Japan, meaning it takes a long time to assess volcanic activity based on the absolute concentration of an element. In order to assess the volcanic activity in a shorter time period, based on a time-series of lake element concentration data, we developed a numerical model to calculate temporal changes in the steady-state concentration, which is proportional to the elemental concentrations of the bulk hydrothermal fluid injected from subaqueous fumaroles and hot springs. We applied our method to Yugama crater lake at Kusatsu–Shirane volcano, Japan, and quantitatively evaluated temporal changes in the hydrothermal input from 1964 to 2020. As a result, we detected changes in the Cl concentrations of the bulk hydrothermal input that were associated with unrest including the phreatic eruption in 1976 and earthquake swarms in 1989–1992 and 2014–2020. The future concentration in the lake water can be predicted from the most recent steady-state concentrations. Comparing the predicted concentration curve with the concentration obtained from lake water samples, it is possible to quickly assess whether the concentration of the bulk hydrothermal input has increased/decreased or remained constant.

Rare Earth Elements Variations in a Hyperacid Crater Lake and Their Relations With Changes in Phreatic Activity, Physico-Chemical Parameters, and Chemical Composition: The Case of Poás Volcano (Costa Rica)

Decades of geochemical monitoring at active crater lakes worldwide have confirmed that variations in major elements and physico-chemical parameters are useful to detect changes in volcanic activity. However, it is still arduous to identify precursors of single phreatic eruptions. During the unrest phase of 2009–2016, at least 679 phreatic eruptions occurred at the hyperacid and hypersaline crater lake Laguna Caliente of Poás volcano (Costa Rica). In this study, we investigate the temporal variations of Rare Earth Elements (REE) dissolved in Laguna Caliente in order to 1) scrutinize if they can be used as a new geochemical tool to monitor changes of phreatic activity at hyperacid crater lakes and 2) identify the geochemical processes responsible for the variations of REE concentrations in the lake. The total concentration of REE varies from 950 to 2,773 μg kg−1. (La/Pr)N-local rock ratios range from 0.93 to 1.35, and Light REE over Heavy REE (LREE/HREE)N-local rock ratios vary from 0.71 to 0.95. These same parameters vary in relation to significant changes in phreatic activity; in particular, the (La/Pr)N-local rock ratio increases as phreatic activity increases, while that of (LREE/HREE)N-local rock decreases when phreatic activity increases. REE concentrations and their ratios were compared with the variations of major elements and physico-chemical parameters of the lake. Calcium versus (La/Pr)N-local rock and versus (LREE/HREE)N-local rock ratios show different trends compared to the other major elements (Na, K, Mg, Al, Fe, SO4, and Cl). Moreover, a higher loss of Ca (up to 2,835 ppm) in lake water was found with respect to the loss of Al, K, and Na. This loss of Ca is argued to be due to gypsum precipitation, a process corroborated by the mass balance calculation simulating the precipitation of gypsum and the contemporaneous removal of REE from the lake water. The observed relations between REE, changes in phreatic activity, and the parameters commonly used for the monitoring of hyperacid volcanic lakes encourage investigating more on the temporal and cause-effect relationship between REE dynamics and changes in phreatic activity at crater lake-bearing volcanoes.

Classification of volcanic tremors and earthquakes based on seismic correlation: application at Sakurajima volcano, Japan

SUMMARY Classification of volcanic tremors and earthquakes is an important part of volcano monitoring. Conventional classification technique relies on visual characterization of the amplitudes, frequency, or duration of seismic signals. In some cases, such classification is difficult to be maintained continuously, especially during high eruptive activities. In the present study, we develop a classification technique based on analyses of seismic wave correlation. The technique is based on a measure of spatial coherence using the distribution width of eigenvalues from the seismic covariance matrix that is referred to as ‘spectral width’. We use the spectral width characteristics to extract the frequencies where the seismic signals are the most spatially coherent. We analyse 6-month continuous seismic data at six seismic stations deployed on Sakurajima volcano, Japan. The classification is performed every 10 min to recognize volcanic tremor and B-type earthquakes, both of which show unclear onsets and are dominant at around 1–4 Hz. Their frequencies of high spatial coherence are different from each other, providing a basis for automatic classification of both types of seismic events. Our classification results show that an increase in seismic activity, particularly volcanic tremors, occurred during high eruptive activity and is well matched with the events that are routinely determined by the Japan Meteorological Agency (JMA) based on visual examination of the data. We discuss the volcanic activities by combining our classification results with those from detections and location methods based on seismic correlation. Most of the source locations of volcanic tremor and B-type earthquakes are distributed at the same region with depths of less than 4 km beneath the active craters, which are almost consistent with the source locations of explosion earthquakes and B-type earthquakes determined from P-wave onsets in the previous studies. Also, the distribution of horizontal locations is spatially elongated at shallow depths above a dyke that is inferred to extend from Aira caldera at a depth of 7–9 km. We obtain deep and shallow sources for the tremor that occurred before and after an explosive eruption, respectively. We also obtain shallow sources beneath the active crater during a continuous tremor that accompanies increasing eruptive activity. The source locations of the classified volcanic tremor and B-type earthquakes may represent the movement of magma and gas from the shallow magma chamber at 5 km depth to the active craters.