Unrest Episodes Research Articles

Sierra Negra, Galápagos: A resurgent-block basaltic caldera

Caldera resurgence is rare at mafic volcanoes. Here we consider the well-exposed resurgence at Sierra Negra caldera (Galápagos) to investigate how resurgence develops at a mafic system. Based on topographic and field analysis, the structure of the resurgence consists of an eastward-tilted block bounded by a fault-propagation fold activated by a steep inward-dipping reverse fault with dip decreasing toward the surface. Extension of the uplifted part of the reverse fault results from gravitational instability and is accommodated by horst-and-graben structures over a zone several hundred meters wide. Extension culminates in the main normal fault responsible for the inward tilt of the lava pile, forming a distinctive ridge. The resurgence results from spatially and temporally distinct unrest episodes, promoted by the shallow accumulation of large volumes of magma. Sierra Negra is the first documented example of piecemeal resurgence, as shown by the recent uplift episodes associated with eruptions in 2005 and 2018. The example of Sierra Negra suggests that the formation of resurgent blocks depends on the initial location of the feeding system, with non-centered feeding systems developing asymmetric (trapdoor) blocks. Finally, Sierra Negra demonstrates that mafic volcanoes without well-developed rift zones may promote resurgence when reaching a mature stage with significant amounts of cumulates, favoring shallow magma accumulation.

Monastic Unrest on the Border of Carolingian Italy: The Strange Case of Abbot Poto of San Vincenzo al Volturno

ABSTRACT In the aftermath of the Carolingian conquest of the Italian peninsula, a commission was held in Rome to judge the case of Poto, Abbot of San Vincenzo al Volturno, who was accused of infidelitas for refusing to pray for Charlemagne’s safety. The papal inquiry reveals the existence of two factions within the monastic community, each of them likely following a different approach to monastic discipline. However, regional politics soon overlapped with internal strife, as the group led by Ambrogio Autpert was supported by Duke Hildeprando of Spoleto and the other led by Poto was supported by Pope Adrian I. This article analyses this episode of monastic unrest by focusing on the opportunities that political uncertainty opened for public authorities in an unsettled border area. It also aims to challenge the ethnic framework (assuming a Lombard faction opposed to a Frankish faction) which historiography has traditionally applied when interpreting Poto’s act of disobedience.

Source characterization of the 20th May 2024 MD 4.4 Campi Flegrei caldera earthquake through a joint source-propagation probabilistic inversion

On May 20th, 2024, an earthquake of magnitude MD 4.4 nucleated at shallow depth (2.6 km) in the Campi Flegrei caldera (Southern Italy), a densely populated area where an increase in seismic activity has been observed since 2019 attributable to an on-going unrest episode. While the magnitude was moderate, the event produced a strong ground shaking with an observed maximum peak ground acceleration of 3.58 m s-2, and several buildings were damaged. Here, we characterize the earthquake source using a probabilistic joint source-propagation spectral inversion in the Fourier space. We estimate a moment magnitude Mw = 3.70 ± 0.13 and a corner frequency fc = 1.11 ± 0.19 Hz. Assuming a circular rupture model, we estimate a source radius r = 400 ± 70 m and a stress drop Δσ = 3.2 ± 2.2 MPa. The estimated stress drop suggests that future earthquakes in the hypocentral region, considering a possible rupture length of 3 km suggested by previous studies, can have magnitude increased by 1.2 ± 0.3 units with respect to May 20th event. A systematic source characterization of the recent seismicity in the caldera would hep in estimating the expected ground motions from future large-magnitude events.

Messina, 1167–68: A Microhistory of Urban Agency in the Twelfth-Century Kingdom of Sicily

ABSTRACT Recent scholarship on the cities of the twelfth-century Kingdom of Sicily continues to nuance our understanding of urban life under the umbrella of a new monarchy. Collectively, these studies demonstrate that this new monarchy, centralising and powerful as it was, did not fully squash its cities’ independence, and that this was not even its aim. This picture has been aided by recent studies that interpret urban identity and agency in a more holistic and polycentric framework, incorporating both its political and economic situations and its socio-cultural, religious, ceremonial and communicative contexts. Consequently, recognition of a vibrant “public arena” within the cities of the medieval world now extends to those in the twelfth-century Kingdom of Sicily. The present article supports and further expands this re-evaluation of the kingdom’s cities through a microhistory. It takes one episode of unrest in one city which offers an unusually close (for the Kingdom of Sicily at least) snapshot of urban dynamics. It interprets this moment via approaches grounded in recent scholarship on urban public politics that foreground the “political turn” in rebellion, and which view such moments of unrest and collective action as a normal part of a continual dialogue within a polycentric political landscape.

Seismic characteristics of the 2022-2023 unrest episode at Taupō volcano, Aotearoa New Zealand

Taupō is a large caldera volcano located beneath a lake in the centre of the North Island of New Zealand and most recently erupted ~1800 years ago. The volcano has experienced at least 16 periods of unrest since 1872, each of which were characterised by increased seismic activity. Here we detail seismic activity during the most recent period of unrest from May 2022 to May 2023. The unrest was notable for the highest number of earthquakes detected during instrumented unrest episodes, and for one of the largest magnitude earthquakes detected beneath the lake for at least 50 years (ML 5.7). Relocated earthquakes indicate seismic activity was focused around an area hosting overlapping caldera structures and a hydrothermal system. Moment tensor inversion for the largest earthquake includes a non-negligible inflationary isotropic component. We suggest the seismic unrest was caused by the reactivation of faults due to an intrusion of magma at depth.

Sulfur isotopic variations in the products of the 1895 CE eruption at Zao volcano (NE Japan): Implications for connecting eruption source and syn-eruptive magmatic-hydrothermal processes

Sulfur isotopic ratio in sulfate and sulfide in subvolcanic hydrothermal systems is a valuable tracer to study the magmatic-hydrothermal processes from the magma source through to volcanic eruptions. Zao volcano is among the most active volcanoes in NE Japan, with historical explosive eruptions occurring during the last thousand years and unrest episodes since 2013. This necessitates a detailed assessment of the potential risk of future volcanic hazards. We investigated the magmatic-hydrothermal processes that occurred during the 1895 CE eruption sequence at Zao volcano by conducting mineralogical and sulfur isotope analyses in the exposed well: (i) six volcanic units (Layers 1–6) of the 1895 CE eruption products (clayish ash deposits with andesitic bombs, lapilli of scoria, and minor altered lithic fragments) deposited on the rim of Okama crater lake; and (ii) clay-altered and silicified rocks from the Nigorikawa alteration zone (NGA) surrounding the Goshikidake cone. Mineralogical data show that the samples mainly consist of alunite, pyrite, and gypsum. Alunite and pyrite occur as fine crystal mixtures associated with mineral assemblages of both advanced argillic alteration (i.e., those of cristobalite and kaolinite) and silicification (i.e., those of cristobalite, tridymite and native sulfur). Gypsum typically appears as isolated euhedral crystals of several millimeters in size. Samples of the 1895 CE eruption products have a narrow range of δ34S values from +3 ‰ to +5 ‰ for gypsum, from +9 ‰ to +13 ‰ for alunite, and approximately −10 ‰ for pyrite. For the NGA samples, the δ34Sgypsum, δ34Snative sulfur, and δ34Spyrite values range from −12 ‰ to −9 ‰, whereas for alunite, these range from +8 ‰ to +18 ‰. This indicates that alunite and pyrite in the 1895 CE eruption products were derived from the advanced argillic alteration and silicification zones that developed under Okama crater, which is exposed as the NGA. Estimated alteration temperatures based on the sulfur isotopic equilibrium between alunite and pyrite pairs are 200 °C–300 °C. By contrast, δ34Sgypsum values in the 1895 CE products are significantly higher than those in the NGA (which are derived from oxidation of pyrite or H2S, or both), ranging between an estimated parental fluid of δ34Sbulk-initial = ca. +1 ‰ and the Quaternary volcanic rocks of the Japan arc. This suggests that gypsum in the 1895 CE eruption products derived from magmatic vapor condensate (anhydrite) formed in the volcanic conduit during the eruption, thus becoming replacement of anhydrite by gypsum after or during the tephra deposition on the Zao summit surface. Our results on sulfur-bearing minerals provide new clues for better understanding (and monitoring) the syn-eruptive processes of volcanic eruptions focused on subvolcanic hydrothermal systems.

Pricing protest: the response of financial markets to social unrest

Abstract We identify start days of 156 episodes of social unrest from textual analysis of media reports and show a systematic negative impact of social unrest on stock market performance. Social unrest on average leads to a 1.4 percentage point drop in cumulative abnormal returns in 2 weeks, more for events that last longer and that happen in emerging markets. Stronger institutions, particularly better governance and more democratic systems, are associated with a smaller adverse impact of social unrest on stock market returns. We argue this reflects the ability of better institutions to provide a more reliable way to reconcile conflicting views and dampen uncertainty after unrest.

The information content of conflict, social unrest and policy uncertainty measures for macroeconomic forecasting

It is widely accepted that episodes of social unrest, conflict, political tensions and policy uncertainty affect the economy. Nevertheless, the real-time dimension of such relationships is less studied, and it remains unclear how to incorporate them in a forecasting framework. This can be partly explained by a certain divide between the economic and political science contributions in this area, as well as the traditional lack of availability of timely high-frequency indicators measuring such phenomena. The latter constraint, though, is becoming less of a limiting factor through the production of text-based indicators. In this paper we assemble a dataset of such monthly measures of what we call “institutional instability”, for three representative emerging market economies: Brazil, Colombia and Mexico. We then forecast quarterly GDP by adding these new variables to a standard macro-forecasting model using different methods. Our results strongly suggest that capturing institutional instability above a broad set of standard high-frequency indicators is useful when forecasting quarterly GDP. We also analyse relative strengths and weaknesses of the approach.

Volcanic soil gas 4He/CO2 ratio: a useful geochemical tool for real-time eruption forecasting

At many dormant volcanoes, magmatic gases are not channeled through preferential degassing routes as fumaroles and only percolate through the flanks of the volcano in a diffuse way. This type of volcanic gas emission provides valuable information, even though the soil matrix contains an important atmospheric component. This study aimed to demonstrate that chemical ratios such as He/CO2 in soil gases provide excellent information on the evolution of volcanic unrest episodes and help forecast the volcanic eruption onset. Before and during the occurrence of the October 2011–March 2012 submarine of El Hierro, Canary Islands, more than 8500 soil He analyses and diffuse CO2 emission measurements were performed. The results show that the soil He/CO2 emission ratio began increasing drastically one month before eruption onset, reaching the maximum value 10 days before. During the eruptive period, this ratio also showed a maximum value several days before the period with the highest magma emission rate. The He/CO2 ratio was also helpful in forecasting the eruption onset. We demonstrate that this tool can be applied in real-time during volcanic emergencies. Our results also encourage a reevaluation of the global He emission from the subaerial volcanism.

Dynamics of the 2021 unrest at Changbaishan Tianchi volcano from ALOS-2/PALSAR-2 and seismic data

The Changbaishan Tianchi intraplate volcano is one of the most active and hazardous volcanoes of NE Asia, characterized by a summit caldera formed after the 946 CE ‘Millennium’ Plinian eruption. From December 2020 to June 2021, the frequency and magnitude of earthquakes at Tianchi were significantly higher than during background periods, with hundreds of earthquakes (46 events per month in average) and reaching a local magnitude of ML 3.1. This study reports a comprehensive deformation analysis and geophysical inversion scheme aimed to unveil the dynamics of this period. Multi-temporal InSAR analysis results of 32 ALOS-2 images from 2018 to 2022 show that the surface deformation is a combination of seasonal fluctuations (± 25 mm in average, with a maximum ± 45 mm) and a long-term positive component. The least squares linear regression of the deformation time series and temperature data, isolates the seasonal fluctuations, revealing a clear uplift-subsidence process from June 2020 to July 2021 in the caldera area. To constrain the Tianchi plumbing system dynamics, a combined inversion scheme consisting of three deformation sources is designed. The inversion results and the seismic records indicate that Tianchi volcano has experienced a low-level unrest episode from December 2020 to June 2021. The shallower plumbing system, located at about 5–9 km depth and modelled by pressurized spheroids, underwent a cumulative volume increase of 26 × 106 m3 from November 2018 to April 2021, followed by a volume decrease of 9 × 106 m3 from April to July 2021. This suggests magma uprising from the 14 km deep storage zone to the shallower plumbing system, followed by depressurization of the plumbing system due to the escape of fluids. This research provides a comprehensive understanding of the magma and fluid migration dynamics within the Tianchi multi-level plumbing system for the first time.

Ground deformations associated with an overpressurized hydrothermal systems at Azuma volcano (Japan) revealed by InSAR data

Inflations at active volcanoes are indicators of overpressure in the subsurface, which is known to be a phenomenon that precedes eruptions. Volcanic overpressure is induced by the injection of magmatic fluids, accumulated magma, or heat supply from greater depths. Azuma volcano (Japan) has experienced several episodes of volcanic unrest with increases in seismicity at the depth of the hydrothermal system, implying a potential increase in phreatic eruptions. The time series of interferometric synthetic aperture radar data, associated with the unrest episodes occurring in 2014–2015 and 2018–2019, revealed spatiotemporal variations of inflation episodes, centered around Oana crater, the most active fumarole of Azuma volcano. The modeled best-fit geometry of the elongated pressure source for the local deformation has the same dip as the overlying topographic slope direction and angle around Oana crater, suggesting the existence of topography-correlated layered structures within the hydrothermal system. In contrast, the broader deformation associated with the 2014–2015 unrest was explained by the overpressure of a horizontal flat source at 360–1500 m below sea level, showing the similar depth of the top as the conductive low-resistivity or low-viscosity body suggested by previous studies. The unrest episodes were thus interpreted as resulting mainly from the supply of magmatic fluids, or the transfer of heat from greater depths. Our study helps in understanding the shallow structure of this volcanic system and contributes to evaluating the potential for forthcoming eruptions in Azuma volcano.Graphical

From the detection of monitoring anomalies to the probabilistic forecast of the evolution of volcanic unrest: an entropy-based approach

Owing to the current lack of plausible and exhaustive physical pre-eruptive models, often volcanologists rely on the observation of monitoring anomalies to track the evolution of volcanic unrest episodes. Taking advantage from the work made in the development of Bayesian Event Trees (BET), here we formalize an entropy-based model to translate the observation of anomalies into probability of a specific volcanic event of interest. The model is quite general and it could be used as a stand-alone eruption forecasting tool or to set up conditional probabilities for methodologies like the BET and of the Bayesian Belief Network (BBN). The proposed model has some important features worth noting: (i) it is rooted in a coherent logic, which gives a physical sense to the heuristic information of volcanologists in terms of entropy; (ii) it is fully transparent and can be established in advance of a crisis, making the results reproducible and revisable, providing a transparent audit trail that reduces the overall degree of subjectivity in communication with civil authorities; (iii) it can be embedded in a unified probabilistic framework, which provides an univocal taxonomy of different kinds of uncertainty affecting the forecast and handles these uncertainties in a formal way. Finally, for the sake of example, we apply the procedure to track the evolution of the 1982–1984 phase of unrest at Campi Flegrei.

‘Throwing in my two cents’: Burundian diaspora youth between conventional and transformative forms of mobilization

ABSTRACT Since its independence in 1962, Burundi has witnessed several episodes of civil war and political unrest. The most recent of these, which started in 2015, triggered the migration of approx. 400,000 Burundians to neighbouring countries and the rest of the world. Based on interviews with first- and second-generation Burundian youths living in France and Belgium, this contribution examines how Burundian diasporic youth mobilizes against the authoritarian regime in Burundi. It explores what motivates their engagement, and how they evade control and navigate threats of repression. It also sheds light on different mobilization paths, between rather conventional types of engagement and transformative forms of activism. It shows how Burundian diasporic youth in Belgium and France have developed a reflexive form of transnational awareness building on their hyphenated identities and challenging simplistic binaries, notably by identifying similarities between structures of power and oppression in Burundi and their countries of residence.

Muslim political dissent in coastal East Africa: complexities, ambiguities, entanglements

ABSTRACT This article stages a comparative analysis of Muslim politics in coastal Kenya and Tanzania between 2010 and 2023. We explore parallels, discontinuities, and entanglements between different expressions of – and responses to – Muslim political dissent. Our insights are drawn from ethnographic fieldwork conducted in Dar es Salaam, Malindi, and Zanzibar City. We begin by investigating a sharp rise of militant jihadi activity across the region, examining responses by Kenyan, Tanzanian, and U.S. governments, as well as the perceptions of ordinary Muslim citizens. We then explore currents of Muslim civic activism, highlighting the different claims, sentiments, and memories that these movements invoke. Merging these discussions, we analyse episodes of civil unrest and violence that are associated with Muslim dissenters, but which are shrouded with uncertainty. We examine the shifting interpretive frames that Muslim residents apply to these events. We demonstrate how these uncertainties and framing practices, alongside state security strategies, impact the capacity for Muslims at large to engage in political dissent. Using our analysis, we argue that forms of Muslim political expression in coastal East Africa, though comparable and sometimes entangled, must be interpreted with close attention to the distinct experiences, demographic configurations, and political landscapes that characterise different (sub-)national contexts.

Nyiragongo Crater Collapses Measured by Multi‐Sensor SAR Amplitude Time Series

AbstractCrater morphology undergoes rapid changes at active volcanoes, and quantifying these changes during volcanic unrest episodes is crucial for assessing volcanic activity levels. However, various limitations, including restricted crater access, cloud cover, haze, and intra‐crater eruptive activity, often impede regular optical or on‐site crater monitoring. To overcome these challenges, we utilize multi‐sensor satellite Synthetic Aperture Radar (SAR) imagery to generate dense time series of quantitative indicators for monitoring crater morphological changes. By combining images from diverse satellites and acquisition modes, we achieve high temporal resolution. Nevertheless, due to variations in acquisition geometries, direct image comparisons become impractical. To address this, we develop PickCraterSAR, an open‐access Python tool that employs basic trigonometry assumptions to measure crater radius and depth from SAR amplitude images in radar geometry. We apply our methodology to study the crater collapse associated with the May 2021 and January 2002 eruptions of Nyiragongo volcano. Following the 2021 collapse, we estimate the maximum depth of the crater to be 850 m below the rim, with a total volume of 84 ± 10 Mm3. Notably, the post‐2021 eruption crater was 270 m deeper but only 15%–20% more voluminous compared to the post‐2002 eruption crater. Additionally, we demonstrate that the 2021 crater collapse occurred progressively while a dike intrusion migrated southward as a consequence of the drainage of the lava lake system. Overall, our study showcases the utility of multi‐sensor SAR imagery and introduces PickCraterSAR as a valuable tool for monitoring and analyzing crater morphological changes, providing insights into the dynamics of volcanic activity.

One decade of b-value variations from volcano-tectonic seismicity as an early indicator of episodes of crisis in a volcano: the case of Copahue, Southern Andes

Volcanoes can enter in episodes of unrest, which might end later in an eruption, with little warning. They are normally produced due to the inner dynamics of the volcano, but can also be triggered by external earthquakes. To detect these periods early, it becomes crucial to understand the dynamics of the different structures (such as fault systems) of the volcano, as they can act as magma pathways and can also generate instabilities on it. In this article we study the seismicity of Copahue (central-south Chile), which sits atop a complex system of faults, and was importantly affected by the 2010 Mw 8.8 Maule earthquake. We focused ourselves in the temporal variations of the b-value of the Gutenberg-Richter law during the 2012–2022 period. During this timeframe the volcano had a series of crises, which led to seven eruptive phases. Our results show that the system does exhibit signs of a future unrest phase weeks to months before there is a change on its alert level, and they are mostly linked to the activity of a N-S fault zone, located not beneath the main crater of Copahue, but around 10 km to the East. Most of the crises start after drops in the b-value of this structure, with sudden variations in b-value being also noticeable as a response to the 2015 Mw 8.3 Illapel earthquake. Our results show a correlation between instances of fluid injection and release in the relevant structures of an active volcano with the variations of the b-value. This allow us to use the temporal variations of the b-value as a tool to anticipate the inner dynamics of the system, particularly when there is a strong structural control on it, such as in the case of Copahue. We also found out that the influence of the 2010 Mw 8.8 Maule earthquake was long-lived, affecting mostly the structures that later destabilized the volcano for the most part of a decade, therefore enhancing magma injection into the whole system. This seems to have changed since 2021, which might indicate a waning in the influence of the megathrust earthquake in Copahue volcano.

Modelling the post-caldera plumbing system of Changbaishan volcano (China) from integrated geochemical, isotopic, geobarometric, and geophysical data

The Changbaishan Tianchi volcano is the largest active volcano of China and the 946 CE VEI 6 “Millennium eruption” (ME) is responsible for the formation of its summit caldera. After the ME, the volcanic activity is characterized by smaller eruptions most of which are recorded only in historical documents. Here we analyze the deposits of the ME and 1403 CE eruption outcropping in the Heishigou section, which is located east of the Chagbaishan Tianchi caldera. We present whole-rock, glass, mineral, isotopic, and geobarometric data to obtain information on the vertical extent of the caldera and post-caldera plumbing system of the volcano. Results show that the plumbing system of the 1403 CE eruption is located between 1.9 and 5.4 km depth, which is also the depth range of the shallower reservoir of the ME. Therefore, the magma reservoir of the ME was active also after the caldera collapse. The 1403 CE magma stored in the ME reservoir evolved by fractional crystallization of the residual ME trachytic magma and underwent the contamination of upper crust material. The 1403 CE event was triggered by the injection of less evolved, mafic magma from depth. The inferred depth of the shallower ME reservoir and that of the 1403 CE eruption is the same of the present-day main geophysical anomalies (low seismic velocity and low resistivity) recorded below the Changbaishan Tianchi caldera. In addition, the source of the deformations related to the uplift during unrest episodes between 2002 and 2020 suggests a refilling of the 1403 CE plumbing system. We conclude that the plumbing system of the 1403 CE, post-caldera activity at Changbaishan Tianchi volcano is the same of the ME eruption and it is still active. This system may be recharged from the arrival of less evolved magma from depth. The proposed conceptual model of the post-caldera volcanism at Changbaishan Tianchi represents a reference framework to interpret the signals of unrest and decipher the dynamics of the present-day plumbing system.

The February 2018 seismic swarm in São Miguel, Azores

The island of São Miguel is among the most seismically active areas of the Azores archipelago. This work focuses on the most significant recent swarm, which occurred on February 2018. We set up an automated procedure to process continuous full seismic waveform data from local stations to generate high-quality earthquake information on the volcano unrest episode. First, we applied an automated detector software, next we located the detected events and then classified the earthquakes based on their waveform similarity, identifying three families of seismic events. We then extended the catalog by template matching. Finally, we computed moment tensors to investigate the source mechanisms of the largest earthquakes. Our results image the ∼2-week swarm evolution. The activity started with a precursory phase with low rate and low magnitude (ML < 2.0) seismicity and the activation of a deeper structure (∼10-15 km). After ∼1 week, a new earthquake family emerged at shallower depths (∼8–12 km) reaching magnitudes up to ML 3.4. Finally, a third slightly shallower family was activated. Moment tensors show mostly normal faulting mechanisms, striking ∼NW-SE, compatible with the orientation of the regional stress field. A surface deformation transient was recorded by geodetic stations, starting with the swarm, and continuing over the following ∼17 months, corresponding to either inflation or extension around the swarm region. The prolonged surface deformation implies a process that was initiated during the swarm and subsequently accommodated mostly aseismically. We interpret the seismicity observed at the early stage of deformation as indicating episodic fluid injection through the crust, related to the local hydrothermal or magmatic systems. We conclude that the Fogo-Congro region continues to be seismo-volcanically active, with both seismic and aseismic deformation observed and requiring close multidisciplinary monitoring. The proposed methology based on the automated analysis of continuous waveform data provides high-quality imaging of the spatio-temporal evolution of seismicity, which can be used elsewhere in the operational monitoring of seismo-volcanic crises to gain insight into the ongoing deformation processes, improve hazard assessment and help in the development of effective mitigation strategies.

Gentrification and Social Unrest: The Blitz, Urban Change and the 2011 London Riots

Many of the world’s major cities have recently seen large episodes of social unrest. What is the relationship between the changes these cities have experienced, particularly in the form of gentrification, and urban riots? We address this question by examining how local gentrification affected participation in the 2011 London riots. We use an instrumental variable strategy that exploits exogenous variation in the amount of local destruction caused by the Blitz; this is a strong predictor of local gentrification in London in 2001–2011. We find that gentrifying neighborhoods had a lower participation rate than other areas; this was a result of changes in the type of resident (a composition effect) and in the context in which the residents made their participation decisions (a contextual effect). Our findings are consistent with qualitative evidence from the United Kingdom and the United States, and highlight the effect that urban change can have on social order.

Identification of Seismo‐Volcanic Regimes at Whakaari/White Island (New Zealand) Via Systematic Tuning of an Unsupervised Classifier

AbstractWe present an algorithm based on Self‐Organizing Maps (SOM) and k‐means clustering to recognize patterns in a continuous 12.5‐year tremor time series recorded at Whakaari/White Island volcano, New Zealand (hereafter referred to as Whakaari). The approach is extendable to a variety of volcanic settings through systematic tuning of the classifier. Hyperparameters are evaluated by statistical means, yielding a combination of “ideal” SOM parameters for the given data set. Extending from this, we applied a Kernel Density Estimation approach to automatically detect changes within the observed seismicity. We categorize the Whakaari seismic time series into regimes representing distinct volcano‐seismic states during recent unrest episodes at Whakaari (2012/2013, 2016, and 2019). There is a clear separation in classification results between background regimes and those representing elevated levels of unrest. Onset of unrest is detected by the classifier 6 weeks before the August 2012 eruption, and ca. 3.5 months before the December 2019 eruption, respectively. Regime changes are corroborated by changes in commonly monitored tremor proxies as well as with reported volcanic activity. The regimes are hypothesized to represent diverse mechanisms including: system pressurization and depressurization, degassing, and elevated surface activity. Labeling these regimes improves visualization of the 2012/2013 and 2019 unrest and eruptive episodes. The pre‐eruptive 2016 unrest showed a contrasting shape and nature of seismic regimes, suggesting differing onset and driving processes. The 2016 episode is proposed to result from rapid destabilization of the shallow hydrothermal system, while rising magmatic gases from new injections of magma better explain the 2012/2013 and 2019 episodes.