Explosive Episodes Research Articles

Modeling Lahars on a Poorly Eroded Basaltic Shield: Karthala Volcano, Grande Comore Island

Lahars impose significant secondary hazards on highly populated volcanic islands by remobilising volcanic ash deposits. Karthala, on Grande Comore Island, is a basaltic shield volcano with sporadic occurrence of ash-forming phreatic eruptions. In 2005, two mildly explosive episodes emplaced tephra blankets on the summit area. Heavy precipitation subsequently triggered the repetitive occurrence of small-volume secondary lahars up to 2012. These lahars impacted the settlements at the foot of Karthala, damaging roads and hundreds of houses, affecting thousands of inhabitants. This study aims at gaining insights into the effectiveness of two simple tools to reproduce the lahars’ extent in this data-poor context. To understand the physical characteristics of the lahars, we first documented the extent and characteristics of the debris deposits at the foot of Karthala volcano and in the ravines that guided the flows. Our observations suggest that the debris was emplaced by small-scale (volumes ≤105 m³), rain-triggered and predominantly low sediment concentration lahars. The spatial extent of the deposits served to calibrate and compare numerical lahar simulations from the widely used volume-limited LaharZ model with results from Q-LavHA, a recent probabilistic flow model originally developed for lava flows. Our comparison demonstrates that LaharZ outperforms Q-LavHA for small-scale lahar simulations. While Q-LavHA has features that mitigate some limitations of LaharZ, such as its ability to simulate flow bifurcations and the transition from constrained to unconstrained flow, it typically underestimates the actual inundated area. Nevertheless, both models show rather poor performance, encountering difficulties in delineating lahar flow paths due to the smooth topography of the volcanic edifice. We therefore also evaluated the potential to increase simulation accuracy by updating a 10-meter resolution Digital Elevation Model (DEM) with channel topography measurements. While this approach led to improved simulation accuracies for both models compared to using un-adapted DEMs, it did not prevent simulations to miss the hazard-prone position of settlements actually affected by the hazard. This study helps inform numerical volcanic flow simulation strategies on young and poorly dissected volcanic edifices, such as basaltic shields, and forms a basis for further developing numerical simulation tools more adapted to these particular environmental settings.

Forecast performance and bubble analysis in noncausal MAR(1, 1) processes

AbstractThis paper examines the performance of nonlinear short‐term forecasts of noncausal processes from closed‐form functional predictive density estimators. The processes considered have mixed causal–noncausal MAR(1, 1) dynamics and non‐Gaussian distributions with either finite or infinite variance. The quality of point forecasts is affected by spikes and bubbles in the trajectories of these processes, which also characterize many financial and economic time series. This is due to deformations of estimated predictive densities from multimodality during explosive episodes. We show that two‐step‐ahead predictive densities of future trajectories based on the MAR(1, 1) Cauchy process can be used as a new graphical tool for early detection of bubble outsets and bursts. The method is applied to the Bitcoin/US dollar exchange rates and commodity futures.

Date-stamping multiple bubble regimes

Abstract Identifying the start and end dates of explosive bubble regimes has become a prominent issue in the econometric literature. Recent research has demonstrated the advantage of a model-based minimum sum of squared residuals estimator, combined with Bayesian Information Criterion model selection, over recursive unit root testing methods in providing accurate date estimates for a single explosive regime. However, in the context of multiple bubbles, a large number of models are possible, making such a model-based method unappealing. In this paper, we propose a two-step procedure for dating multiple explosive regimes. First, recursive unit root tests are used to identify a ‘date window’ in which an explosive episode starts and ends. Second, a model-based BIC approach is used to precisely estimate the regime change points within each date window. In addition, our method allows us to distinguish between different types of explosive episode, such as whether or not each explosive regime crashes before reverting back to a unit root process, and date any crash regimes. Monte Carlo simulations highlight the effectiveness of our procedure when compared to existing methods of dating. The value of the new methodology is also demonstrated through an empirical application to housing markets.

Price bubbles and market integration in global sugar futures markets

ABSTRACT We use a Supremum Augmented Dickey–Fuller test to detect price bubbles in the world’s most important sugar futures markets (ZCE, NYBOT, and LIFFE) from 2006 to 2017. Results show 19 bubbles with characteristics similar in quantity, duration, and price variation. We explore whether sugar futures prices in ZCE, NYBOT, and LIFFE are integrative in a full sample with an improved hybrid method of directed acyclic graphs and structural vector autoregression. Based on the bubble test, we examine market integration in the sugar futures markets during explosive and unexplosive episodes. We find the impact of price bubbles on market integration and explore the cause of price bubbles in a macro-economic environment. Empirical results show futures markets are more integrative when price bubbles occur. We find sugar futures price bubbles reflect supply and demand imbalance, market participants are extremely sensitive, and market information exchanges frequently during the bubble period.

Unravelling the links between seismo-acoustic signals and eruptive parameters: Etna lava fountain case study

Deriving eruption source parameters from geophysical data is critical for volcano hazard mitigation, yet remains a challenging task in most volcanoes worldwide. In this work, we explored the temporal relationship between geophysical signals and eruptive parameters measured during six explosive episodes from the New South-East Crater of Mt. Etna (Italy). The quadratic reduced seismic velocity and pressure were calculated to track the temporal variation of volcanic elastic radiation, and the lava fountain height was estimated by thermal camera image processing. The temporal relationships between these geophysical and eruptive time series were studied. In particular, the first considered lava fountain exhibited a “clockwise hysteresis” pattern: higher seismic amplitude with respect to the fountain height during the waxing phase as compared to the waning phase. We also calculated the regression parameters for both linear and power laws, linking seismo-acoustic and eruptive time series. For the linear regressions, we found fairly constant values of the scaling factors in five out of six eruptive episodes, which can be considered as a promising step to derive eruption source parameters from geophysical data in real-time. Regarding power law regressions, a clear relationship was observed between the exponents determined for the power law linking quadratic reduced velocity and lava fountain height, and the time interval duration from the previous eruption. These results suggest that the condition of the uppermost part of the plumbing system (e.g. viscosity of residing magma and conduit conditions) play a key role in the seismic energy generation during the eruptions.

D/H ratios and H2O contents record degassing and rehydration history of rhyolitic magma and pyroclasts

Volcanic eruptions of rhyolitic magma often show shifts from powerful (Vulcanian to Plinian) explosive episodes to a more gentle effusion of viscous lava forming obsidian flows. Another prevailing characteristic of these eruptions is the presence of pyroclastic obsidians intermingled with the explosive tephra. This dense, juvenile product is similar to the tephra and obsidian flow in composition, but is generally less degassed than its flow counterpart. The formation mechanism(s) of pyroclastic obsidians and the information they can provide concerning the extent to which magma degassing modulates the eruptive style of rhyolitic eruptions are currently subject to active research. Porous tephra and pyroclastic and flow obsidians from the 1060CE Glass Mountain rhyolitic eruption at Medicine Lake Volcano (California) were analyzed for their porosity, ϕ, water content, H2O, and hydrogen isotopic composition, δD. H2O in porous pyroclasts is correlated negatively with δD and positively with ϕ, indicating that the samples were affected by post-eruptive rehydration. Numerical modeling suggests that this rehydration occurred at an average rate of 10−23.5±0.5 m2s−1 during the ∼960 years since the eruption, causing some pyroclasts to gain up to 1 wt% of meteoric water. Pyroclastic and flow obsidians were not affected by rehydration due to their very low porosity. Comparison between modeled δD-H2O relationships in degassing magma and values measured in the Glass Mountain samples supports the idea that rhyolitic magma degasses in closed-system until its porosity reaches a value of about 65±5%, beyond which degassing occurs in open-system until quench. During the explosive phase, rapidly ascending magma fragments soon after it becomes permeable, creating porous lapilli and ash that continue to degas in open-system within an expanding gas phase. As suggested by recent studies, some ash may aggregate and sinter on the conduit sides at different depths above the fragmentation level, partly equilibrating with the continuously fluxing heavier magmatic vapor, explaining the wide range of H2O contents and high variability in δD measured in the pyroclastic obsidians. Using only H2O and δD, it is impossible to rule out the possibility that pyroclastic obsidians may also form by permeable foam collapse, either syn-explosively near the conduit sides below the fragmentation level or during more effusive periods interspersed in the explosive phase. During the final effusive phase of the eruption, slowly ascending magma degasses in open-system until it reaches the surface, creating flows with low H2O and δD. This study shows that H2O measured in highly porous pyroclasts of a few hundred years or more cannot be used to infer syn-eruptive magma degassing pathways, unless careful assessment of post-eruptive rehydration is first carried out. If their mechanism of formation can be better understood, detailed analysis of the variations in texture and volatile content of pyroclastic obsidians throughout the explosive phase may help decipher the reasons why rhyolitic eruptions commonly shift from explosive to effusive phases.

Driving mechanisms of subaerial and subglacial explosive episodes during the 10th century Eldgjá fissure eruption, southern Iceland

The 10th century Eldgjá fissure eruption is the largest in Iceland in historical time. It erupted 21.0 km3 of magma, with 1.3 km3 as tephra in at least 16 explosive episodes from subaerial and subglacial vents, producing magmatic and phreatomagmatic deposits respectively. Grain-size distributions for these end-members show distinct differences at comparable distances from source: the former are coarser and unimodal; the latter are finer and bimodal. These distributions appear to record different primary fragmentation histories. In contrast, the vesicle-size distributions of pyroclasts from each type of deposit show the pyroclasts underwent similar vesicle nucleation and growth prior to fragmentation. This indicates that the role of glacial water was comparatively late-stage, re-fragmenting an already disrupting magma by quench granulation. The presence of microlite-rich domains within clasts reveals a history of complex conduit evolution, during the transition from a continuous dyke to focussed, discrete vents.

Testing for Single Bubble Episode in the Nigerian Stock Market: An Empirical Investigation

This study tested for a single bubble episode in the Nigerian Stock Exchange (NSE) by utilizing monthly data on nominal and real all-share index (ASI) from January 2010 to December 2017. Analysis of data based on Sup Augmented Dickey-Fuller (SADF) test for bubble detection suggested non-existence of a bubble in the NSE between 2010 and 2017. Though there was an indication of one explosive episode in September 2011 at which the Dickey-Fuller statistic lied above the critical values sequence line. However, it was not a bubble but a short deviation from trend. The study also estimated a time-varying long memory parameter, using a fractionally-integrated autoregressive model to check the robustness of the SADF test and it provided further evidence on the absence of a bubble. These findings showed that the behaviour of stock prices was not driven by a bubble in the Nigerian Stock Exchange (NSE). The study, therefore recommended that a time-to-time bubble diagnostic check on the exchange so that symptoms of a bubble can be early detected and managed to avoid losses that may result from the bust.

Dust explosion hazard in the textile industry

The textile industry is not considered among those at high dust explosion risk, as official statistics (CSB 2010, Abbasi and Abbasi 2007, Yuan et al. 2015) account for a low incidence of episodes. Despite these data, there is also evidence that dust explosions may affect the textile industry with severe consequences. The Harbin linen explosion in China ( Hailin, 1988, Bowen 1988, Eckhoff 2003) is probably recognized as having been the most severe episode. Marmo et al. (2010), Piccinini (2008), Salatino et al. (2012) and the U.S. Fire Administration (1995) have discussed dust explosion episodes involving the textile compartment, with nylon flock and wool dust explosions. Amyotte summarized the state-of-the-art knowledge about flocculent materials (which mainly originate from the textile industry), explosion hazards, and the related studies in chapter thirteen of his well-known book (Amyotte 2013). The textile industry has two particular features regarding the dust explosion risk:a.The dust produced as a by-product of the textile process often belongs to the non-traditional dust category, as a result of the type of materials being handled, which are characterized by a marked flocculent attitude. Amyotte and his co-workers discussed flocculent dust behavior and the related risks in detail (Amyotte et al. 2011, 2012a, 2012b);b.The textile industry often needs control of the ambient moisture, which is generally accomplished using ventilation and environment humidification; the latter naturally implies a reduction in the risk of explosions. These particular features suggest that the risk of dust explosions in the textile industry deserves a dedicated discussion. The number of flocculent dust studies is somewhat limited. The role played by the particle size and aspect ratio (namely the diameter-to-length ratio) in explosibility proneness is still unclear. Worsfold et al. (2012) reported the importance of enlarging the knowledge on the hazards related to unconventional dust since most studies have focused on traditional dust. This work investigates the explosion hazards related to dust generated in the textile industry. Moreover, it presents some experimental results obtained by the authors, and detailed in Marmo et al. (2016, 2018), and illustrates the perspective of the study to increase the safety culture of these particular materials.

Detecting West Texas Intermediate (WTI) Prices’ Bubble Periods

Oil prices have had considerable surges and bursts since the first oil crisis of 1973. Until then its price was stable, with almost zero volatility. Since then, apart from the two oil crises of 1973 and 1978/9, oil prices had consecutive bubble episodes like the surges up to 2008 and 2014 and their successive bursts, respectively. The trace of these bubble periods is of crucial importance for policymakers, since their drivers and consequences impact global economic developments. Phillips et al. and Phillips et al. methodologies are applied to detect whether West Texas Intermediate prices experienced bubble periods. Both methodologies suggest that WTI prices experienced explosive episodes, which could be fundamentally, speculatively, or politically attributed. Some suggested periods coincide for both methods, but the second methodology seems to be more sensitive than its predecessor is, leading to better bubble detection but also to identification of non-existent bubbles. The identified bubble periods are compared to relevant research in the literature concerning their presence, duration, and explosiveness. The main goal of the research, apart from the detection of bubbles’ presence and duration, is to identify the causal underlying reasons for each explosive episode. Further, we compare the start and endpoints of each bubble episode with time-points when structural changes occurred. The contribution of the paper is that it clearly defines the bubble episodes with their corresponding drivers. The paper identifies the importance of market fundamentals’ swifts in explaining the bubble periods. The findings of the papers can help policymakers and other stakeholders to monitor oil price shifts and their underlying reasons, and then proceed with prompt actions. Since bubble episodes are fundamentally explained, then the practical utility is that by focusing on the market fundamentals, stakeholders can avoid actions that could result in market failures.

Mild explosivity in recent crude oil prices

This paper provides an analysis of oil prices during and in the aftermath of the Global Financial Crisis, concentrating on the 2007–08 price spike and the 2014–16 price decline. The mildly explosive/multiple bubbles testing strategy by Phillips, Shi and Yu (2015, International Economic Review 56(4), 1043–1133) is used to test for price departures from an underlying stochastic trend and to assess whether any such departures can be explained by fundamentals or other proxy variables. The test dates two significant time periods in both Brent and WTI nominal and real front-month futures prices: a mildly explosive episode during the 2007–08 spike, prior to the peak of the Global Financial Crisis; and a significantly shorter, negative such episode during the 2014–16 price decline, whose commencement is dated around a key OPEC meeting in November 2014. Evidence using other commodity prices points to explanatory factors beyond commodity markets. A global economic activity proxy is found to be decisive in the episode in mid-2008; excess speculation is not. U.S. shale oil production, though contributing to the post-June 2014 price decline, is not seen to have been decisive. Against some recent work tying the CBOE Volatility Index (VIX) to oil futures prices, we find no evidence that the VIX decisively affected oil price levels during the sample period. The results are compared and contrasted with those obtained by Baumeister and Kilian (2016, Journal of the Association of Environmental and Resource Economists 3, 131-158) via a forecasting approach based on a structural vector autoregressive model without financial variables. Taken altogether, the results herein provide new evidence based on formal statistical testing that helps resolve a number of recent controversies in the oil price literature. • Phillips, Shi and Yu test strategy used to provide a comprehensive analysis of oil prices around the Global Financial Crisis • Mildly explosive episode dated near peak of Global Financial Crisis ; shorter episode after key OPEC meeting in November 2014 • New evidence to show global economic activity decisive in the run-up in prices during the 2007-08 spike ; excess speculation was not • No evidence the VIX or excess speculation decisively affected oil price levels during the sample period

Pressure-driven gas flow in viscously deformable porous media: application to lava domes

The behaviour of low-viscosity, pressure-driven compressible pore fluid flows in viscously deformable porous media is studied here with specific application to gas flow in lava domes. The combined flow of gas and lava is shown to be governed by a two-equation set of nonlinear mixed hyperbolic–parabolic type partial differential equations describing the evolution of gas pore pressure and lava porosity. Steady state solution of this system is achieved when the gas pore pressure is magmastatic and the porosity profile accommodates the magmastatic pressure condition by increased compaction of the medium with depth. A one-dimensional (vertical) numerical linear stability analysis (LSA) is presented here. As a consequence of the pore-fluid compressibility and the presence of gravitation compaction, the gradients present in the steady-state solution cause variable coefficients in the linearized equations which generate instability in the LSA despite the diffusion-like and dissipative terms in the original system. The onset of this instability is shown to be strongly controlled by the thickness of the flow and the maximum porosity, itself a function of the mass flow rate of gas. Numerical solutions of the fully nonlinear system are also presented and exhibit nonlinear wave propagation features such as shock formation. As applied to gas flow within lava domes, the details of this dynamics help explain observations of cyclic lava dome extrusion and explosion episodes. Because the instability is stronger in thicker flows, the continued extrusion and thickening of a lava dome constitutes an increasing likelihood of instability onset, pressure wave growth and ultimately explosion.

Volcanic Tremor of Mt. Etna (Italy) Recorded by NEMO-SN1 Seafloor Observatory: A New Perspective on Volcanic Eruptions Monitoring

The NEMO-SN1 seafloor observatory, located 2100 m below sea level and about 40 km from Mt. Etna volcano, normally records a background seismic signal called oceanographic noise. This signal is characterized by high amplitude increases, lasting up to a few days, and by two typical 0.1 and 0.3 Hz frequencies in its spectrum. Particle motion analysis shows a strong E-W directivity, coinciding with the direction of sea waves; gravity waves induced by local winds are considered the main source of oceanographic noise. During the deployment of NEMO-SN1, the vigorous 2002–2003 Mt. Etna eruption occurred. High-amplitude background signals were recorded during the explosive episodes accompanying the eruption. The spectral content of this signal ranges from 0.1 to 4 Hz, with the most powerful signal in the 0.5–2 Hz band, typical of an Etna volcanic tremor. The tremor recorded by NEMO-SN1 shows a strong NW-SE directivity towards the volcano. Since the receiver is underwater, we inferred the presence of a circulation of magmatic fluids extended under the seafloor. This process is able to generate a signal strong enough to be recorded by the NEMO-SN1 seafloor observatory that hides frequencies linked to the oceanographic noise, permitting the offshore monitoring of the volcanic activity of Mt. Etna.

Paroxysmal Explosions, Lava Fountains and Ash Plumes at Etna Volcano: Eruptive Processes and Hazard Implications

Between 2011 and 2015, 49 paroxysmal explosive episodes occurred at two of Mt Etna's five summit craters: the New South-East Crater (NSEC) and the Voragine (VOR). At the NSEC, each event was usually preceded by Strombolian activity, increasing in intensity with time, and climaxed with powerful lava fountains, often accompanied by lava overflows from the crater rim and by the development of ash plumes that spread several kilometres above and hundreds of kilometres away from the crater. In turn, the ash plume caused fallout and often disruption to airports and traffic on the motorways well beyond the area of the volcano itself, impacting the stability of buildings and caused public health issues. The four episodes at VOR were more intense and of greater magnitude compared to the NSEC events, resulting in a higher ash plume, and were produced by a deeper source region. In this paper, we examine the features of the 40 episodes occurring at the NSEC during 2011-2013, and of the 4 events at VOR in December 2015. We study these paroxysmal explosions using geophysical monitoring data, characterise the episodes, and analyse all available data statistically, with the aim of recognising the eruptive processes and extracting common parameters that may prove useful for hazard assessment and risk mitigation.

The Quaternary history of effusive volcanism of the Nevado de Toluca area, Central Mexico

Andesite and dacite lava flows and domes, and intermediate-mafic cones from the Nevado de Toluca area were classified into five groups using field data and 40Ar/39Ar geochronology constraints. Thirty-four lava units of diverse mineralogy and whole-rock major-element geochemistry, distributed between the groups, were identified. These effusive products were produced between ∼1.5 and ∼0.05 Ma, indicating a mid-Pleistocene older-age for Nevado de Toluca volcano, coexisting with explosive products that suggest a complex history for this volcano. A ∼0.96 Ma pyroclastic deposit attests for the co-existence of effusive and explosive episodes in the mid-Pleistocene history. Nevado de Toluca initiated as a composite volcano with multiple vents until ∼1.0 Ma, when the activity began to centralize in an area close to the present-day crater. The modern main edifice reached its maximum height at ca. 50 ka after bulky, spiny domes erupted in the current summit of the crater. Distribution and geochemical behavior in major elements of lavas indicate a co-magmatic relationship between different andesite and dacite domes and flows, although unrelated to the magmatism of the monogenetic volcanism. Mafic-intermediate magma likely replenished the system at Nevado de Toluca since ca. ∼1.0 Ma and contributed to the eruption of new domes, cones, as well as effusive-explosive activity. Altogether, field and laboratory data suggest that a large volume of magma was ejected around 1 Ma in and around the Nevado de Toluca.

Magma dynamics within a basaltic conduit revealed by textural and compositional features of erupted ash: the December 2015 Mt. Etna paroxysms

In December 2015, four violent explosive episodes from Mt. Etna’s oldest summit crater, the Voragine, produced eruptive columns extending up to 15 km a.s.l. and significant fallout of tephra up to a hundred km from the vent. A combined textural and compositional study was carried out on pyroclasts from three of the four tephra deposits sampled on the volcano at 6 to 14 km from the crater. Ash fractions (Φ = 1–2) were investigated because these grain sizes preserve the magma properties unmodified by post- emplacement processes. Results were used to identify processes occurring in the conduit during each single paroxysm and to understand how they evolve throughout the eruptive period. Results indicate that the magmatic column is strongly heterogeneous, mainly with respect to microlite, vescicle content and melt composition. During each episode, the heterogeneities can develop at time scales as short as a few tens of hours, and differences between distinct episodes indicate that the time scale for completely refilling the system and renewing magma is in the same order of magnitude. Our data also confirm that the number and shape of microlites, together with melt composition, have a strong control on rheological properties and fragmentation style.

Tests for an end-of-sample bubble in financial time series

ABSTRACTIn this paper, we examine the issue of detecting explosive behavior in economic and financial time series when an explosive episode is both ongoing at the end of the sample and of finite length. We propose a testing strategy based on a subsampling method in which a suitable test statistic is calculated on a finite number of end-of-sample observations, with a critical value obtained using subsample test statistics calculated on the remaining observations. This approach also has the practical advantage that, by virtue of how the critical values are obtained, it can deliver tests which are robust to, among other things, conditional heteroskedasticity and serial correlation in the driving shocks. We also explore modifications of the raw statistics to account for unconditional heteroskedasticity using studentization and a White-type correction. We evaluate the finite sample size and power properties of our proposed procedures and find that they offer promising levels of power, suggesting the possibility for earlier detection of end-of-sample bubble episodes compared to existing procedures.

Identifying Price Bubble Periods in the Energy Sector

In this paper we test for the existence of single and multiple bubble periods in three energy sector indexes and five energy sector spot prices using the Supremum Augmented Dickey-Fuller (SADF) and the Generalized SADF (GSADF). These techniques allow us to estimate the beginning and the end of the bubble periods. Our results provide strong statistically significant evidence of speculative bubbles in all energy sector price series tested with the exception of the natural gas price series where we observe the weak explosive episodes. Our results are valuable to energy analyst, investors, and oil importing and exporting countries.

Explosive Behaviour in Australian Housing Markets: Rational Bubbles or Not?

Using recently developed econometric procedures (Phillips, Wu and Yu, 2011; Phillips, Shi and Yu, 2015), we find evidence of temporary episodes of explosive behaviour in price-to-rent ratios for established houses, in five of Australia’s largest cities. One interpretation of our results is that stochastic, rational bubbles were a feature of Australia’s major housing markets; particularly during the early to mid-2000s. However, further analysis of each city’s price-to-rent ratio indicates a very different pattern of behaviour in Sydney and Perth to that experienced in Brisbane, Adelaide and Canberra. For the latter three cities, we present evidence suggesting the explosive root tests are likely capturing the effects of a one-time structural break in their respective price-to-rent ratios. In any event, based on the estimated timing of the explosive episodes in Australia’s housing markets, there is little evidence that what might be identified as house price bubbles had any important negative consequences for the wider economy. Despite the ability of the econometric procedures to provide a real-time signal of explosive behaviour, results from Australian housing markets, suggest policymakers need to be cautious in responding too aggressively to a positive signal from the tests.

Strombolian surface activity regimes at Yasur volcano, Vanuatu, as observed by Doppler radar, infrared camera and infrasound

In late 2008 we recorded a continuous multi-parameter data set including Doppler radar, infrared and infrasound data at Yasur volcano, Vanuatu. Our recordings cover a transition in explosive style from ash-rich to ash-free explosions followed again by a phase of high ash discharge. To assess the present paradigm of Strombolian behavior in this study we investigate the geophysical signature of these different explosive episodes and compare our results to observations at Stromboli volcano, Italy. To this end we characterize Yasur's surface activity in terms of material movement, temperature and excess pressure. The joint temporal trend in these data reveals smooth variations of surface activity and regime-like persistence of individual explosion forms over days. Analysis of all data types shows ash-free and ash-rich explosive styles similar to those found at Stromboli volcano. During ash-free activity low echo powers, high explosion velocities and high temperatures result from the movement of isolated hot ballistic clasts. In contrast, ash-rich episodes exhibit high echo powers, low explosion velocities and low temperatures linked to the presence of colder ash-rich plumes. Furthermore ash-free explosions cause high excess pressure signals exhibiting high frequencies opposed to low-amplitude, low-frequency signals accompanying ash-rich activity. To corroborate these findings we compare fifteen representative explosions of each explosive episode. Explosion onset velocities derived from Doppler radar and infrared camera data are in excellent agreement and consistent with overall observations in each regime. Examination of infrasound recordings likewise confirms our observations, although a weak coupling between explosion velocity and excess pressure indicates changes in wave propagation. The overall trend in explosion velocity and excess pressure however demonstrates a general correlation between explosive style and explosion intensity, and points to stability of the uppermost conduit on timescales shorter than at Stromboli volcano.