First Japan-Singapore Volcanology PhD Symposium

14 – 15 October 2019 | Singapore

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First Japan-Singapore Volcanology PhD Symposium 14 – 15 Oct 2019

WORKSHOP PROGRAMME

14 October 2019, Monday

0900 – 0930 Welcoming of participants, N2-01a (EOS’ lobby)

0930 – 1030 Fabio Manta's PhD defense open to all. 1030 – 1200 Visit of the laboratories 1200 – 1300 Lunch (buffet provided) 1300 – 1500 Petrology and Geology

Hajime Taniuchi New genesis of felsic magma: the unmixing of slab-derived supercritical liquid

Yuya Ikenaga Transition of the “An’ei eruption” of Izu-Oshima volcano

Ayumu Nishihara Genesis of magmas causing the 30ka Aira catastrophic caldera-forming eruption inferred from geochemical characteristics of plagioclase phenocrysts in the essential clasts

Sri Budhi Utami Correlating petrology with unrest records for short-lived, explosive eruptions at Kelud volcano, Indonesia

Damia Benet The components of volcanic ash: what do they tell us?

1500 – 1600 Poster 1600 – 1700 Volcanic Gas Charlotte Barrington Using volcanic gases to understand open vent volcanoes Takato Ono Development of a field auto-sampling tool for volcanic waters 1700 – 1800 Posters Leaving for dinner together

15 October 2019, Tuesday

0900 – 1100 Seismo/Acoustic

Kazuya Yamakawa Experiments of Small Aperture Infrasonic Array Kyoka Ishii

Estimation of the attenuation of infrasound signals observed around volcano based on diffraction theory

Dan Muramatsu

Analysis of infrasound accompanying the 2018 small phreatic eruption from Iwo-Yama volcano, Kirishima Volcanic Complex, Japan

Kimiko Taguchi New approach to estimate source properties of volcanic long-period events based on formulas for crack resonance frequencies and quality factors

Luo Yizhou

Evaluate the Influence of Instrumental Bias on Recognising Volcanic Unrest Signals

1100 – 1200 Poster 1200 - 1300 Lunch (buffet provided) 1300 – 1400 Experiment/Volcanic Hazard/Paleo-Mag Noriko Teshima

Experimental study on precursory pressure oscillation in the laboratory geyser system

Lior Kamhaji Magnetic Evidence for Paleo-OCC structure at Troodos Ophiolite, Cyprus Vanesa Burgos Delgado How often do volcanoes with no previous record erupt? 1400 – 1600 Posters 1600 – 1700 Visit labs and analogue and technical

First Japan-Singapore Volcanology PhD Symposium 14 – 15 Oct 2019

PARTICIPANTS Ayumu Nishihara Kobe University Genesis of magmas causing the 30ka Aira catastrophic caldera-forming eruption inferred from geochemical characteristics of plagioclase phenocrysts in the essential clasts (Petrology/Geology) The 30 ka Aira catastrophic caldera-forming eruption (CCFE) discharged voluminous silicic magma with a small amount of less silicic magma. Essential clasts of this eruption consist of three types; white pumice (WP; 74-78 SiO2 wt.%), dark pumice (DP; 67-75 wt.%), and scoria clasts (SC; 59 wt.%). In order to understand the genesis of magmas for the Aira CCFE, geochemical characteristics of plagioclase phenocrysts (Pl) in the clasts were examined. An-contents (An) of Pl cores in the WP and DP show bimodal distributions with peaks of An45 and An85. 87Sr/86Sr of the high-An Pl cores (0.7068±12, 2SE) are higher than that of the low-An Pl cores (0.7058±4, 2SE). The SC contains solely high-An Pls, 87Sr/86Sr of which increase from 0.7054 to 0.7078 with decreasing An from An93 to An71. The highest-An core in the SC has 87Sr/86Sr identical to the low-An cores in the WP. Bimodalities of An and 87Sr/86Sr of the Pl cores in the WP suggest that the voluminous silicic magma was produced by mixing between low-87Sr/86Sr felsic (LF) and high- 87Sr/86Sr mafic (HM) magmas with the low- and high-An Pls, respectively. The HM magma is andesitic and contains SC. Variation of 87Sr/86Sr of the HM magma may be caused by variable degrees of assimilation with higher 87Sr/86Sr upper crust materials such as sediments and granites having 87Sr/86Sr >0.7089. This process may generate the dacitic magma of the DP. 87Sr/86Sr of the highest-An Pl in the HM magma is identical to that of the LF magma, suggesting that the HM and LF magmas were generated from an identical source material. These magmas have lower 87Sr/86Sr than the basement rocks and higher than mantle-derived basalts (87Sr/86Sr=0.7042-0.7047), suggesting that the source material for the voluminous magmas of the Aira CCFE are derived from a lower crust rock. Charlotte Barrington Earth Observatory of Singapore

Using volcanic gases to understand open vent volcanoes (Volcanic gas) Open vent volcanoes are characterized by a persist plume and release kilotons of gas per day. These persistently degassing volcanoes are widely distributed globally and include the most active volcanoes on Earth (e.g. Etna, Stromboli, Popocatepetl, Satsuma-Iwojima, Masaya). Forecasting the eruptive activity of open conduit volcanoes remains challenging because they typically show limited signs of unrest, such as volcanic-tectonic earthquakes and deformation prior to magmatic eruptions. This was the case at Mayon volcano where a change in activity was only recognized after a new summit lava dome was observed on 12 August 2014. Mayon is a steep-sided stratovolcano, the most active volcano in the Philippines and capable of producing dangerous pyroclastic flows, particularly from the collapse of summit lava domes. In an attempt to forecast eruptions at Mayon, emissions of Sulphur dioxide (SO2) are quantified by Differential Optical Absorption Spectroscopy (DOAS) using a series of scanning mini-DOAS instruments. Mean rates of SO2 degassing are in the order of 500 t/d but display considerable variation. Here I propose to take an alternative route and exploit the full wavelength range of the mini-DOAS spectrometers and retrieve quantities of bromine monoxide (BrO) together with SO2. Whist BrO is not emitted directly from volcanoes, it may be used as a tracer for hydrogen bromide (HBr), a known volcanic species. Although the geochemical behavior of Br species in melts and gases is still poorly known it is believed to be similar to other halogens, such as Chlorine (Cl) and therefore indicate dry, shallow degassing. Constraining BrO/SO2 ratios at the time of the lava dome emplacement at Mayon, should provide a new opportunity to identify precursory changes in the relative contributions of these gases and also, to better understand the role of bromine in volcanic systems. Damia Benet Earth Observatory of Singapore The components of volcanic ash: what do they tell us? (Petrology/Geology) Volcanic ash is made of particles of diameter finer than 2 millimeters. These particles come from a wide range of origins, from older rocks of the volcano building, to the hydrothermal system, up to those that form directly from the magma, the so-called juvenile particles. Juvenile particles may be indicative of a magma reservoir at shallow depth, and hence have hazard implications. However, identification of these particles is not straightforward due to the lack of unique, textural features. As part of my PhD, I am designing a procedure to classify ash particles as objectively as possible by means of binocular observations. I will be presenting the observational criterion employed for classification of ash particles vented during the eruptive cycle (2016-2018) of Nevados de Chillán Complex. This criterion relies on parameters such as color, luster, vesicularity, among others, and will be illustrated with particle images. A number of particle images will be described, classified and labelled. Then, new particle images will appear, and the attendees will classify and infer the course of the eruption as a brief hands-on exercise. I will finish with the application of Artificial Intelligence to ash classification and our current approach.

First Japan-Singapore Volcanology PhD Symposium 14 – 15 Oct 2019

Dan Muramatsu SEVO, Kyushu University Analysis of infrasound accompanying the 2018 small phreatic eruption from Iwo-Yama volcano, Kirishima Volcanic Complex, Japan (Acoustic) Iwo-yama volcano, part of the Kirishima Volcanic Complex which is located in southern part of Kyushu Island, produced a small phreatic eruption on 19 April, 2018. We analyzed infrasonic signals accompanied the eruption. Spectral analysis of the signal shows that the dominant frequency clearly lowered at about 4 hours after the onset of the eruption. After that, low-frequency content (around ~1 Hz) of the signal significantly intensified. At the time of the dominant frequency change, it is observed many isolated infrasonic pulses, named ‘A-type’ pulse in this study. The A-type pulse shows a high frequency (10-20 Hz) oscillation and resembles an infrasonic pulse observed at Stromboli volcano. On the other hand, low-frequency (1-2 Hz) N-shape pulses named ‘B-type’ occurred continuously every ~2 seconds after the frequency change. According to the spectral features of the pulses, we interpret that the A-type pulse is a bursting sound of a bubble rising in viscous mud, and the B-type is produced by a bubble vibration in a mud-water pool formed at the vent. These results may provide insights to the transition of the surface phenomena of the phreatic eruption from Iwo-yama. Hajime Taniuchi Department of Natural History Science, Graduate School of Science, Hokkaido University New genesis of felsic magma: the unmixing of slab-derived supercritical liquid (Petrology/Geology) Felsic magmas produced at subduction zones have played an important role in the generation and evolution of the continental crust. For the origin of felsic magmas, processes such as fractional crystallization of mafic magmas with or without crustal assimilation (DePaolo, 1981), partial melting of crustal materials (Huppert and Sparks, 1988), and partial melting of subducting slabs (Defant and Drummond, 1990) have been proposed. Here, we study calc-alkaline andesitic and dacitic lavas from Rishiri volcano, to elucidate the origin of felsic magmas. Rishiri volcano is located at the Kuril arc, and is 300 km above the Wadati-Benioff Zone. The calc-alkaline andesitic products and the following dacitic lavas, investigated in this study, comprise Middle stage of the volcanic activity (Ishizuka, 1999). Our new 40Ar/39Ar age data for the dacitic and andesitic lavas are 35.5 ± 1.4 ka and 34.6 ± 3.0 ka, respectively. The andesitic lavas have disequilibrium petrographic features, which can be explained primarily by mixing of felsic magma and primitive mafic magma which generated by fluid-fluxed melting of mantle peridotite. On the other hand, the dacitic lavas are characterized by high Sr/Y and La/Yb ratios, no Eu anomaly, and no evidence of magma mixing. The absence of Eu anomaly negates a primary role of fractional crystallization in the magma genesis. In addition, the adakitic signature cannot be produced by crustal melting, because the underlying crust is too thin (Niu et al., 2016) for garnet to be stable (Rapp and Watson, 1995). Therefore, it is suggested that the dacitic magma was derived from the subducting slab. One possible origin of the dacitic magma would be direct partial melting of the subducted slab. However, extremely high temperatures with >1300ºC (Mibe et al., 2011) is required to melt the slab at ~300 km depth, which is highly unlikely. The petrological and geochronological observations suggest that the slab-derived dacitic melt coexisted with slab-derived aqueous fluid in the upper mantle beneath Rishiri at ~35 ka. Therefore, the dacitic magma might represent hydrous melt, which was separated from the ascending slab-derived supercritical liquid above the depth corresponding to the critical end-point (e.g. Kawamoto et al., 2012), leaving behind fluids that induced the generation of the primitive basaltic magma. Kazuya Yamakawa Earthquake Research Institute, Tokyo University, Japan Experiments of Small Aperture Infrasonic Array (Acoustic) Signal detection and directions of arrival (DOAs) estimation in infrasound observations are efficiently performed by array techniques [e.g. Ripepe et al., 2002]. For volcano monitoring, infrasonic array observations typically with >100-m aperture have been operated to identify volcanic signals. Expanding the aperture and increasing the elements of an infrasonic array improves its performance, however it loses its mobility and easiness of the installation and maintenance. Recently, an infrasonic array having an aperture of a few tens of meters has started to be used in the field [Johnson & Palma, 2015]. With such a small aperture, the practicality of the system would be significantly increased. Because demand for small aperture infrasonic arrays is increasing [e.g. Marchetti et al., 2013], it is important to clarify its performance. We tested the performance of an infrasonic array consisting of three or four microphones with ~20-m aperture mainly at Stromboli volcano, Italy. There were four or five active vents separated by ∼10 degrees. We employed multiple signal classification (MUSIC) to estimate DOA of the detected signals. It was shown that our array could distinguish the active vents, but that DOA estimations were the frequency dependent: DOA of the eruption signal in the same time window but in different frequency window differed systematically up to ~8 degrees from the vent. Possible DOA estimation errors were evaluated by numerical tests with gaussian noises, but it was difficult to explain the frequency dependence of DOA estimations. In future work, we will investigate what causes this frequency dependence of DOA estimation and evaluate the performance of our array properly.

First Japan-Singapore Volcanology PhD Symposium 14 – 15 Oct 2019

Kimiko Taguchi Graduate School of Environmental Studies, Nagoya University, Nagoya, Japan New approach to estimate source properties of volcanic long-period events based on formulas for crack resonance frequencies and quality factors (Seismology) Long-period (LP) seismic events are thought to arise from the oscillations of a fluid-filled resonator. The fluid properties and resonator geometry of LP sources have been estimated by comparing observed frequencies and quality (Q) factors to those calculated by numerical simulations of the crack model. However, simultaneous determinations of these crack model parameters using numerical simulations took extensive computational time especially for LP events with large Q factors. To resolve this problem, we developed an alternative method based on an analytical formula for the crack resonance frequencies and an empirical relation between Q factors and the crack model parameters. We applied it to LP events at Kusatsu-Shirane volcano, Japan, and Galeras volcano, Colombia, which have been thought to be generated by resonances of cracks filled with misty and dusty gases, respectively. Then all the crack model parameters were successfully estimated by this simple method and we found the following common features in these estimated parameters at the two volcanoes: (1) Crack volumes and gas-weight fractions of water vapor in the cracks showed simultaneous temporal changes, and (2) mist and dust volumes were proportional to the product of crack aperture and crack length or width. From these features, we inferred that the LP events were triggered by volumetric changes associated with condensation of water vapor exsolved from magma at Kusatsu-Shirane and by fragmentation of vesiculated magma intruded into the crack at Galeras. These results demonstrate that our method is useful to investigate the state of fluids and triggering processes of LP events beneath volcanoes. Kyoka Ishii Graduate School of Science, Kyoto University Estimation of the attenuation of infrasound signals observed around volcano based on diffraction theory (Acoustic) Observation of infrasound signals generated from volcanic eruptions provides us with important information directly reflecting explosion processes at the source. Amplitudes of signals observed at stations are attenuated by topography on the propagation paths (Lacanna & Ripepe, 2013). In order to evaluate the attenuation, some studies adopt a 3-D numerical simulation of wave propagation considering the effects of topography (e.g., Kim & Lees, 2014). However, many studies ignore the topographical effect because calculation cost of such simulation is quite high. In this study, we attempted to easily estimate the amplitude of infrasound signals by applying the sound diffraction theory. This research targets explosions at Sakurajima, Japan as an example case. We regarded the topographical barrier on the propagation path as an equivalent single screen. Then, we calculated the amplitudes at stations from a classical mathematically solution of sound diffraction (Macdonald, 1915) and compared them with observed amplitudes of 31 Vulcanian explosions in September 2017. For comparison, the amplitude value was converted to the amplitude ratio to the value at the KUR station that is located in the line-of-sight with the source. As a result, the estimated amplitude ratios correspond to the observed ones at most stations. Therefore, the estimation based on the diffraction theory is practical for a first-order investigation for infrasound amplitudes. Based on this result, we estimated the attenuation at each node of 200-m mesh grid around Sakurajima and obtained the distribution of the attenuation of amplitudes. The distribution has a significant contrast between the eastern and western sides of Sakurajima. This contrast can only cause from the variation in the relative height of the screen to signal wavelength. Thus, it suggests that the central summit area becomes an effective barrier to signal wavelength. Lior Kamhaji Hebrew University of Jerusalem ISRAEL Title: Magnetic Evidence for Paleo-OCC structure at Troodos Ophiolite, Cyprus (Paleo-magnetism) Troodos Forest Fault (TFF) juxtaposes a dome-shaped mantle serpentinite with lower crust gabbro exposures at the Troodos massif. The fault forms a part of the lower cretaceous hydrothermal system of the Solea paleo-spreading center. A paleomagnetic survey is designed to test the hypothesis of the TFF role in OCC development at Troodos. Dense Sampling included 38 sites at the gabbro suite along two main transects:East-West across the suggested OCC dome; North-East - South-West along Solea spreading axis towards the Arakapas paleo-transform. Comparing ChRM vector at each site with the Troodos mean vector (TMV) reveals significant rotations at the vicinity of the fault, and small rotations under the detachment surface to the west (at the suggested breakaway). The rotation pattern broadly suits the expected OCC footwall deformation at different depths of the lower crust. At the hanging wall, rotations were documented mainly at the TFF zone. This research completes the last piece at a complex deformation map of the lower crust in oceanic ridge-transform intersection, a map charted by previous research; the in-situ magnetic information, reported here for the first time, will constrain tectonic models based on geophysical data from mid-oceanic ridges. Luo Yizhou Earth Observatory of Singapore, Asian School of Environment, Nanyang Technological University Evaluate the Influence of Instrumental Bias on Recognising Volcanic Unrest Signals (Seismology) During traditional volcano monitoring, some eruptions show less signs of precursor while some have stronger indications. Whether the discrepancy was due to the type of activity, the type of instrument or to the type of processing remains to be tested. In our study, we start to shed light on those questions by investigating the design of the monitoring system and the type of analysis that is carried out. By standardising the type of analysis performed on continuous monitoring data retrieved from multiple sensors located at various distances from the vent, we looked for consistent patterns that allow us to quantitatively evaluate the capability of the analysis in detecting the event.

First Japan-Singapore Volcanology PhD Symposium 14 – 15 Oct 2019

The type of analysis that we began with is Self-Organising Maps (SOM), an artificial neural network that implements unsupervised machine learning to organise data by feature similarity. SOM followed by Clustering, this processing technique groups similar data (e.g. frequency content of seismic data) together in clusters and gives clear visualisation on subtle changes. We also used Kolmogorov-Smirnov test (KS test), a statistical way to evaluate the difference between cumulative distribution functions of the frequency content from different time windows, to quantify any frequency content changes in seismic data. In the events we examined, we found that seismic data from some sensors presented clearer precursory signals (change in frequency content clusters) than the others. It could be associated with their strategic locations such as the distance from the vent; or the type of instrument used, given it is short-period or broadband sensor. In order to perform a quantitative evaluation on the influence of instrumental bias, more events will be considered and examined. The scope of this work could also be extended to other analytical techniques for further investigation on their bias. Noriko Teshima Tohoku University Experimental study on precursory pressure oscillation in the laboratory geyser system (Experiment) Geysers show characteristic behaviors such as cyclicity and precursory seismic events. Tremors observed in natural geysers are similar to volcanic earthquakes (Kieffer, 1984) and have causal relationship with pressure pulses occurring inside of the conduit (Kedar et al., 1996). However, the source and mechanism of the pressure pulses are still unclear. Natural geysers show complex phenomena, thus simplified laboratory experiments allowing us to directly observe what are happening in the interior may be useful. In this study, using our experimental data and a fluid oscillation model, we tried to reveal the mechanism of the pressure oscillation and factors controlling its dominant frequency. We developed an experimental setup mainly consisting of a flask and a conduit, which reproduces cyclic eruptions of natural geysers. We measured pressure and temperature in the flask, and erupted mass of each eruption. We took videos of flask interior and water surface in the conduit, synchronizing them with pressure observation in the flask. For examining the effects of geometric conditions on characteristics of pressure oscillation, we conducted experiments changing three geometric parameters: 1) flask volume, 2) initial water level in the conduit, and 3) inner diameter of the conduit. In interpreting experimental results, we used a fluid oscillation model to calculate theoretical value of the resonance frequency and compared it with the measured value of the pressure oscillation frequency. The pressure oscillation measured in the flask consisted of 1) lower-frequency fluctuation and 2) higher-frequency pulses superimposing it. The lower-frequency pressure fluctuation had strong inversed correlation with the vertical water surface movement in the conduit. The higher-frequency pressure pulses coincided with bubble formations in the flask. In an eruption cycle, the dominant frequency of pressure oscillation decreased systematically toward the next eruption, which was explained by the reduction of the effective bulk modulus of fluid in the flask. The initial value of oscillation frequency differed from each eruption cycle, which was explained by the differences in 1) the geometric parameters of the experimental system and 2) the erupted mass of the preceding eruption. Sri Budhi Utami Asian School of Environment, Nanyang Technological University Correlating petrology with unrest records for short-lived, explosive eruptions at Kelud volcano, Indonesia (Petrology/Geology) Many volcanoes show month- to year-long pre-eruptive unrest periods; this is not the case of Kelud volcano (Indonesia), which proceeds very quickly from unrest towards eruption. A typical explosive event can escalate from unrest to VEI 4 eruption within hours to days. Such explosive eruptions at Kelud have generated a range of hazards, including lahars, ash fall, and pyroclastic flows. Hence forecasting eruptions from Kelud is crucial for public safety, but remains challenging owing to its quick evolution from unrest to eruption, and the limited monitoring records with which to interpret pre-eruptive processes and timescales. Here, we seek to ameliorate this situation by adding the petrologic record of Kelud magmas through constraining pre-eruptive magmatic processes and their timescales. We focus on the main-stage pumices from the 1990 VEI 4 eruption and propose a conceptual model of Kelud’s plumbing system, which includes the pre-eruptive storage conditions, processes, and their timescales that we attempt to correlate with available unrest records. The measured mineral compositions and applied thermodynamic modeling indicate that the erupted basaltic andesite magma was stored in a volatile-rich environment at about ~1000°C, with >4wt.% water and >500ppm CO2 in the melt. Assuming volatile saturation, these conditions correspond to 1-2 kbar and a depth of 3-4 km below the volcano. However, we suspect there is also a deeper magma storage area. Chemical zoning patterns of pyroxenes and plagioclase are reverse and normal, respectively but the chemical variations can be limited to a few mol%. This suggests a dynamic system, yet no clear evidence of interaction or mixing between different magmas. Thus, the high explosivity and short timescales of unrest might be more related to progressive volatile accumulation and building of overpressure rather that the classic interpretation of repeated magma intrusions. Diffusion timescales from orthopyroxene give < 10 days, suggesting diffusion likely occurred during magma ascent. Implications of this study could inform future monitoring efforts at Kelud and other volcanoes exhibiting similar eruption styles.

First Japan-Singapore Volcanology PhD Symposium 14 – 15 Oct 2019

Takato Ono Grad. School of Sci., The University of Tokyo Development of a field auto-sampling tool for volcanic waters (Volcanic gas) Temporal changes in volcanic water chemistry are conventionally observed by repetitive samplings and subsequent laboratory analyses, and are good indicators for changes in volcanic activities. Time intervals of the repetitive samplings have been usually about a week to a year due to campaign-style observations and were not sufficient for quantitative discussion for related volcanic activity changes. In this study, we developed a field auto-sampling tool for collecting volcanic water discharges at crater area to improve the frequency of water sampling. The base of the developed auto-sampling tool is a commercially available XY-plotter, a drawing robot, which can move a pen to an arbitrary coordinate to draw digital artworks. In this study, the pen of the XY-plotter was replaced by a plastic dripping pipe connected to a small peristaltic pump to automatically drip water into arranged sampling bottles in turn. A field test using the developed sampling tool was carried out for a hot spring discharge at crater area of Hakone volcano, Japan, at 3-hour sampling interval for 40 consecutive days from February 7 to March 19, 2019. The sampling tool successfully worked without stopping for entire observation period and collected total of 332 water samples automatically and stably. With the high time resolution samples obtained by the sampling tool, we observed short-term significant positive SO4 peaks having duration of about a day, which have not been detected by repetitive man-power samplings in previous studies at this volcano. Our results clearly show an importance of high temporal sampling even during quiescent volcanic activity period to clarify the characteristic of the observing hot spring. The sampling tool can probably contribute for better understanding of volcanic activities by high-frequency water sampling. Vanesa Burgos Delgado Earth Observatory of Singapore How often do volcanoes with no previous record erupt? (Volcanic hazards) The compilation of a comprehensive eruptive history for hazard assessments and eruption forecasting can be challenging. The incompleteness of the catalogue, the eruption recurrence or the lack of detailed studies are some factors that prevent us from having a complete eruptive history, which leads us to complement the catalogue with analogue volcanoes. Although the number of recorded eruptions has increased exponentially in the last centuries, there are many volcanoes for which the available information from past eruptive behaviour is very scarce. Around 30% of the GVP catalogue are volcanoes with only one eruption reported during the Holocene, and we still witness volcanoes that reported the first eruption in modern times, as the VEI 4 Nabro (Eritrea) eruption in 2011, a caldera considered extinct. The goal of this work is to estimate how often we can be surprised by eruptions at volcanoes with no previous record of eruption, what we define as ‘new volcanoes’. The frequency and recurrence are estimated as function of time, geographical regions, size of the eruption (VEI) and other volcanological characteristics. We have found that these parameters are strongly influenced by the length of the record selected. Considering that the last 500 years of the catalogue is complete, we obtained that a ‘new volcano’ erupt every 10 years for a VEI 3, every 56 years for a VEI 4 and every 250 years for a VEI 5 or 6. The average recurrence interval is also highly variable within the subsets considered for the tectonic setting, rock composition and morphology. Comparing the occurrence of eruptions at ‘new volcanoes’ with the total number of eruptions, we obtained that for the last 25 years, 2.2% of VEI≤3 eruptions and 4.5% of VEI≥4 eruptions are from ‘new volcanoes’. These results support the importance of designing a robust and reproducible method to define analogues in order to better understand and forecast the hazards associated with little-known volcanoes. Yuya Ikenaga Earthquake Research Institute, The University of Tokyo Transition of the “An’ei eruption” of Izu-Oshima volcano (Petrology/Geology) Izu-Oshima is one of the well-known active volcanoes in Japan. This volcano has caused large-scale eruptions every 100-150 years (Nakamura 1964). There is concern that next large eruption will soon come because it has been over 200 years since the last large eruption occurred. The last large eruption is the An’ei eruption that continued from 1777 to 1792. This eruption was described in historical documents, and these documents were analyzed by previous research. According to these documents, the An’ei eruption started with the “scoria and lava period” of about a year and ended after the “ash period” of 9 years (Tsukui et al. 2009). In this study, detailed transition of the eruption in the former period was estimated by geological and petrological method. The scoria layer can be divided into 3 units (unit A～C) by the characteristics, especially scoria size and the amount of plagioclase phenocrysts, and distributions of scoriae. From scoria size and thickness of each unit, it can be said that both intensity and volume of unit C were highest in 3 units. In addition, it was estimated that unit C had been produced in November 1778 as compared to previous research on historical documents. About the full-rock compositions, the Al2O3 content of scoria increased as time passed. It was also revealed that the melt composition, the mineral composition and the amount of minerals other than plagioclase didn’t change through the eruption. It can be said that the full-rock compositions of the 3 units are different only because of the amount of plagioclase crystals. From these results, it is assumed that some mechanism that caused only increase in the amount of plagioclase (such as accumulation of crystals or mixing of magmas having same melt compositions) existed in the magma system.