19 eruption of Kadovar Volcano, Papua New Guinea, analyzed by multi-sensor satellite imagery

Journal Pre-proof Growth and collapse of a littoral lava dome during the 2018/19 eruption of Kadovar Volcano, Papua New Guinea, analyzed by Multi-Sensor Satellite Imagery Simon Plank, Thomas R. Walter, Sandro Martinis, Simone Cesca

PII:

S0377-0273(19)30313-0

DOI:

https://doi.org/10.1016/j.jvolgeores.2019.106704

Reference:

VOLGEO 106704

To appear in: Received Date:

31 May 2019

Revised Date:

28 October 2019

Accepted Date:

30 October 2019

Please cite this article as: Plank S, Walter TR, Martinis S, Cesca S, Growth and collapse of a littoral lava dome during the 2018/19 eruption of Kadovar Volcano, Papua New Guinea, analyzed by Multi-Sensor Satellite Imagery, Journal of Volcanology and Geothermal Research (2019), doi: https://doi.org/10.1016/j.jvolgeores.2019.106704

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Growth and collapse of a littoral lava dome during the 2018/19 eruption of Kadovar Volcano, Papua New Guinea, analyzed by Multi-Sensor Satellite Imagery

Simon Plank1* [email protected], Thomas R. Walter2, Sandro Martinis1, Simone Cesca2

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German Aerospace Center (DLR), German Remote Sensing Data Center, D-82234

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Oberpfaffenhofen, Germany;

GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

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Corresponding author: Tel.: +49-8153-28-3460; Fax: +49-8153-28-1445.

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Highlights

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Abstract

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Investigation on the rarely observed rapid growth and collapse of a peripheral lava dome and destabilization episode of an island and dome sector Study of the first historical confirmed eruption at Kadovar Volcano, a small volcanic island north of Papua New Guinea Multi-sensor satellite imagery based analysis

Growing volcanic islands and lava domes become structurally unstable, associated with sectoral collapses, explosive volcanism and related hazards. This article describes the rare case of a growing and collapsing lava dome at Kadovar Volcano, a small inhabited volcanic island located north of Papua New Guinea. The eruption began on January 5, 2018 and was monitored by multi-sensor satellite imagery, including optical, thermal and synthetic aperture 1

radar (SAR) sensors. Results show that Kadovar began a new episode of volcanic activity at the central crater and then also at the eastern coast of the island. SAR amplitude imagery has made it possible to monitor the birth of a new peninsula on the eastern coast. This new peninsula has a blocky appearance and is associated with a localized thermal anomaly, which is indicative of an emerging lava dome. We analyzed the changes on the island and the peripheral lava dome and found that after a great increase in the size of the area, parts of the island and the lava dome then collapsed eastwardly into the ocean on February 9, 2018. The

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sector collapse caused small tsunami waves that hit the neighboring islands. A subsequent slower re-growth of the lava dome is evident from the satellite data, which has reached a final area of ~40,000 m². This study provides details on the rapid growth and collapse of a

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peripheral lava dome and a destabilization episode in an island and dome sector, and

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underlines the great value of remote sensing data on remote volcanic islands.

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Keywords: lava dome; landslide; flank instability; monitoring; remote sensing

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1. Introduction

About 200 active dome-building volcanoes are known worldwide and have generated

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particularly hazardous explosive eruptions (Sheldrake et al. 2016). Lava domes grow above defined conduit systems, most commonly near the summit of andesite to dacite volcanoes.

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Lava domes grow both vertically and horizontally. They may develop oversteepened flanks, resulting in the partial collapse of the dome. This is associated with pyroclastic flows that travel down the slopes (Voight et al. 2000). Therefore, close monitoring of the orientation and possible vertical and lateral growth of lava domes is essential (Walter et al. 2015, 2018; Zorn et al. 2019). Such close monitoring naturally presupposes that the location of the emerging dome is known, and that geodetic (e.g., Dzurisin et al. 2019; Salzer et al. 2014), seismic and 2

camera networks (Salvage and Neuberg 2017; James et al. 2007; Major et al. 2009) are used to acquire data. However, in some places, the growth of a lava dome occurs without being recorded by any instruments, because the volcano has long been dormant or is in a remote location (or both). In such cases, satellite synthetic aperture radar (SAR) remote sensing is of particular importance, especially at sites where cloud cover is frequent (Pallister et al. 2013; Wadge et al. 2011; Wang et al. 2015). Only very few volcanoes have been reported worldwide where a lava dome has developed beneath the sea or on the flank of an ocean

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island (Reynolds et al. 1980; Goto and Tsuchiya 2004) and where it has been possible to monitor this lateral growth and destabilization. Here, we provide the first recorded evidence

of volcano activity at Kadovar Island (Figure 1) associated with lava dome growth, island and

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dome collapse into the ocean and continued magmatic activity.

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Many of the world’s volcanoes are located in remote areas. Especially during explosive eruptions, volcanoes are not readily accessible, so that it is difficult to gain in situ

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information. Satellite remote sensing provides extensive capabilities to support volcano monitoring and contribute to investigations of active volcanoes (Francis and Rothery 2000).

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Volcano monitoring by satellite remote sensing often relies on optical and thermal imagery and on SAR analysis. High-resolution optical satellite imagery is ideally suited for the

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detailed analysis of, for example, pyroclastic density currents and lava flows (e.g., Solikhin et al. 2015; Arnold et al. 2017). With the beginning of the Landsat Thematic Mapper series in

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1982, thermal remote sensing became an established volcano monitoring technique (e.g., Francis and Rothery 1987; Rothery et al. 1992; Wright et al. 2001; Blackett 2017). Important developments in automated thermal hotspot detection approaches are based on the Moderate Resolution Imaging Spectrometer (MODIS) provided by MODVOLC (Wright et al. 2016) and the Middle InfraRed Observation of Volcanic Activity (MIROVA) system (Coppola et al. 2015). Hotspot detection algorithms have also been developed for processing data from the 3

Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), as demonstrated for example by the ASTER Volcano Archive (AVA 2019) and by the ASTER Volcanic Thermal Output Database for Latin American Volcanoes (Reath et al. 2019). Plank et al. (2018) presented a detailed analysis of thermal volcanic activity using high-resolution thermal data from the FireBIRD small experimental satellite mission. However, only SAR systems provide useful amplitude imagery day and night and almost completely independent of the weather. With SAR data, it is possible to observe the surface of

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a volcano location in tropical regions, and also during explosive eruption events when the visibility and applicability of optical sensors are limited by clouds. SAR data have been extensively used for volcano monitoring by exploiting the SAR amplitude information

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(Pallister et al. 2013; Wadge et al. 2011; Walter et al. 2013; Wang et al. 2015; Walter et al.

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2018) and by interferometric processing of both amplitude and phase information (e.g., Massonnet et al. 1995; Salzer et al. 2014; Ebmeier et al. 2018). While interferometric

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processing can be used to monitor terrains free of vegetation and snow which deform only slowly, SAR amplitude data also provide information when major changes occur on the

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surface of the volcano, e.g. due to explosive eruptions (Arnold et al. 2018). A combined analysis of different Earth observation disciplines can make a significant

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contribution to the understanding of volcanic processes (Francis and Rothery 2000). For example, Bignami et al. (2014) analyzed SAR and optical data to study the Puyehue-Cordon

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Caulle eruption in 2011, and Goitom et al. (2015) investigated the Nabro Volcano eruption in 2011 by DInSAR and hotspot analysis. Our study is based on the analysis of multi-sensor satellite imagery, combining SAR with thermal and optical data. The satellite data allow the detailed monitoring and analysis of remote volcanic activity and make it possible to identify peripheral dome growth and

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collapse. In addition, seismic data from global seismic catalogs and from four broadband

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stations were investigated, together with field photographs.

Figure 1 Study area. a) Location north of Papua New Guinea (PNG), seismicity has been

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evident since 1980 (source NEIC). b) Close up of Schouten Islands, Kadovar is located south of Blup Blup and south of the sinistral Bismarck Sea lineament. Submarine landslide deposits according to Silver et al. (2009). c) Eruption photograph taken on January 14, 2018, note the white steam plume originating from two locations (denoted by black arrows near Summit and Flank in the figure). d) Kadovar Island now vegetated (Digital Globe Image 2015), the Village of Gewai is located on the northern slope (outlined by a dashed white line). The 5

amphitheater separates the old edifice (Somma) from the new edifice (Central cone). The 2018 eruption sites are indicated roughly by red stars. e) Slope map of Kadovar showing the amphitheater structure and the central cone off-center to the south with respect to the island perimeter. Digital Elevation Model from ALOS.

2. Regional setting

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Kadovar Island is one of the Schouten Islands (Figure 1), which are six volcanic islands running parallel to the north of Papua New Guinea and forming the western end of the

Bismarck Volcanic Arc (Llanes et al. 2009). All these volcanoes consist of low-silica dacite

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and andesite; no basalt has been found on the islands (Johnson et al. 1972, 1977).

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Kadovar Volcano (3°36’24”S, 144°35’18”E) is 365 m high. It is the summit of the 1.5 km wide and circular Kadovar Island, located on a ~1 km deep sea bed. It is now considered to be

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one of the fifteen historically active volcanoes that belong to Papua New Guinea and are known to have erupted during the last 100 years (cf. Cooke and Johnson 1978). Volcanism at

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Kadovar is producing mainly andesitic scoria and lavas, but also major dome building episodes (Johnson et al. 1972). The geological history of Kadovar is not well known; three

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phases of development have been described, (i) building of a steep andesite cone, (ii) development of a summit crater that breached to the south, and (iii) formation of an andesite

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dome inside the breached crater (Wallace et al. 1981). The nature of this breached crater was questioned in later studies and attributed to a sector collapse of the island (Silver et al. 2009), forming an amphitheater of unknown age open to the south (Figure 1), in which a new cone has since grown (Wallace et al. 1981). We refer to the old edifice as the somma, and the new edifice that grew inside the landslide amphitheater as the central cone. The region where the volcano sector collapse occurred is densely vegetated and can only just be identified by a 6

horseshoe-shaped morphology (Figure 1), but is also confirmed by submarine studies using side scan sonar, which has provided evidence of 20 km² of low backscatter submarine debris avalanche deposits to the south of Kadovar (Silver et. al. 2009). Llanes et al. (2009) and Silver et al. (2009) reported that the submarine flanks of Kadovar are marginally dissected by submarine canyons and only some minor channels and isolated seamounts disrupt the submarine slope of Kadovar. The last historic activity of Kadovar Volcano may have been in 1700, as reported by the sailor

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William Damier (Llanes et al. 2009). Fumarole activity has repeatedly been reported, such as in the early 1900s and also in 1976, even prompting evacuations and fears of an imminent

eruption. It led to diminishing vegetation in 1976-78 (Wallace et al. 1981), and more activity

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was reported in March 1981. A short period of seismic unrest in 2015 was observed at

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Kadovar by the Rabaul Volcanological Observatory - RVO (Global-Volcanism-Program – GVP 2018).

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The first recorded eruption at Kadovar began on January 5, 2018 at 02:20 UTC, as observed in Himawari-8 imagery. At 03:30 UTC an ash cloud moving WNW from Kadovar was

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detected in Visible Infrared Imaging Radiometer Suite (VIIRS) data by the NOAA/CIMSS Volcanic Cloud Monitoring system (GVP 2018). All the residents of Kadovar Island were

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evacuated to the nearby Blup Blup Island (GVP 2018). On January 22, 2018 the International Charter “Space and Major Disasters” (www.disasterscharter.org/) was activated to support the

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National Disaster Center of Papua New Guinea.

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ro of -p re lP na ur Jo Figure 2. Photo documentation before and during initial eruptions on January 6 to 8, 2018, from a boat looking due west to Kadovar Island. a) Pre-eruption illustration showing 8

the dense vegetation. Note the lack of vegetation on the S flank at the site of venting gas which had intensified earlier that year. The location of the central cone, the somma and the amphitheater separating those two is indicated for reference. b) Major eruption associated with base surge, air fall and eruption cloud drifting to the N. c) Only 10 minutes later, the destruction of vegetation became visible. Active vent location indicated. d) Two days later, vent location was found to have shifted northward along the amphitheater. Note the eroded

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ravines, and linear features seen on the E-flank.

3. Data and Methods

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3.1. Data

Field photographs were taken during the beginning of the eruption. We describe a detailed

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analysis based on optical, thermal, and SAR satellite data (Table 1). In addition, we also

established on Kadovar Island.

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consider seismic data available from distant stations; no local monitoring network was

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3.1.1. We study photographs taken in the field (Figure 2). Photographs were taken by mobile phone (iPhone 5s) from a small boat sailing past the island, before the eruption on February 7,

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2017. The beginning of the eruption is documented by photographs from the sea on January 6 and January 8, 2018, and later again on February 7, 2018 and April 14, 2018. These

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observations help to identify and confirm structures observed in our remote sensing data. 3.1.2. Optical and thermal monitoring: We consider 29 high-resolution (HR) optical Sentinel2 (S2) and seven Landsat-8 (L8) datasets. We note that a much larger dataset is available in the catalogs, but this could not be investigated further due to cloud cover. S2 and L8 have a repetition rate of five and 16 days, respectively. The spatial resolution of the S2 bands used in the analysis (cf. section 3.2) is 10 m (bands 3 and 8) and 20 m (band 12). The spatial 9

resolution of the L8 data analyzed is 30 m. Daily hotspot data from the thermal sensors MODIS and VIIRS were analyzed (FIRMS 2019). For thermal hotspot detection, the 1 km resolution MODIS bands 21/22 (center wavelength λ = 3.959 µm) and band 31 (λ = 11.03 µm) as well as the 375 m resolution VIIRS bands I4 (λ = 3.74 µm) and I5 (λ = 11.45 µm) were used. Kadovar is recorded four times a day by MODIS and twice a day by VIIRS. 3.1.3. SAR monitoring: Due to the small size of the island, high-resolution SAR sensors are found to be particularly useful. We exploit seven Fine-mode Advanced Land Observing

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Satellite-2 (ALOS-2) and Phased Array L-band (λ = 22.9 cm) Synthetic Aperture Radar-2

(PALSAR-2) with approx. 6.6 m spatial resolution, as well as two HighResolution SpotLight (HS) TerraSAR-X (TSX) acquired in X-band (λ = 3.1 cm) with 1.25 m spatial resolution and

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one Enhanced-HighResolution (EH) KOMPSAT-5 (K5) SAR acquisition (X-band, λ = 3.2

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cm) with 1 m spatial resolution. The largest dataset is available from the C-band sensor (λ = 5.5 cm) Sentinel-1 (S1), for which we considered 32 images, at 20 m resolution. We

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investigated the SAR amplitude images, which show the signal strength of the radar waves scattered back from the surface and recorded by the SAR sensor. The rougher the surface, the

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higher is the SAR backscatter. Due to the side-looking imaging geometry of SAR systems, slopes facing towards the SAR sensor appear shortened (foreshortening) and brighter in the

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SAR image, while slopes facing away from the SAR sensor appear elongated and darker in the image. In the case of steep slopes facing towards the sensor, layover of SAR backscatter

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signals from different objects on the ground occurs. This complicates the visual interpretability of the image in those regions. Steep slopes facing away from the sensor are affected by radar shadowing. A 30 m resolution ALOS Global Digital Surface Model (GDSM) (JAXA 2018) was used for terrain correction of the SAR imagery. 3.1.4. In addition, for the period January 1 to June 30, 2018, (i) data from global seismic catalogs (Global CMT, USGS, Geofon) were investigated and (ii) data from four broadband 10

stations (MANU and RABL stations of the Australian National Seismograph Network, GENI operated by Geofon, and PMG operated by the Global Seismograph Network, GSNIRIS/USGS) were reprocessed and analyzed in detail.

Table 1 Satellite imagery analyzed Acquisition date 31 Dec. 2017 – 30 Apr. 2019

Sensor MODIS

Acquisition date 10 May 2018

Sensor S2

Acquisition date 2 Oct. 2018

Sensor L8 & S2

14 May 2018

S1

5 Oct. 2018

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31 Dec 2017 – VIIRS 15 May 2018 S2 17 Oct. 2018 S1 30 Apr. 2019 25 May 2018 S2 27 Oct. 2018 S2 24 Sept. 2017 ALOS-2a) 26 May 2018 S1 29 Oct. 2018 S1 26 Dec. 2017 S2 07 Jun. 2018 S1 1 Nov. 2018 S2 2 Jan. 2018 S1b) 14 Jun. 2018 S2 11 Nov. 2018 S2 10 Jan. 2018 ALOS-2 19 Jun. 2018 S1 22 Nov. 2018 S1 14 Jan. 2018 S1 1 Jul. 2018 S1 26 Nov. 2018 S2 23 Jan. 2018 TSXc) 13 Jul. 2018 S1 11 Dec. 2018 S2 24 Jan. 2018 ALOS-2 24 Jul. 2018 S2 16 Dec. 2018 S1 & S2 26 Jan. 2018 S1 25 Jul. 2018 S1 5 Jan. 2019 S2 5 Feb. 2018 K5d) 29 Jul. 2018 S2 9 Jan. 2019 S1 7 Feb. 2018 S1 30 Jul. 2018 L8 21 Jan. 2019 S1 19 Feb. 2018 S1 6 Aug. 2018 S1 22 Jan. 2019 L8 21 Feb. 2018 ALOS-2 8 Aug. 2018 ALOS-2 & S2 30 Jan. 2019 S2 3 Mar. 2018 S1 18 Aug. 2018 S1 2 Feb. 2019 S1 21 Mar. 2018 ALOS-2 & S2 28 Aug. 2018 S2 7 Feb. 2019 L8 24 Mar. 2018 L8 30 Aug. 2018 S1 14 Feb. 2019 S2 27 Mar. 2018 S1 7 Sept. 2018 S2 26 Feb. 2019 S1 8 Apr. 2018 S1 11 Sept. 2018 S1 1 Mar. 2019 S2 10 Apr. 2018 S2 12 Sept. 2018 S2 22 Mar. 2019 S1 15 Apr. 2018 S2 16 Sept. 2018 L8 27 Mar. 2019 L8 20 Apr. 2018 S1 22 Sept. 2018 S2 15 Apr. 2019 S1 & S2 25 Apr. 2018 S2 23 Sept. 2018 S1 30 Apr. 2019 TSX 2 May 2018 ALOS-2 & S1 27 Sept. 2018 S2 a) ALOS-2: HH/HV polarization, orbit ascending 113; b) S1: VV/VH, descending 133; c) TSX: HH, ascending 49; d) K5: HH, descending.

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3.2. Methods

3.2.1. Optical and thermal data handling methods: To obtain a first visual overview, all cloudfree Sentinel-2 and Landsat-8 satellite images listed in Table 1 were analyzed in a GeoInformationSystem (GIS) with respect to the following criteria: (1) Is volcanic activity at Kadovar visible in the HR optical data? (2) If so, where is this activity? We used the satellite bands Short Wave Infrared (SWIR), Near Infrared (NIR) and GREEN, which are the S2 bands 12, 8 and 3 and the L8 bands 7, 5 and 3. A false-color RGB representation of the 11

SWIR, the NIR and the GREEN channel shows high thermal activity areas in red colors, e.g. the central crater and the newly created peninsula. There, the SWIR channel is mostly influenced by the high thermal irradiation of volcanic activity rather than by reflected sunlight as is the case everywhere else in the satellite image (Rothery et al. 1992). By visual interpretation, we distinguish activity at two locations: at the summit of the island or at the eastern coast (see also Figure 1e). In addition, the evolution of the radiant power over Kadovar in the course of time, as detected by MODIS and VIIRS, was analyzed. The radiant

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power was calculated according to Wooster et al. (2003). As both sensors perform several overflights per day, the radiant power of the overflight with the highest radiant power value within each day was considered in the analysis. MODIS and VIIRS are low-resolution

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sensors. Therefore, they do not make it possible to identify the exact location of the thermal

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activity. This information is used for analyzing the changes over time instead. 3.2.2. SAR data handling methods: The SAR observations, mainly ALOS-2 and Sentinel-1

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(S1) SAR images were co-registered to data stacks of the two SAR missions, using the ESA SNAP toolbox. Based on the above-mentioned ALOS GDSM, the ALOS-2 and the S1 data

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stacks were orthorectified and projected onto the corresponding map projection Universal Transverse Mercator 55 South. The orthorectification and map projection processing steps

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were repeated for the K5 and TSX images. Next, change detection within the ALOS-2 and S1 was performed by constructing a two-layer composite map from two consecutive images. In

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addition, the area of the newly created peninsula at the eastern coast of Kadovar was measured on the basis of each single SAR image (Table 1). 3.2.3. Seismic data handling took the form of two steps: (i) The data from the global seismic catalogs described above provide a first insight into the seismic activity of Kadovar Island and its surrounding region. (ii) In order to search for weaker seismic events in the Kadovar region, we reprocessed continuous seismic data from January 1 to June 30, 2018 from the seismic 12

stations mentioned in section 3.1. Seismic stations are located at distances of up to 850 km with good azimuthal coverage and almost complete seismic data, with a few gaps in May 2018 only. However, since the closest station MANU is ~350 km from Kadovar, only relatively large seismic signals can be recorded with a signal-to-noise ratio sufficient to enable a reliable source location to be inferred. We searched for seismic signals potentially generated by the volcanic unrest and mass movement, applying a waveform-based detector (Lopez Comino et al. 2017). We scanned for seismic sources in a region of 200 x 200 km centered at

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Kadovar Island, processing data in the frequency band 0.1 – 1.0 Hz to detect P phases, and 0.04 – 0.1 Hz to detect surface waves. All automated detections were revised manually.

The reprocessing of the data from four broadband stations enabled us to identify a catalog of

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30 seismic events occurring in the Kadovar region (Figure 5); these include seismic events

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present in global seismic catalogs (NEIC, Geofon and Global CMT) but also previously undetected weaker events. Due to the long distance of the seismic stations to Kadovar and

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because of other seismic events (earthquakes), it was not possible to detect clear evidence of seismic readings of Kadovar unrest or clear seismic evidence following the dome collapse

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4. Results

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between February 7 and 9, 2018.

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Magmatic activity at Kadovar could be located at two main sites: a new peripheral lava body grew on the eastern flank near the coast, and effusion and explosions occurred and continued at the summit of the island. 4.1. Growth, collapse and re-growth of a peripheral lava dome Photographs taken from a small boat near the eastern flank of the island show the pre-eruption island and the initial explosion on January 6, 2018, with an eruption site located near the 13

amphitheater scarp on the upper eastern flank of the central volcano. The eruption site migrates northward by January 8, 2018, and the eastern flank shows signs of strong erosion, and the emergence of linear features (Figure 2). 4.1.1. Optical and thermal monitoring: For the early 2018 period, five clear sky Sentinel-2 images and one Landsat image were available. The December 26, 2017 Sentinel-2 image shows the initial shape of the island. Green-colored areas represent living vegetation, while brown colors show areas free of vegetation and/or covered by tephra and volcanic ash. The

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first cloud-free optical HR satellite image acquired after the beginning of the eruption was

obtained on March 21, 2018 and shows that the eastern part of Kadovar Island was already

covered by ash and tephra (Figure 3). All the other acquisitions in early 2018 were obscured

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by clouds. Furthermore, the summit of the island is often obscured by light-colored steam or

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darker ash-laden clouds, but the false color representation shows high thermal activity located in the eastern coastal area and also at the summit of the central cone located in the

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amphitheater (see the Sentinel 2 image of April 10, 2018). From March 21 to May 15, 2018,

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volcanic activity is visible at both sites (Figure 3).

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Figure 3. Initial lava dome growth, collapse and regrowth. a-d) Sentinel-2 images

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showing vegetation cover (green), the tephra deposition (brown) and changes after the first eruption in January 2018 as well as thermal activity (red) at the pre-eruptive gas vent (arrow) (a), at the central crater and at the littoral dome (e.g., b, c, d), from December 26, 2017 to April 25, 2018. e-l) SAR images by ALOS-2 showing clear views of island perimeter changes, dome growth, collapse and re-growth, from September 6, 2017 to May 2, 2018. h-i) Close-up views of SAR images. m-q) Change in the corresponding acquisitions (post–pre): increase (cyan), decrease (red), no change (yellow) in the SAR backscattering. 15

4.1.2. SAR monitoring: Figure 3e-l shows six ALOS-2 acquisitions from September 6, 2017 to May 2, 2018 and the change between successive images. Between the pre-eruptive image (Figure 3e) and the first co-eruptive image (Figure 3f), we see a decrease in the SAR backscattering on the eastern flank, with the strongest decrease appearing in the middle part of the flank. We note that a sharp lineament is visible (Figure 3g) along with a new ravine on the east flank, possibly related to erosion and/or localized fracturing. Between January 10 and

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24, 2018 (Figures 3f and g) the area of SAR backscattering at the eastern flank extended and a new peninsula at the eastern coast of Kadovar became visible in that ALOS-2 time series.

This newly created peninsula was imaged for the first time on January 14, 2018 by Sentinel-1

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nine days after the beginning of the eruption. The peninsula has a blocky appearance

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resembling a blocky lava flow (Rhodes et al., 2018) to blocky lava dome (Zorn et al., 2019) surfaces elsewhere. We therefore interpret the SAR data as implying that a new lava dome

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grew in the periphery of Kadovar Island, breaking above the surface of the sea just ~100 m east of the coast. The exact timing when the subaerial region of the dome became connected

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to Kadovar Island is difficult to determine because of foreshortening effects. Figures 4 and 5 show a threefold increase in the area of the peninsula from around 21,600 m² (January 14,

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2018) to over 66,500 m² (January 24, 2018). The average growth rate was ~4,490 m²/day. SAR data-based measurements showed a lava dome extension of ~150 m out from the coast

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by January 14 and ~250 m by January 24, 2018. Figure 4 shows a 1 m high-resolution TSX image of Kadovar on January 23, 2018. A zoom onto the peninsula at the eastern coast shows the blocky structure typical of lava domes, and crevasses indicative of an eastward flow of its eastern sector (Figure 4). Until February 7, 2018 the lava dome extended continuously at an average growth rate of ~380 m²/day to an area of over 71,800 m².

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On Kadovar Island, the SAR images allow us to locate two main eruption sites, one at the summit region and another one on the eastern flank. The SAR data reveal that the summit eruption sites are forming pronounced and gradually deepening craters, accompanied by increasing shadowing (dark pixel regions) and foreshortening effects (brighter pixel regions facing the satellite). One of the new craters is located just on the upper northern flank of the central cone, and the other more active vent is located to the east near the undated amphitheater headwall.

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On the next available SAR image acquired on February 19, 2018, ~80% of the peripheral lava dome has disappeared. Furthermore, the eastern flank south of the peninsula shows a strong decrease in SAR backscattering. The affected area, which is bordered by straight lines,

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extends from the old central crater down to the coast (Figure 3i). We infer that the newborn

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peripheral lava dome as well as a considerable part of Kadovar Island has slumped into the ocean, affecting a coastal stretch about 250 m in length, moving the coastline 50 m inland,

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and eroding up to a height of about 200 m above sea level. This material loss is confirmed by the Rabaul Volcano Observatory, which reported that the littoral lava dome collapsed on

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February 9, 2018. This phenomenon was associated with five to six minor tsunamis with <1 m height reported on the neighboring island Blup Blup (RVO 2018), and with reported

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fractures in the area of mass wasting on the island. The period following the collapse of the littoral lava dome shows a re-growth of the lava

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dome until March 21, 2018, at a lower growth rate of ~560 m²/day. By March 21, 2018 we measured an area of around 37,400 m² in ALOS-2 data (Figure 3k). Then, a small decrease in the size of the peninsula was observed, with a minimum on April 8, 2018 again (~31,200 m²); this was followed by an increase (at ~ 370 m²/day) to the stable extent of the peninsula of ~40,000 m² as observed from May 2, 2018 onwards (Figure 5).

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Over the next eight months, all the subsequent SAR acquisitions showed no change in the peninsula until January 9, 2019, when a decrease in the area of the peninsula to ~32,700 m² was observed. On the last very high resolution TSX image analyzed in this study, we

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measured a peninsula area of ~28,500 m² (Figures 5 and 7).

Figure 4. a) Radar images illustrate growth of peripheral dome and summit craters, TerraSAR-X HS acquisition (January 23, 2018) of Kadovar. b) Zoom onto the eastern coast showing the growth of the lava dome and its re-growth after the collapse of the majority of the dome and parts of the slope south of the dome. c) Zoom onto the two new summit craters, labeled C1 and C2. 18

4.2 Evolution of the volcano activity over time From a combined investigation of optical, thermal and SAR satellite data captured during the 2018/19 Kadovar eruption, we can derive the evolution of the new peninsula over time, which was created by a peripheral lava dome located on the eastern coast of the island. Figure 5 shows a synopsis of the eruption’s evolution over time: (1) the area of the newly created

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peninsula derived from TSX, ALOS-2, S1 and K5 SAR data; (2) the maximum radiant power during one satellite overflight per day measured over the whole of Kadovar by the VIIRS and MODIS thermal sensors; (3) the visible volcanic activity at the eastern coast or at the central

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crater derived from HR optical S2 and L8 data.

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Figure 5. Synopsis of the evolution over time (December 31, 2017 to April 30, 2019). The figure shows a) the peninsula area, b) the visibility of volcanic activity on the eastern coast or

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the central crater, c) the maximum radiant power during one satellite overflight per day and d)

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the cumulative number of earthquakes detected in the neighborhood of Kadovar (for the period January to June 2018). The detected seismic signals (d) originate close to Kadovar Island and could be related to the volcano activity, but might also be the result of tectonic processes in the surrounding region. Figure 5c shows the maximum radiant power measured by thermal sensors over the whole of Kadovar. Because of the small size of Kadovar Island (1.5 km diameter) and the low spatial 20

resolution of VIIRS and MODIS of 375 m and 1 km respectively, it is not possible to distinguish whether a thermal hotspot detected by one of these sensors is located at the central crater or at the coastal vent. The evolution of the radiant power over time, as observed by MODIS and VIIRS, tallies well with the temporal-spatial evolution of the peninsula. The first thermal hotspot was detected on January 15, 2018. The MODVOLC (Wright 2016) and MIROVA (Coppola et al. 2015) systems also reported a thermal hotspot on this date (GVP 2018). Radiant power values of up

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to 22 MW (February 7, 2018) were observed during periods of strong peninsula growth. In the time period after the collapse of the peninsula (on February 9, 2018), no thermal hotspot was detected. Then, from February 24 onwards, thermal activity was observed again with

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continuously increasing power, reaching a maximum of 11 MW on June 20, 2018. After that,

VIIRS (~1.4 MW) on August 8, 2018.

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the thermal activity decreased with the last thermal hotspots during this period detected by

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The still high thermal activity at the peninsula observed in HR optical data, which was still present three months after the end of its growth, shows that the vent at the eastern coast of

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Kadovar was still active (Figure 6). From May 25 to August 8 only the littoral dome shows a thermal signal. The size and shape of the newly created peninsula seems to be constant in all

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the co-eruptive optical images. However, we note that the thermal anomaly is located more towards the eastern part of the dome. As stated above, the growing of the lava dome at the

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eastern coast vent ceased before the first cloud-free optical HR image became available. From August 8, 2018 onwards, the volcanic activity at Kadovar focused on the central crater. This thermal activity was only visible in the HR optical imagery and was too low to be detected by the low-resolution sensors MODIS and VIIRS. From August 28, 2018 onwards, we see thermal activity only at one of the small new craters on the central cone. From September 29,

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2018 onwards, most of the western part of Kadovar also shows an increase in the ash and tephra coverage (Figure 6). On October 2, 2018, a new eruption phase (Figure 5) began. MODIS and VIIRS measured thermal activity with even higher values than during the previous eruption phase, associated with lava flows on the island. Radiant power values of up to 40 MW (October 12) were measured. HR optical data showed activity at the eastern central vent (Figure 6). On December 11 and 16, 2018 as well as on March 1 and 27, 2019, lava flows were visible that

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originated from this vent and flowed down the eastern flank of Kadovar. Comparing the HS

TSX images acquired on January 23, 2018 (Figure 3) with the one acquired on April 30, 2019 (Figure 7), we see a strong change in the summit region of Kadovar. At the eastern central

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crater the above-mentioned lava flow is clearly visible. Continuous activity was observed at

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Kadovar until the end of the observation period at the end of April 2019.

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Figure 6. Time series of optical high-resolution Sentinel-2 and Landsat-8 imagery from

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December 26, 2017 to November 1, 2018. False color SWIR/NIR/GREEN representation.

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Figure 6 (continued). Time series of optical high-resolution Sentinel-2 and Landsat-8 imagery from November 11, 2018 to April 15, 2019. False color SWIR/NIR/GREEN

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representation.

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Figure 7. Radar image after peripheral dome emplacement. a) TerraSAR-X HS acquisition (April 30, 2019) of Kadovar. b) Zoom onto the eastern coast showing the

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peripheral lava dome. c) Zoom onto the summit craters showing the lava flow at the eastern

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central crater.

5. Discussion

The remote sensing data provided here are in line with observations made by residents and recorded in field photos taken during initial periods of the eruption prior to the peripheral dome growth (Figure 2) and after the dome growth (Figure 8), and also by the Rabaul 25

Volcanological Observatory (RVO). On January 13, 2018, the RVO (2018) observed a steam plume that rose from the eastern coastal vent. The RVO reported three vents located in the summit crater and another two at the eastern coast. The RVO (2018) estimated that on January 18–22 the lava dome extended 150 to 200 m out from the coast, which agrees well with our observations. However, since the first cloud-free optical HR S2 and L8 images could not be obtained until 75 days after the beginning of the eruption, if it had been necessary to rely on the HR optical satellite data, the entire evolution of the growth, collapse and re-growth

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of the lava dome at the newly created peninsula would have been missed, i.e. the amount of lava that was erupted would have been severely underestimated. This demonstrates the

importance of SAR sensors, which make it possible to observe and analyze surface changes

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on volcanoes with high frequency and almost completely independently of the weather

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(Arnold et al. 2018). The S1 mission in particular is a great step forward.

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Figure 8. Photo documentation of peripheral dome growth on April 14, 2018 at Kadovar volcano. a) View due west showing the new ravine on the eastern flank of the island and the regrown lava dome. b) The dome and the island connected. c) View of the lava dome shows its blocky appearance (shown in close up), which is typical of lava domes. d) Lava dome from the east, showing two generations, the remnants of the first growth episode (right, labeled I)

5.1. Theoretical model of the 2018/19 eruption period

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and the darker new lava dome (left-hand side, labeled II).

Based on the observations described in section 4, we have derived a theoretical model of the

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evolution of the 2018/19 Kadovar eruption over time (Figure 9). During the first phase of the eruption, which began on January 5, 2018, activity was observed at the central crater and a

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new lava dome was formed at the eastern coast. We observed a decrease in the SAR

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backscattering during this early period, possibly related to the accumulation of tephra on this previously vegetated flank. This is confirmed by aerial photographs taken by Ricky Wobar on a Samaritan Air flight on January 5 and 6, 2018 (GVP 2018), but also by a missionary sailing

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past the island (Brandon Buser, see Figure 8 and also Kadovar activity bulletin report [04/2018 BGVN 43:04] at https://volcano.si.edu), which primarily show ash coverage on the

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eastern flank of the volcano and fractures and deposits of pyroclastic flows especially in the

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middle part of that flank.

The first signs of renewed activity were an increase in venting at a site on the S/SE flank of Kadovar Island, which was indicated by a loss of vegetation and thermal anomaly in optical and thermal satellite images (Figure 2a, 6a). Photographs taken during the beginning of the eruption on January 6, 2018 indicate summit activity first, with an eruption vent migrating by January 8, 2018 along the former amphitheater headwall, and encarvement on the east flank 27

of the island, associated with extended fractures and ravine formation (Figure 2). The two eruption vents are distinguished in high-resolution TerraSAR-X images (Figure 4c). For the first time, the lava dome on the east coast was visible above sea level in satellite data on January 14, 2018. Dome growth continued and reached a maximum on February 7, 2018 when the newly created littoral dome covered an area of ~71,800 m² (Figure 9b). On February 9, 2018 the littoral lava dome collapsed. At the same time parts of Kadovar’s eastern slope also collapsed. These events caused a series of tsunamis that hit the neighboring

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island of Blup Blup as reported by RVO (2018) (Figure 9c). The second phase of the eruption was characterized by a regrowth of the littoral lava dome, well identified in both our second field survey on April 14, 2018 and in satellite imagery, until the peninsula reached its final

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size at the beginning of May 2018 (Figure 5). Activity at the summit crater (C2) now

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involved the formation of lava flows running downwards towards the littoral dome along the collapsed east flank of Kadovar (Figure 7b, c). In the third phase, from May to the beginning

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of August 2018, the thermal activity at the littoral lava dome slowly ceased. During this period, no thermal activity was observed at the central crater. Then, from August 8 to the end

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of September 2018, only the central crater showed thermal activity (see Figures 5 and 6; these two periods are not shown in Figure 9).

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Finally, on October 2, 2018, a new eruption phase began at the central eastern vent with much stronger activity compared to the previous eruption phases. Lava flows were observed in mid-

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December 2018 and in March 2019 that originated from this vent (Figure 9d). Due to the small size of Kadovar Island (1.5 km diameter) we assume that the vents both at the central crater and at the littoral lava dome have the same magma sources. An interesting observation is the long stability of the littoral lava dome after its re-growth in February 2018. From May 2018 onwards, over a period of eight months, almost no change in the peninsula area was observed. A small decrease in the size of the peninsula was observed on January 9, 28

2019. This is assumed to have been caused by coastal erosion. The area of the littoral lava dome visible above sea level seems to have been stable since January 2019 (see Figure 5). Possibly the lava dome is built of viscous lava (GVP 2018) and is located on a stabilizing

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marine terrace of the island.

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Figure 9. Theoretical model of the 2018/19 Kadovar eruption. a) Activity at the summit crater. Erosion and fracture formation on E/SE flank. b) Growth of the peripheral dome,

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which reached its maximum size as measured on February 7, 2018. c) February 9, 2018: Collapse of the littoral dome and parts of the island’s east slope south of the dome causing a

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tsunami. d) The collapsed littoral dome (labeled I) later grew back (II) until the beginning of May 2018. Activity ceased at the littoral dome and a summit crater emitted lava flows

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traveling along the previously created ravine to the east. This vent was continuously active from October 2, 2018 onwards.

5.2. Estimation of lava dome growth rate

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The bulk of lava dome growth occurred in two episodes, one before and one after the February 9, 2018 collapse. Considering the perimeter area of the lava dome and the timing (Figure 5), the first episode was faster (~2,000 m² area per week) and shorter (three weeks), whereas the second episode was slower (~285 m² per week) and longer (eight weeks). The ~400,000 m³ volume of the subaerial part of the lava dome was estimated as follows for time period around April 14 to 20, 2018: First, the area and also the N-S and E-W extension of the lava dome were measured in the SAR images (Sentinel-1 of 20 April 2018, Table 1). Second,

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additionally using the ratio of the horizontal to the vertical extension of the dome, as derived from the photographs (Figure 8), the vertical extension of the dome was calculated. In order to estimate the lava dome volume, its semi-ellipsoid shape was taken into consideration (see

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Figure 8). The growth rate of a lava dome is often dependent on the macroscopic viscosity, higher growth rates (>6 m³/day) being observed at Bezymianny, Mount St. Helens, or

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Soufriere Hills with silica contents below 60% and viscosities below 105 Pa∙s, whereas those domes with lower rates (<5 m³/day) are observed at Usu or Novarupta with silica contents

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above 65% and viscosities near or above 106 Pa∙s (Yokoyama, 2005). In the authors’ view, the

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viscosities.

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growth rates observed herein for both episodes suggest relatively high silica contents and

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5.3. Peripheral lava domes

Lava domes typically grow at the summit region or even in the summit craters of steep-sided volcanoes (Voight et al. 2000). Growing domes have now been recorded at over 120 volcanoes worldwide (Ogburn et al. 2015), commonly developing high up above the central conduit of an active volcano. Due to the steep topography, a gravitational collapse of the hot dome material may develop into block and ash flows travelling downhill at high speeds, which is one of the main hazards with volcanoes (Witham 2005). The 1902 pyroclastic 30

density currents at Mt. Pelee, Martinique, reached the coast of the island and caused 28,000 deaths within minutes (Tanguy 1994). The 2010 collapse of the Merapi lava dome, Indonesia, travelled 15 km southward and prompted the evacuation of a third of a million of people (Cronin et al. 2013; Surono et al. 2012). Those domes which are located on slopes are notoriously unstable and are especially susceptible to gravitational collapse (Voight et al. 2000), which is why the understanding of peripheral lava domes, i.e. those growing at some distance from the main conduit on the volcano slopes, is especially vital.

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A number of domes have been reported to grow on slopes, such as the Soufriere Hills dome, Montserrat, growing from 1995 to 1998 (Watts et al. 2002) or also to some extent the Mount St. Helens dome, which grew in different directions from 2004 to 2006 (Vallance et al. 2008).

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The growth of a dome on a slope leads to flow-typical structures and crevasses, and grades

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into a coulée if there is a transition between symmetrical lava domes and flow-like streams (Blake 1990). This is why the Kadovar peripheral dome, as seen in the high-resolution TSX

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image shown in Figure 4, partly resembles a coulée on its eastern sector. Another question is: Why did the dome grow in the periphery of the Kadovar central cone?

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Asymmetrical or directional growth of lava domes may also result from heterogeneous loading and diverging of the magma feeder conduit (Zorn et al. 2019) or by clogging of ascent

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pathways (Husain et al. 2014). The peripheral lava dome growth on Kadovar Island is occurring after more than 300 years of inactivity, so that it is possible that the central conduit

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was initially clogged, or that new magma pathways have opened, possible associated with the fracture formation and collapse on the island’s east flank caused by the peripheral dome growth.

5.4. Edifice collapse and interactions 31

Volcanic islands often grow on weak substrata, or are inherently unstable, leading to gravitydriven destruction (Delaney et al. 1992). For this reason, volcanic islands often display signs of flank instability, or bear remnants of earlier landslides (Stix et al. 1991). These are amphitheater-like morphological depressions, open to one side, and (submarine) depositions of avalanche debris, often many kilometers away from the island, implying highly energetic mass movements that are potentially tsunamigenic (Krastel et al. 2001). Volcano-induced tsunamis are not very well studied and are usually not considered in early warning measures.

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A prominent and very recent example is the Anak Krakatau flank collapse on 22 December 2018 (Walter et al. 2019), which was caused by the collapse of about one third of the

subaerial volume of the island into the sea, leading to 430 fatalities. The Anak Krakatau

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tsunami was anticipated years before, but no dedicated monitoring system was put in place (Giachetti et al. 2012). Volcano landslides into the ocean and associated tsunamis are

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commonly underestimated, since they only represent 5% of all tsunami events worldwide

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(Paris et al. 2014). However, they are thought to be responsible for one fourth of all fatalities related to volcanic activity, underlining the importance of a closer investigation. In Southeast Asia alone, 17 volcanoes have been identified as tsunami sources, triggering deadly waves

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same period.

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since 1600 AD (Paris et al. 2014), and globally about 130 tsunami events are reported in the

Furthermore, the increasing volcanic activity at Kadovar initially led to concerns and

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warnings about a possible tsunami either due to explosions or landslides, as warned by the RVO. 736 people in total were affected by evacuation (according to International Federation of Red Cross and Red Crescent Societies). Details of the mechanisms of the tsunami that occurred remain insufficiently monitored, however. As regards the location of the peripheral lava dome let us consider its growth on the coastal flank of the edifice. Our analysis of the satellite imagery suggests that the collapse involved 32

both the lava dome and also parts of Kadovar Island at a location that was already subject to erosion along a linear radial fissure. The dome largely disappeared and the coastline of the island changed. The eastern flank of the island was subject to erosion and possible collapse during the eruption, allowing us to speculate whether this directivity was the pathway of least effort for magmas in an edifice sector that was weaker than other parts. In this view, particular importance may be attached to the most active eruption vent, located on the eastern flank of the central cone (Figure 4), very close to the inferred ancient collapse headwall, and very

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close to the flank that collapsed eastward in 2018. Possibly loading, erosion or intrusion into this flank has contributed to flank instability, similar to what has been inferred for other sites elsewhere (Elsworth 1996). Whether the collapse of the dome was caused by loading of the

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unconsolidated volcano flank, or whether it was triggered by one of the powerful explosions,

answered from these observations.

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or whether magma intrusion into a hypothesized decollement triggered the collapse cannot be

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The simultaneous activity at the summit and at the eastern flank is of particular interest, as the style of eruption does not appear to be the same. While explosive activity with strong

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steaming and tephra production was initially observed at the summit, the coastal activity is dominated by effusive lava dome growth. However, high resolution TSX SAR observations

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from early 2018 and early 2019 reveal that short traveling coulée lavas were emitted at both sites, revealing that they in fact are similar. The shallow bifurcation of a conduit into a coastal

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and a summit branch therefore appears realistic. As field observations and photo analysis (Figures 2 and 8) show major deforestation and erosion, and lineaments that may be indicative of normal faulting extending in a north-south direction, we may hypothesize that these two eruption sites may in fact be connected by a planar dike-like feature striking eastwest and opening north-south. As the lineaments (Figure 2d) converge near or just below sea level, the upper tip of such a hypothetical dike may also be located close to that level. 33

No possible external triggers were identified. The region is a seismically and volcanically active location, as evidenced by the eruption of the neighboring Manam Island in 2018 (GVP 2019), or the occurrence of a major magnitude 7.5 tectonic earthquake on February 25, 2018 (UTC, February 26, 2018 local time) just south of Papua New Guinea (

). The M7.5 earthquake reportedly

triggered many landslides (https://www.who.int/westernpacific/emergencies/papua-newguinea-earthquake), but no significant change in activity at Kadovar volcano could be derived

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from the available data investigated by us and would demand further investigations.

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6. Conclusions

This article presents the rare case of a growing, collapsing and re-growing littoral lava dome

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at Kadovar Volcano, a small volcanic island located north of Papua New Guinea. The

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eruption was not recorded instrumentally. We therefore worked on remote sensing data. In addition, data from global seismic catalogs (Global CMT, USGS, Geofon) and, in more detail,

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from the four broadband stations located next to Kadovar were investigated. Satellite imagery, including optical (Sentinel-2 and Landsat-8), thermal (MODIS and VIIRS) and SAR (Sentinel-1, ALOS-2, TerraSAR-X and KOMPSAT-5) sensors were analyzed to monitor the

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first confirmed historical eruption at Kadovar Volcano, which began on January 5, 2018. We

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showed that Kadovar initiated a new episode of volcanic activity at the central crater and then also at the eastern coast of the island. SAR amplitude imagery made it possible to monitor the birth of a new peninsula at the eastern coast, due to lava dome growth. From January 14 to 24, 2018 the peninsula grew at a rate of ~4,490 m²/day and reached its maximum size of over 71,800 m² on February 7, 2018. The estimated volume of the subaerial part of the lava dome was ~726,000 m³. Two days later, ~80% of the littoral lava dome as 34

well as a part of the island’s slope about 250 m long and 50 m wide south of the peripheral dome collapsed and caused a tsunami that hit the neighboring islands. This was the first ever observed dome collapse in shallow waters. Then the peninsula grew back to its final area of around 40,000 m² as observed on May 2, 2018, with an estimated subaerial volume of the lava dome of ~400,000 m³. Observations from high-resolution optical data confirmed thermal activity at the central crater and the coastal vents until May 15, 2018. Next, until August only the coastal vent showed

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activity. Then, until the end of September 2018, activity was visible only at the central crater. Finally, on October 2, 2018, a new eruption phase with much higher thermal activity began at

the observation period at the end of April 2019.

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the eastern central crater. The activity at this vent was observed continuously until the end of

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We have shown that detailed analysis of volcanic eruptions is only possible with very frequent monitoring. SAR sensors play a key role, since they are independent of the weather.

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As presented in Section 4, if only optical data had been available, the amount of lava produced during the 2018/19 Kadovar eruption would have been severely underestimated, as

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the growth, collapse and re-growth of the peninsula created by a lava dome would have been missed. On the other hand, the change in thermal activity between the littoral lava dome and

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the central crater could only be monitored by using HR optical satellite data. Because of the long distance of Kadovar from the closest station (~350 km), no clear seismic evidence

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following the dome collapse between February 7 and 9, 2018 could be found.

Data Availability TerraSAR-X data are available through a science proposal to the TerraSAR-X Science Service System (https://sss.terrasar-x.dlr.de/). Sentinel data are available through the 35

European Space Agency Copernicus open access hub (https://scihub.copernicus.eu/). ALOS2 data are available through a science proposal to JAXA (https://auig2.jaxa.jp/ips/home) and KOMPSAT-5 data through a science proposal to KARI (http://ksatdb.kari.re.kr/arirang). Landsat-8 data are available at the USGS Earth Explorer (https://earthexplorer.usgs.gov/). MODIS and VIIRS thermal hotspot data are available at NASA FIRMS

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(https://firms.modaps.eosdis.nasa.gov).

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Conflicts of Interest: The authors declare no conflict of interest.

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Acknowledgements

The field photographs were all kindly provided to us by Brandon Buser, a missionary for

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Ethnos360 (https://ethnos360.org/), which are greatly appreciated, as are his detailed descriptions. The TerraSAR-X, the KOMPSAT-5 and the ALOS-2 data were kindly provided

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by DLR, KARI and JAXA (Proposal number MTH1153, PI number 3043), respectively. This is a contribution to VOLCAPSE, a research project funded by the European Research Council

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646858].

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under the European Union's H2020 Programme / ERC consolidator grant No. [ERC-CoG

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