*Article* **The 2014 Effusive Eruption at Stromboli: New Insights from In Situ and Remote-Sensing Measurements**

**Federico Di Traglia 1,\* Sonia Calvari 2, Luca D'Auria 3,4,5, Teresa Nolesini 6, Alessandro Bonaccorso 2, Alessandro Fornaciai 7, Antonietta Esposito 3, Antonio Cristaldi 2, Massimiliano Favalli <sup>7</sup> and Nicola Casagli <sup>1</sup>**


Received: 15 November 2018; Accepted: 12 December 2018; Published: 14 December 2018

**Abstract:** In situ and remote-sensing measurements have been used to characterize the run-up phase and the phenomena that occurred during the August–November 2014 flank eruption at Stromboli. Data comprise videos recorded by the visible and infrared camera network, ground displacement recorded by the permanent-sited Ku-band, Ground-Based Interferometric Synthetic Aperture Radar (GBInSAR) device, seismic signals (band 0.02–10 Hz), and high-resolution Digital Elevation Models (DEMs) reconstructed based on Light Detection and Ranging (LiDAR) data and tri-stereo PLEIADES-1 imagery. This work highlights the importance of considering data from in situ sensors and remote-sensing platforms in monitoring active volcanoes. Comparison of data from live-cams, tremor amplitude, localization of Very-Long-Period (VLP) source and amplitude of explosion quakes, and ground displacements recorded by GBInSAR in the crater terrace provide information about the eruptive activity, nowcasting the shift in eruptive style of explosive to effusive. At the same time, the landslide activity during the run-up and onset phases could be forecasted and tracked using the integration of data from the GBInSAR and the seismic landslide index. Finally, the use of airborne and space-borne DEMs permitted the detection of topographic changes induced by the eruptive activity, allowing for the estimation of a total volume of 3.07 ± 0.37 × 106 m3 of the 2014 lava flow field emplaced on the steep Sciara del Fuoco slope.

**Keywords:** Stromboli volcano; landslides; effusive activity; Ground-Based InSAR; infrared live cam; seismic monitoring; PLEIADES; Digital Elevation Models; optical sensors; volcano remote sensing

#### **1. Introduction**

Stromboli volcano (Italy; Figure 1), a stratovolcano located at the easternmost end of the Aeolian Archipelago, experienced a flank eruption from August–November 2014 [1–7]. In this paper, in situ and remote-sensing measurements at Stromboli between May and November 2014 are presented. Data comprise videos recorded by the visible and infrared cameras network [8,9] (Figure 1a), ground displacement recorded by the permanent sited Ku-band, Ground-Based Interferometric Synthetic Aperture Radar (GBInSAR) device [10,11], ground displacements in the seismic band (0.02–10 Hz) [12,13] (Figure 1a), and high-resolution Digital Elevation Models (DEMs) reconstructed based on tri-stereo PLEIADES-1 imagery and Light Detection and Ranging (LiDAR) data, respectively. Data allowed us to characterize the precursors and the phenomena that occurred during the eruption, as well as an estimation of the erupted volume.

Stromboli is characterized by persistent Strombolian activity from several vents within a crater terrace area [9,14] (Figure 1b). The volcanic edifice is frequently affected by landslides [15], mainly within the Sciara del Fuoco (SdF; Figure 1b), a collapse depression on the NW flank of the island formed during the last 13 ka [16,17]. Landslides triggered tsunamis on average every 20 years [18], especially during flank eruptions or paroxysmal explosions [12,19,20]. Small to large volcano slope instability characterized the initial phases of the last four flank eruptions (1985–86, 2002–03, 2007, 2014) [21–24], triggering tsunamis only during the 2002–03 event [22,25].

The last flank eruption started at Stromboli on 7 August 2014, preceded by 2 months of increased Strombolian activity and several lava overflows from the craters expanding along the SdF [2,26]. Overflows were often accompanied by landslides along the SdF [7], described as rock-falls and/or gravel slides, evolving down slope to gravel flows (Figure 2a). The onset of the 2014 flank eruption (6–7 August) involved the breaching of the summit cone with emplacement of a landslide along the SdF (Figure 2b), the opening of an eruptive fissure on the NE flank of the cone [24], and the effusion of lava from the crater rim at first and from the eruptive fissure later, feeding the 2014 lava flow field [4,5,7]. The eruption was characterized by the lava effusion in the SdF from a fissure at 650 m above sea level (a.s.l.) that lasted until 13 November 2014 [4].

The 2014 lava flow field was similar to others erupted from high-elevation vents (as described for the 2002–2003 lava flow field by [27]), comprising: (i) a series of tumuli and lava flows around the effusive vent at ∼650 m a.s.l. (proximal shield); (ii) a medial zone fed by small flows, and characterized by frequent lava crumbling down slope and producing a debris field; (iii) a basal toe composed of the debris flow field emplaced above the stacked lava delta [28]. The erupted volume has been estimated to be 7.4 × <sup>10</sup><sup>6</sup> <sup>m</sup><sup>3</sup> by [4] and ∼5.5 × 106 <sup>m</sup><sup>3</sup> by [5].

#### **2. Materials and Methods**

### *2.1. The Camera Monitoring Network (Istituto Nazionale di Geofisica e Vulcanologia, Osservatorio Etneo - INGV-OE)*

The camera monitoring network comprises thermal infrared and visible cameras located at Il Pizzo, at ~918 m elevation and ~250 m from the craters, plus visible and thermal infrared cameras (SQV400 and SQT400, respectively) at 400 m elevation on the N flank of the SdF and ~800 m from the craters (Figure 1a). The SQV400 and SQT400 cameras allow a view from NE of the North-East Crater NEC area (Figure 1a) and of the upper eastern sector of the SdF. An additional thermal camera installed in July 2014, is located at the lower eastern end of the SdF (~190 m a.s.l., Figure 1b) and is focused on the lower portion of the slope between ~400 m and the N coast line. To obtain a description of the eruptive activity, the total number of explosive events that occurred during each day of cloud-free observation was manually counted and plotted as an integer versus time. On average, between 24 July 2014 and 10 August 2014, ~30% of the days were affected by clouds and/or by system failure. In such cases data are lacking.

**Figure 1.** (**a**) Geographic setting of the Island of Stromboli, located at the easternmost end of the Aeolian Archipelago, and names of the summit vents within the crater terrace. NEC: North-East Crater area; CC: Central Crater area; SWC: South-West Crater area (**b**) 3D-view of the investigated sectors observed from north, highlighting the location of the measurement stations used in this work. STR1, STR8, and STRA: broadband seismic stations. SQT400 and SQV400: thermal and visible live cam, respectively. In both (**a**) and (**b**), the line of sight (LOS) displacement map produced by the GBInSAR apparatus between 30 May 2014 and 6 August 2014 is also shown, highlighting the strong inflation of the crater terrace and the stability of the Sciara del Fuoco before the 2014 flank eruption. The measure-areas of the time-series are indicated by white circles. All Digital data were collected in the Projected coordinate system: WGS 1984 UTM zone 33 Projection: Transverse Mercatore. Maps were generated using ESRI ArcGIS CAMPUS (Università degli Studi di Firenze Licence; http://www.siaf.unifi.it/vp-1275-arcgis-licenza-campus.html).

**Figure 2.** (**a**) Gravel flow associated with the 7 July 2014 overflow. The landslide is composed by mixed breccia from the overflow front and the (mainly fine-grained) debris eroded along the Sciara del Fuoco (photo taken by the Università degli Studi di Firenze personnel during field survey). (**b**) Landslides associated with the collapse of the NEC-hornito, occurred on 6 August 2014 (photo taken by UNIFI personnel during boat survey).
