Geophysical precursors of the July-August 2019 paroxysmal eruptive phase and their implications for Stromboli volcano (Italy) monitoring

Abstract

Two paroxysmal explosions occurred at Stromboli volcano in the Summer 2019, the first of which, on July 3, caused one fatality and some injuries. Within the 56 days between the two paroxysmal explosions, effusive activity from vents located in the summit area of the volcano occurred. No significant changes in routinely monitored parameters were detected before the paroxysmal explosions. However, we have calculated the polarization and the fractal dimension time series of the seismic signals from November 15, 2018 to September 15, 2019 and we have recognized variations that preceded the paroxysmal activity. In addition, we have defined a new parameter, based on RSAM estimation, related to the Very Long Period events, called VLP size, by means of which we have noticed significant variations through the whole month preceding the paroxysm of July 3. In the short term, we have analyzed the signals of a borehole strainmeter installed on the island, obtaining automatic triggers 10 minutes and 7.5 minutes before the July 3 and the August 28 paroxysms, respectively. The results of this study highlight mid-term seismic precursors of paroxysmal activity and provide valuable evidence for the development of an early warning system for paroxysmal explosions based on strainmeter measurements. Stromboli (Aeolian Archipelago, Italy) is an open conduit volcano with persistent explosive activity. It is located in the Mediterranean Sea, not far from the coasts of Sicily and Calabria (Fig. 1). The persistent explosive Strombolian activity consists of several hundred of moderate-intensity events per day. Typical Strombolian explosions eject pyroclastic fragments at the height of some tens of meters, which fall a short distance from the eruptive vent. Explosions occur in numerous eruptive vents located in the summit area of the volcano that can change over time both in number and position. However, the eruptive vents can be grouped into three areas (Fig. 1), northeast (NE), central (C) and southwest (SW), and are distributed along the dominant structural direction (NE-SW) of a graben-like collapsed area at the top of the volcanic edifice 1-3. Major explosions 4,5 eject pyroclastic material over a hundred meters high, which can fall outside the crater terrace in the area visited by tourists. The frequency of these phenomena varies in time, with an average of 2 events per year 5-7. Paroxysms, violent explosions that produce eruptive columns more than 3 km high and are often accompanied by pyroclastic flows, can also occur at Stromboli 8-13. Ballistic blocks associated with these explosions can reach up to 2 m in diameter. Strombolian paroxysms are rare and their occurrence frequency varies over time.

The summer 2019 basaltic Vulcanian eruptions (paroxysms) of Stromboli

“…Some tourists, who were swimming, told us of a strong seaward current which made it impossible for them to reach the seashore for some seconds. The eruption started at 14:45, with only a very short-term instrumental precursor, recorded as an inflation of the edifice which lasted 10 min before the eruption (Giudicepietro et al 2020; see also LGS 2019a).…”

“…. The only precursory phenomena recognized was ground deformation started just 10 min before the eruption (Giudicepietro et al 2020).…”

The cascading origin of the 2018 Kīlauea eruption and implications for future forecasting

“…Stromboli, for instance, has been studied for centuries, and the onset of flank effusion is now recognized by Italy’s volcano monitoring agency as a possible precursor to paroxysmal explosions 81 , 82 —an outstanding example of eruption forecasting based on monitoring data, the historical record, and an understanding of the volcanic system. Nonetheless, two paroxysmal explosions in 2019 (one fatal) occurred in the absence of flank activity 93 . As with Kīlauea’s 2018 eruption, these events highlight the limits of current understanding even at relatively well-studied volcanic systems.…”

On The Use of Dense Seismo-Acoustic Network to Provide Timely Early Warning of Volcanic Eruptions

“…Records exhibiting abnormal noise level likely related to sensor malfunction have been discarded. Time is relative to the origin of the main explosion (14:45:42 and 10:17:16 for the July and August eruptions, respectively) [27] . Aligning the envelops of band-pass filtered data between 0.5-2 Hz using a constant celerity of 300 m/s highlights the signature of stratospherically ducted air-to-ground phases up to a propagation range of ~600 km [21] .…”