# The 2014 Effusive Eruption at Stromboli: New Insights from In Situ and Remote-Sensing Measurements
## 4 Discussion  
The run-up phase of the 2014 flank eruption was characterized by very vigorous explosive activity and episodic lava overflows from the crater terrace. This activity was preceded by an increase in the displacement rate towards the sensor of the crater terrace at anomalous level since the end of May 2014, and by frequent landslides along the SdF. Ground deformation of the crater terrace strongly increased on 26 July 2014 and was followed by increase of the volcanic tremor and explosion-quake amplitude, starting approximately on 2 August 2014. The explosive activity peaked on 5-6 August 2014, anticipating by -1 day the increase in the displacement rate towards the sensor (inflation of the crater terrace). Different authors [1; 3; 6] suggested that the 2014 eruption was triggered by a gradual recharge of the shallow plumbing system and upper conduits. The SO\({}_{2}\) flux in the persistent Stromboli plume has been measured for two months before the onset of the eruption, detecting a strong increase above normal activity in the SO\({}_{2}\) pulses (puffing and explosions) before the effusion onset. This is consistent with the increase in gas bubble supply and magma transport rate feeding the uppermost storage system at Stromboli [47]. Ground displacement is consistent with the presence of a very shallow  
Figure 9: Stromboli topographic changes detected comparing 2012 and 2017 DEMs. Dashed black line encloses the area covered by the 2014 lava flow. Dashed yellow lines, labelled with P1, P2 and P3, show the position of the profiles along the flow represented in the insets. reservoir below the crater terrace [2]. Long-term (month to weeks) increase in the ground deformation recorded by the GBInSAR in the crater terrace area, as well as the long-term rise of the VLP seismic source towards the surface [5; 6], are associated with the accumulation of magma in the uppermost storage system at Stromboli [2]. Contrarily, the detected increase in short-term (days to hours) ground deformation and amplitude of volcanic tremor are associated with the impulsive bulk degassing from the shallow plumbing system [35; 48], whereas the increase in the explosion number and intensity and amplitude of explosion quakes are indicative of the increase in the release rate and overpressure intensity of individual explosions. One of the most interesting aspects of the data presented here is the relationship between the instability events and the effusive activity at Stromboli volcano (Figure 10). Other authors have already shown the critical state of Stromboli during the very early stages of effusive eruptions [2; 5; 11; 22; 49]. During the run-up to the 2014 flank eruption, gravel flows consisting of mixed breccia and loose volcaniclastic deposits were generated mainly in two ways: (i) by the crumbling of the overflows, or (ii) by the collapse of some portions of the external part of the crater terrace and apex of the hornitos (Figure 2). The second type of collapse is a recurring phenomenon at Stromboli, occurring when there is a large accumulation of magma below the crater terrace (causing overflows, increase in the frequency of explosions, spattering activity, inflation of the whole summit). Furthermore, weakness of the crater wall (e.g., induced by the strong explosive activity on 5 August 2014), and mechanical erosion caused by vent opening and by lava fingering, may have triggered the NEC-hornito collapse on 6 August 2014 [48]. Here we have documented for the first time that, after the NEC-hornito collapse, eruptive vents migrated from south to north (Figure 10a). Overflows and mass-wasting in the NEC-hornito area preceded the formation of fracture below the NEC1 (Figure 10b) by \(\sim\)11 h, and the onset of the flank eruption and NEC1 collapse by \(\sim\)15 h, suggesting that 11-15 h before the onset of the effusive eruption, the crater terrace was full of magma.