Inflation Leading to a Slow Slip Event and Volcanic Unrest at Mount Etna in 2016: Insights From CGPS Data

Abstract

Global Positioning System (CGPS) data from Mount Etna between May 2015 and September 2016 show intense inflation and a concurrent Slow Slip Event (SSE) from 11 December 2015 to 17 May 2016. In May 2016, an eruptive phase started from the summit craters, temporarily stopping the ongoing inflation. The CGPS data presented here give us the opportunity to determine (1) the source of the inflating body, (2) the strain rate parameters highlighting shear strain rate accumulating along NE Rift and S Rift, (3) the magnitude of the SSE, and (4) possible interaction between modeled sources and other flank structures through stress calculations. By analytical inversion, we find an inflating source 5.5 km under the summit (4.4 km below sea level) and flank slip in a fragmented shallow structure accommodating displacements equivalent to a magnitude M w 6.1 earthquake. These large displacements reflect a complex mechanism of rotations indicated by the inversion of CGPS data for strain rate parameters. At the scale of the volcano, these processes can be considered precursors of seismic activity in the eastern flank of the volcano but concentrated mainly on the northern boundary of the mobile eastern flank along the Pernicana Fault and in the area of the Timpe Fault System. Plain Language Summary In this manuscript we provide a detailed analysis of a period of inflation at Mount Etna. During this inflation, a slow slip event occurred on Mount Etna's eastern flank. We model both the inflation phase and the slow slip event to investigate the sources and the possible relationships between these sources and the volcanic and seismic activity at Mount Etna.

Gravitational collapse of Mount Etna’s southeastern flank

“…The observed differences in fault slip mode during the observation period, i.e., continuous creep onshore and slow slip offshore, can result from variations in fault properties, such as temperature, fluid pressure, or fault gouge material ( 29 ), while still representing the same overall dynamics. Nevertheless, onshore deformation at Etna’s unstable flank also manifests in slow slip events along the coastline, as monitored by continuous GPS ( 8 ).…”

“…Yet, magmatic activity also influences flank movement as episodic accelerations of onshore flank movement have been related to dyke intrusions and magma ascent repeatedly ( 8 , 25 ). Analyses of onshore seismic and ground deformation data show a clear decoupling of the shallow and deep strain regimes beneath the eastern flank at a depth of 2 km during an inflation period ( 31 ).…”

“…Numerous hypotheses have been proposed to explain flank sliding at Mount Etna, including increases in magma pressure ( 6 ), eruptive activity ( 7 ), repeated dyke intrusions ( 8 ), basement uplift ( 9 ), gravitational spreading ( 10 ), gravitational reorganization ( 11 ), gravity-driven instability accelerated by inflation and/or lateral intrusions ( 12 ), or combined magmatic inflation and continental margin instability ( 13 ). All hypotheses are derivatives of two basic processes capable of triggering flank instability: horizontal pushing of ascending magmatic intrusions or gravitational pull.…”

“…Uncertainties regarding the causes of flank sliding originate from the lack of information on the dynamics of the submarine part of Etna volcano. Onshore geodetic measurements have documented large-scale continuous seaward motion at an average rate of 3 to 5 cm per year since the early 1980s ( 8 , 10 , 17 , 18 ), immediately evidencing the highest rates at the coast ( 19 , 20 ). However, no information on the movement of the submarine part of the flank existed before this study.…”

Combined Seismic and Geodetic Analysis Before, During, and After the 2018 Mount Etna Eruption

“…Daily E‐W components of ESAL (black points), EPOZ (blue points), and ELAC (red points) GNSS stations on the eastern flank of Mount Etna (see location in Figure 6c) from January 2011 to 28 February 2019. The time series show both the secular southeastward trend of the eastern flank of the volcano (Bruno et al, 2012) and the slow slip events (SSE) (shaded yellow rectangles) that occasionally occur on the lower sector of the flank (e.g., Bruno et al, 2017; Mattia et al, 2015). After 24 December 2018 the eastern flank was characterized by increased seaward velocity, clearly shown in the slope of the time series (shaded red rectangle).…”