Abstract. The characterization of triggering dynamics and
remobilized volumes is crucial to the assessment of associated lahar hazards.
We propose an innovative treatment of the cascading effect between tephra
fallout and lahar hazards based on probabilistic modelling that also
accounts for a detailed description of source sediments. As an example, we
have estimated the volumes of tephra fallout deposit that could be
remobilized by rainfall-triggered lahars in association with two eruptive
scenarios that have characterized the activity of the La Fossa cone (Vulcano,
Italy) in the last 1000 years: a long-lasting Vulcanian cycle and a
subplinian eruption. The spatial distribution and volume of deposits that
could potentially trigger lahars were analysed based on a combination of
tephra fallout probabilistic modelling (with TEPHRA2), slope-stability
modelling (with TRIGRS), field observations, and geotechnical tests. Model
input data were obtained from both geotechnical tests and field measurements
(e.g. hydraulic conductivity, friction angle, cohesion, total unit weight of
the soil, and saturated and residual water content). TRIGRS simulations show how
shallow landsliding is an effective process for eroding pyroclastic deposits
on Vulcano. Nonetheless, the remobilized volumes and the deposit thickness
threshold for lahar initiation strongly depend on slope angle, rainfall
intensity, grain size, friction angle, hydraulic conductivity, and the
cohesion of the source deposit.
In September 2021, the La Fossa crater at Vulcano, in Italy, entered a new phase of unrest. We discuss a set of monitoring parameters included in the INGV surveillance network, which closely tracked the sequence of effects related to the crisis. The low-frequency local seismicity sharply increased, while the GPS and tiltmeter networks recorded the inflation of the cone, as an effect of fluid expansion in the hydrothermal system. Gravity variations were probably the effects of fast processes within shallow sources. The anomalies in soil CO2 flux, fumarole temperature, and in plume SO2 flux marked the strong increase in the vapor output from crater fumaroles. The signs of the impending crisis had been evident in the chemical and isotopic composition of fumarole gases since July 2021. These geochemical anomalies were clearly indicative of the enhanced input of gases from a magmatic source. In October, the massive degassing also influenced the areas at the base of the cone. In some areas, soil CO2 degassing and the thermal aquifer recorded strong anomalies. By early November, the crisis reached its acme. Afterward, the monitored parameters started a slow and discontinuous decreasing trend although remaining, some of them, sensibly above the background for several months. The multidisciplinary approach proved decisive for the interpretation of the underlying processes acting in the different phases of the unrest, thus allowing a consistent evaluation of the multiple hazards.
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