Table 2 Synthetic state of the art regarding hazard quantification

Opening of new vents

Subsection Opening of new vents in STEP 2

Intensity Measures:

- Occurrence (Yes/No)

Note: Phreatic eruptions are discussed in Subsection Instability of the hydrothermal system and phreatic explosions in STEP 2

Last observation:

Arso (EST Sector)

Largest known observation:

-

Quiescence: -

Unrest: -

Eruption: certain

Note: All models have estimate independent from the eruptive style.

Type: semi-quantitative

Spatial probability:

Sector E: high

Sector N: medium / high

Sector W: medium low

Sector S: medium low

Sector EPOMEO: low

Sector OFFSHORE: low

Intensity: -

Note: We report only a relative rank of the different sectors for the probability of vent opening given an eruption. This is based on a qualitative merging of different (qualitative and quantitative) models. These estimates should be better merged through a quantitative ensemble.

- phreatic explosions

- MRC or large impulsive landslides (debris avalanches)

- earthquakes (weak)

Cascade:

- eruptions

- deformations and, consequently, landslides (also large)

-tsunamis (submarine explosions, or as a consequence of eruptions)

- earthquakes

- explosions

Simultaneous:

-

Volcano generated missiles

Subsection Volcano generated missiles in STEP 2

Intensity Measures:

- Kinetic energy

- Range

Last observation:

Arso (1302 AD)

Largest known observation:

Cretaio (90 ± 87 AD)

Quiescence: -

Unrest: rare, in case of phreatic eruptions.

Eruption: certain in explosive eruptions; frequent in effusive eruptions.

Type: qualitative

Spatial probability:

Sector E: high

Sector N: medium

Sector W: medium

Sector S: low

Sector EPOMEO: low

Sector OFFSHORE: medium

Intensity: -

Note: qualitative indication only for eruptions; in unrest, even if it is not possible to exclude phreatic eruptions, there are not known deposits of this type, therefore an evaluation is not possible in this case.

Trigger:

- vent opening

- phreatic explosions

Cascade:

- fire

Simultaneous:

- tephra emission

Tephra fallout

Subsection Tephra fallout in STEP 2

Intensity Measures:

- Loading (kg/m²)

- Thickness (m)

- Concentration in air (kg/m³)

Last observation:

Arso (1302 AD, strombolian phase)

Largest known observation:

Cretaio (90 ± 87 AD)

Quiescence: -

Unrest: -

Eruption: certain in explosive eruptions (from from eruptive or co-ignimbritic plumes); possible in effusive eruptions (small quantities).

Type: qualitative

Spatial probability:

Sector E: high

Sector N: medium

Sector W: medium

Sector S: low

Sector EPOMEO: low

Sector OFFSHORE: low

Intensity: from strombolian to subplinian.

Note: for small eruptions, only proximal tephra is expected (the spatial distribution may be based on the vent opening distribution). Tephra fallout from larger eruptions depend on wind (mainly toward East in this area).

Trigger:

- vent opening

Cascade:

- PDCs

- atmosphere phenomena

- lahar

Simultaneous:

- volcano generated missiles

- lava flows (mixed eruptions)

Atmosphere phenomena

Subsection Atmosphere phenomena in STEP 2

Intensity Measures:

- Dynamic pressures

Last observation:

-

Largest known observation:

-

Note: studies or chronicles for atmosphere phenomena from volcanic activity at Ischia do not exist.

Quiescence: -

Unrest: -

Eruption: almost certain in explosive eruptions; rare in effusive eruptions (due to interaction with water).

Type: qualitative

Spatial probability:

Sector E: high

Sector N: medium / high

Sector W: medium low

Sector S: medium low

Sector EPOMEO: low

Sector OFFSHORE: low

Intensity: -

Note: solely based on possible vent opening. For the intensity, not different from atmosphere phenomena from non volcanic sources.

Trigger:

- vent opening

- eruptive or co-ignimbric plumes (tephra fallout)

- lava/water interaction (lava flows)

Cascade:

- fire

Simultaneous:

-

Lava flows and domes

Subsection Lava flows and domes in STEP 2

Intensity Measures:

- Invasion (Yes/No)

Note: volume//maximum distance are used to indicate the size of the source

Last observation:

Arso (1302 AD)

Largest known observation:

- Trippodi (domo; volume)

- Arso (lava flow; length)

Quiescence: -

Unrest: -

Eruption: certain for effusive eruptions.

Type: qualitative

Spatial probability:

Sector E: high

Sector N: medium

Sector W: very low

Sector S: very low

Sector EPOMEO: very low

Sector OFFSHORE: -

Intensity: -

Note: based only on past deposits: quantitative hazard studies are not available.

Trigger:

- vent opening

Cascade:

- PDC (from dome collapse)

- tsunami (small)

- fire

- explosions

- gas emissions

Simultaneous:

- volcano generated missiles

- tephra fallout

Pyroclastic density currents (PDC) SubsectionPyroclastic density currents in STEP 2

Intensity Measures:

- Dynamic pressure

- Temperature

- invasion (Yes/No)

- concentration in air

Last observation:

ash-surge of Fondo di Bosso (1301 AD)

Largest known observation:

ash-surge of Piano Liguori (5.5–5.0 ka)

Quiescence: -

Unrest: -

Eruption: very frequent in explosive eruptions (plume collapse or phreatomagmatic activity); very rare for effusive eruptions (dome collapse).

Note: for effusive eruptions, never observed in Ischia.

Type: qualitative

Spatial probability:

Sector E: high

Sector N: medium

Sector W: medium

Sector S: low

Sector EPOMEO: very low

Sector OFFSHORE: -

Intensity: -

Note: probability maps are available only for 2 “reference” scenarios (Alberico et al. 2008).

Trigger:

- vent opening

- phreatic explosion

- plume / dome collapse

Cascade:

- tephra fallout (co-PDC column)

- atmosphere phenomena

- lahar

- tsunami

- fire

Simultaneous:

- volcano generated missiles

- lava flows

- tephra fallout

Instability of the hydrothermal system and phreatic explosions

Subsection Instability of the hydrothermal system and phreatic explosions in STEP 2

Intensity Measures:

- occurrence (Yes/No)

- impacted area

Last observation:

Geyser like activity (1940)

Largest known observation:

Geyser like activity (1940)

Quiescence: possible

Unrest: possible, with intensification of the phenomena

Eruption: possible, with intensification of the phenomena

Type: qualitative, only for quiescence

Spatial probability:

Sector E: low

Sector N: medium

Sector W: medium / high

Sector S: medium / high

Sector EPOMEO: low

Sector OFFSHORE: -

Intensity: -

Note: based on the condition of the system in quiescence periods, in which the are with the highest propensity is South West. During unrest/eruption cannot be evaluated. A magmatic intrusion would significantly increase the probability of explosive events with large intensity

Trigger:

- increase of magmatic fluids

- sudden decompression and vapour separation (large landslides, earthquakes)

Cascade:

- anomalous CO₂ emissions

- volcano generated missiles

- PDC

- landslide

- vent opening and eruptions

Simultaneous:

-

Volcanic gas and aerosol

Subsection Volcanic gas and aerosol in STEP 2

Intensity Measures:

- concentration in atmosphere

- flux from soil

Last observation:

the phenomenon is continuous. The last measure campaign is in May 2007 for diffuse CO₂, March 2009 for Rn indoor e in water. Local campaigns were organized in August 2017 (earthquakes) and planned for June 2018.

Largest known observation:

The largest ongoing emission is at the fumarole of Donna Rachele (sector West), with Rn in aria =3983 Bq m^− 3; Rn in water = 98 Bq l^− 1.

Note: paroxysm are not known, nor observations during unrest/eruptions.

Quiescence: possible

Unrest: frequent, with intensification of the phenomena

Eruption: frequent, with intensification of the phenomena

Type: qualitative, only for quiescence

Spatial probability:

Sector E: medium

Sector N: medium

Sector W: high

Sector S: high

Sector EPOMEO: low

Sector OFFSHORE: medium/high

Intensity: -

Note: based on the condition of the system in quiescence periods. During unrest/eruption cannot be evaluated. A magmatic intrusion would significantly increase the flux and the degassing areas.

Note: diffuse degassing and fumaroles are known in the offshore sector.

Trigger:

- increase in input volcanic-hydrothermal fluids

- eruptive plumes

- lava flows

Cascade:

-

Simultaneous:

-

Deformations

Subsection Ground deformation in STEP 2

Intensity Measures:

- horizontal and/or vertical displacements

Last observation:

the phenomenon is continuous. The last 30 years are characterized by a general subsidence with a maximum rate of 1 cm/a observed in S and central-W sectors.

Largest known observation: The Monte Epomeo resurgence occurred at a mean velocity of about 3 cm/a, for approximately 30 ka with a total of 1000 m cumulated deformations. Possible peaks are not known.

Quiescence: -

Unrest: frequent / almost certain

Eruption: certain

Note: we refer to uplift. In quiescence, significant uplift would indicate unrest.

Type: qualitative

Spatial probability:

Sector E: medium

Sector N: medium

Sector W: high

Sector S: high

Sector EPOMEO: low

Sector OFFSHORE: medium/high

Intensity: in quiescence, few cm/a (from observations); in unrest, in the order of cm/d (from analogues); in eruption, up to m/d (from analogous).

Trigger:

- overpressure of aquifers

- magmatic intrusions in shallow or deep layers

- vent opening

- earthquakes

Cascade:

- landslides

- macroscopic fractures

- eruptions (rare)

Simultaneous:

- local earthquakes

Local Seismicity

Subsection Local seismicity in STEP 2

Intensity Measures:

- PGA in g

- Macroseismic scale

Last observation:

- 31/08/2016 (negligible intensity)

Largest known observation:

Sector E: VIII MCS (1302)

Sector N: XI MCS (1883)

Sector W: IX MCS (1883)

Sector S: VII MCS (1883)

Sector EPOMEO: IX MCS (1883)

Sector OFFSHORE: VI-VII MCS (1883

Quiescence: possible

Unrest: frequent / almost certain

Eruption: certain

Type: qualitative, quantitative but regional

Spatial probability:

Sector E: low

Sector N: high

Sector W: medium /high

Sector S: medium / low

Sector EPOMEO: medium / high

Sector OFFSHORE: -

Intensity: from the regional study, 0.125–0.175 g (mean of 10% in 50 years); <  0.3 g (84^mo percentile of 1% in 50 years).

Note: The regional study provides fairly low intensity with respect to past observations. The spatial probability applies in quiescence, while do not exist studies for unrest/eruption periods.

Trigger:

- magmatic intrusions in shallow or deep system

Cascade:

- tsunami

- landslides (all types)

- eruptions (rare)

- phreatic explosions

Simultaneous:

- deformations

Slope instability of Type 1A (MRC)

Subsection Gravitational instability on slopes in STEP 2

Intensity Measures:

- invasion (Yes/No)

Note: volume/area/maximum distance are used to indicate the size of the source

Last observation:

the creeping is continuous and it is ongoing at Monte Nuovo, in the West sector of Monte Epomeo.

Largest known observation:

the Rock Avalanche of Falanga, in sector W.

Quiescence: rare

Unrest: possible / likely

Eruption: possible / likely

Note: the creep is certainty, so here we refer to acceleration with evolution to rock avalanche. The probability of rock avalanche cannot be defined with precision, but the mean recurrence time in Sector WEST may be smaller than 1000 year.

Type: qualitative, from observation

Spatial probability:

Sector E: low

Sector N: low

Sector W: medium

Sector S: low

Sector EPOMEO: high

Sector OFFSHORE: low

Intensity: documented rock avalanche from MRC may reached 8 km of run out.

Note: the spatial ranking is qualitative, reported in a relative sense (from low to high), and conditional upon a MRC existence.

Trigger

- uplift (re-start of resurgence)

- eruptions

- regional earthquakes

- alteration or other effects of hydrothermal fluids

Cascade:

- rock avalanche

- tsunami

- phreatic explosions

Simultaneous:

-

Note: we refer to acceleration episodes.

Slope instability of Type1B (debris avalanches)

Subsection Gravitational instability on slopes in STEP 2

Intensity Measures:

- invasion (Yes/No)

Note: volume/area/maximum distance are used to indicate the size of the source

Last observation:

IDA, involving Sectors South, West and offshore

Largest known observation:

IDA, involving Sectors South, West and offshore

Note: It is not clear if IDA is a single large event, or the accumulation of smaller events

Quiescence: very rare (not documented)

Unrest: Possible

Eruption: Likely (even if not specifically documented)

Note: the likelihood of these events cannot be defined, since there is not documentation on past episodes and about their possible recurrence.

Trigger:

- regional earthquakes

Cascade:

- tsunami

Simultaneous:

-

Slope instability of Type 2: shallow impulsive landslides

Subsection Gravitational instability on slopes in STEP 2

Intensity Measures:

- dynamic pressure

- invasion (Yes/No)

Note: volume/area/maximum distance are used to indicate the size of the source

Last observation:

Colata 2015 – Barano; events induced by the 2017 earthquakes.

Largest known observation:

Sector E: Rosato Landslide (2.6–2.3 ka)

Sector N: Debris flow of San Francesco

Sector W: Debris Flow of Citara

Quiescence: possible (documented recurrence from 1 to 50 years) Unrest: Possible

Eruption: Likely / very frequent (even if not specifically documented)

Type: qualitative, from observation

Spatial probability:

Sector E: medium-low

Sector N: medium

Sector W: low

Sector S: low

Sector EPOMEO: high

Sector OFFSHORE: low

Intensity: the maximum distance for debris flow for Ischia are of 5 km.

Note: the spatial ranking is qualitative, reported in a relative sense (from low to high), and conditional upon a impulsive landslides.

Trigger:

- meteorological

- events explosive eruptions with generation of rain

- local seismicity

Cascade:

- floods

Simultaneous:

-

Tsunami

Subsection Tsunami in STEP 2

Intensity Measures:

- water depth at coast (m)

- flow depth (m)

- flow velocity (m/s)

Last observation:

No observations in the Euro-Mediterranean Tsunami Catalogue (Maramai et al. 2014). Historical-archaeological data testify a tsunami event in Sector N (Pithecusa at Lacco Ameno; Buchner 1986).

Largest known observation:

A large tsunami has been associated to IDA in several studies, but deposits associated to this tsunami are not known.

Quiescence: very rare (from landslides or Type 1)

Unrest: rare / possible (earthquakes or landslides or Type 1)

Eruption: rare / possible (earthquakes or landslides or Type 1 or submarine explosions or PDC).

Type: qualitative, based on scenarios

Spatial probability:

Sector E: medium

Sector N: medium

Sector W: high

Sector S: high

Sector EPOMEO: low

Sector OFFSHORE: -

Intensity: up to tens of meters (~  40/50 m) in all the sectors where the tsunami is generated. Significant impact (meters) on the mainland may be possible.

Note: the spatial ranking is qualitative, reported in a relative sense (from low to high),conditional upon a tsunami, and it is referred to landslide originated tsunamis. Studies of tsunamis from eruptions or earthquakes do not exist.

Trigger:

- landslides of any type

- PDC reaching the sea

- vent opening offshore

- local seismicity with significant co-seismic deformation at sea

Cascade:

-

Simultaneous:

-