NHESSNatural Hazards and Earth System ScienceNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus GmbHGöttingen, Germany10.5194/nhess-15-409-2015Brief Communication: The effect of submerged vents on probabilistic hazard assessment for tephra falloutToniniR.roberto.tonini@ingv.ithttps://orcid.org/0000-0001-7617-7206SandriL.https://orcid.org/0000-0002-3254-2336CostaA.https://orcid.org/0000-0002-4987-6471SelvaJ.https://orcid.org/0000-0001-6263-6934Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma 1, Rome, ItalyIstituto Nazionale di Geofisica e Vulcanologia, Sezione di Bologna, Bologna, ItalyR. Tonini (roberto.tonini@ingv.it)4March20151534094151August201428November201411February201511February2015This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://nhess.copernicus.org/articles/15/409/2015/nhess-15-409-2015.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/15/409/2015/nhess-15-409-2015.pdf
Many volcanic systems are partially or entirely submerged, implying that
vents may open underwater. The effect of submerged vents on probabilistic
volcanic hazard assessment (PVHA) for tephra fallout has always been
neglected, introducing potentially uncontrolled biases. We present a strategy
to quantify the effect of submerged vents on PVHA for tephra fallout, based
on a simplified empirical model in which the efficiency of tephra production
decreases as a function of the water depth above the eruptive vent. The
method is then applied to Campi Flegrei caldera, comparing its results to
those of two reference end-member models and their statistical mixing.
Introduction
Several very hazardous volcanic systems are located very close to seas,
oceans or lakes worldwide and their vents can be partially submerged by
water. As a consequence, the vent of possible future eruptions for such
volcanoes could be both inland or offshore, inferring the need of considering
the possible different eruptive behaviour of the submerged opening vents with
respect to the subaerial ones. Notorious examples of high-risk volcanoes with
potentially submerged vents are, among many others, the Auckland Volcanic
Field (New Zealand), Rabaul caldera (Papua New Guinea), Santorini (Greece),
and the Campi Flegrei caldera (CFc, Italy).
The high risk associated with volcanic activity at some of these partly
submerged volcanoes motivated many efforts to estimate the hazard posed on
the surrounding high-density populated areas, for different possible
hazardous volcanic outcomes e.g.at the Auckland Volcanic
field. Several hazardous phenomena are associated with eruptions
in shallow waters; however, in the present communication, the focus is on
tephra fallout hazard which can impact very large areas far from the vent.
Tephra fallout hazard assessment is commonly achieved by using different
methodologies ranging from mapping the geological record
e.g. to modelling a few representative scenarios
e.g. or, more recently, applying probabilistic methods
e.g.. However, in all of the studies on tephra fallout,
the effect of potentially submerged vents on the computed hazard has never
been explored. This motivated us to propose two new possible strategies to
analyse the tephra fallout probabilistic volcanic hazard assessment (PVHA),
so as to take into account the effect of the water above a submerged vent on
subaerial tephra production. In particular, on one hand, we propose a new
model consisting of a statistical mixing of the two PVHAs
based on two end-member assumptions on the efficiency of submarine vents to
produce subaerial tephra: (i) the effect of the sea as null, i.e. as if every
possible vent behaves as subaerial, and (ii) the effect of the sea as a cap
that totally inhibits the injection of tephra into the atmosphere. On the
other hand, we propose an empirically based simple model in which the
efficiency of submerged eruptions in producing subaerial tephra linearly
decreases as a function of the water depth above the eruptive vent, up to
a maximum depth (Dmax) at which such production is totally suppressed.
The goal of the paper is to explore the sensitivity of PVHA results when
considering the inhibiting effect of the overlying water on subaerial tephra
production, in the case that the vent opens offshore. Such sensitivity is evaluated by
comparing the two PVHAs resulting from the proposed models with the two
end-member PVHAs. We also check the sensitivity of the PVHA results of the
empirically based model against the value of Dmax, and against increasing
values of such parameter as the size of the eruption increases. Magma–water
interaction at very shallow waters has also the potential to increase the
efficiency of explosion and the production of very fine ash and ash
aggregates e.g.; however, here we neglect these
effects in order to keep our empirical model simple and computationally
cheap. This assumption is justified by the fact that the input values used
to feed computational models for tephra fallout hazard assessment at many
partially submerged volcanoes are commonly obtained from field data that
typically account for these effects.
In order to evaluate such sensitivity, in practice we apply the proposed
models to CFc, a caldera system which is approximately half-submerged, being
formed by two nested calderas originated by two major collapses, the first
related to the Campanian Ignimbrite eruption, which occurred about 39 ka ago
e.g., and the Neapolitan Yellow Tuff eruption, which occurred
about 15 ka ago e.g.. In the last 15 ka, CFc volcanic
activity has been very intense producing about 50 eruptions
, the last (forming Monte Nuovo tuff-cone) occurring in
AD 1538 . Recently, the centre of CFc has been affected
by a few major bradyseismic events, the latter two respectively in the early
1970s and 1980s, which generated almost 2 m of maximum ground
deformation each . Recent compositional anomalies of
fumaroles together with major and minor bradyseismic events might suggest
a new volcanic unrest at CFc .
Our PVHAs are based on the Bayesian Event Tree for Volcanic Hazard
BET_VH, see. Like in , we use
a finite number of eruptive scenarios to represent the full variability of
the next possible eruption, by defining the possible vent locations
seven hundreds, and a range of expected eruptive
styles/sizes (dome-forming effusive, small, medium and large explosive
eruptions) for CFc . By means of BET_VH, we
properly weight each eruptive scenario with its own probability of
occurrence. As in , tephra dispersal for each eruptive
scenario is described using the simulation results
obtained by applying the analytical tephra deposition model HAZMAP
, for all the potential vent locations and explosive
eruptive styles/sizes, and considering a statistically significant set of
wind profiles. Compared to , here we also consider the
probability of eruption occurrence at CFc in 50 years as inferred in
, obtaining the unconditioned PVHA for tephra fallout.
As we mentioned above, for the sake of simplicity, here we neglect the
possible enhancement in explosivity due to magma–water interaction
e.g.. However, we remark that the large proportion of fine tephra observed in
CFc eruptive products (due to magma–water interaction) is, in principle,
considered in the empirical total grain size distribution used as input
for the HAZMAP model .
The final results of each PVHA are presented as Bayesian probability maps,
showing the probability of exceeding a threshold of 3 kPa of tephra
load in the target domain and within a time window of 50 years. We
then check the sensitivity of the effect of water in the case of CFc by
comparing the PVHA resulting from the presented model with the reference
PVHAs and with their statistical mixing.
General event tree scheme for BET_VH after
(upper panel). Campi Flegrei caldera (red rectangle) and vent opening
probability map of the 700 vents after (bottom panel).
Isobaths at 30, 60, 90 and 120 m depth in the Gulf of Pozzuoli are shown
(contour lines from white to dark blue).
Water depth and subaerial tephra production: modelling approaches
In this section we describe how the effect of the sea has been quantitatively
taken into account. In order to distinguish a possible different behaviour
between inland and submarine eruptions, we introduce a variability of the
probability in subaerial tephra production as a function of the vent
position. In other words, the probability of tephra production at node 6 (see
Fig. , upper panel) of BET_VH depends on
the water depth above the submerged vent. In particular, we define four
different hypotheses (namely H1, H2, H3 and H4): the first two (H1 and H2)
represent the end-member models, H3 is the statistical mixing of H1 and H2,
and H4 introduces the inhibiting effect of the overlaying water for offshore
vents by using some empirical considerations .
In this case we do not take into account the presence of
water above the offshore vents; in other words, we rely on the assumption
that both inland and offshore vents have the same capability to produce
subaerial tephra. In general, this corresponds to set a uniform best guess
probability at node 6 (PN6=P0) for all the vents. This is the
same assumption adopted, for example, by for CFc and is the
most common approach applied in tephra fallout hazard assessment up to now.
In this case we assume that if waters are deeper than
10 m, the production of subaerial tephra is totally suppressed. The
cut-depth of 10 m is here assumed as a possible order-of-magnitude
size of uplift precursor to explosive eruptions
e.g., and also a typical size of cones that form in
the first phases of eruptions. In other words, if the vent is at such depth
or shallower, in a short time before (in case of precursor uplift) or after
(due to cone formation) the eruption onset, the activity will turn into
subaerial, and the overall effect of water on subaerial tephra production
will be negligible (neglecting the effects of an increase in explosivity due
to magma–water interaction discussed above). Following this assumption, this
hypothesis considers two typologies of eruptive vents only:
inland or in shallow water (i.e. water shallower than
10 m) having a maximum capability of producing subaerial tephra
(PN6=P0);
offshore deep water (i.e. water deeper than 10 m)
having a totally null capability of producing subaerial tephra (PN6= 0).
This hypothesis consists in assuming that the hazard can
be modelled by a statistical mixing of the two opposite end-members described
in H1 and H2 hypotheses. In this view, the results obtained from H1 and H2
are statistically combined into H3 by representing the latter with a sample
composed by the union of two randomly sampled subsets of values (one subset
from H1 and one from H2). The relative numerosity of the two subsets is
a proxy of the relative weight assigned to H1 and H2, and might be assigned
according to the credibility of the two hypotheses for the considered
volcano: for example, depending on the knowledge of the local bathymetry, if
the sea is very shallow throughout the submerged part, one might want to
assign a higher weight to H1, and vice versa.
This empirical hypothesis is based on the set of
observations on subaqueous eruptions described by , who
reported very few cases of subaqueous eruptions from depths greater than
100 m that have breached the water surface, and none for water depth
over 400 m. To account for this empirical observations we simply
assume that, for submarine vents, PN6 linearly decreases with the
water depth D, from a minimum depth Dmin up to a maximum depth
Dmax, at which the water column completely suppresses the
production of subaerial tephra. This empirical relationship can be expressed asPN6=P0D<DminP01-D-DminDmax-DminDmin≤D≤Dmax0D>Dmax.Here we set Dmin= 10 m, for the same reason explained in
H2, and Dmax= 300 m, after a sensitivity analysis
performed at CFc and described in Sect. . It is important to
remark that, even though we use Eq. () in the BET_VH model,
nevertheless the formula is general and can be used in other probabilistic
frameworks to compute the probability of tephra production given an explosive
eruption in submerged environments.
PVHA based on H1, H2, H3 and H4 hypotheses at CFc are shown from top
to bottom respectively (CF1, CF2, CF3 and CF4). The middle column panels show
the best guess (average) value for the probability of observing a tephra load
larger than 3 kPa in 50 years due to CFc magmatic eruptions, according to
the PVHA model adopted. Left and right column panels show respectively the
corresponding 10th and 90th percentiles.
Percent variation (%) between CF1 and CF2 (top left panel), between
CF1 and CF3 (middle left panel) and between CF1 and CF4 (bottom left panel)
relative to CF1 (in terms of average probability to overcome a threshold
equal to 3 KPa in a time window of 50 years). Similarly, percent variations
between CF3 and CF2, and CF4 and CF2 relative to CF2 are given in top and
middle right panels, respectively. Bottom right panel shows the percent
variation between CF4 and CF3 relative to CF4.
Application to CFc case study: PVHA input
As mentioned above, for our PVHAs at CFc we rely on the model BET_VH
, which is based on the event tree described in
Fig. (upper panel). Our target domain is
a 70 km × 70 km area including CFc and the whole area in
front of the Gulf of Naples (Fig. , bottom panel) where a few
millions of people live. In the following we summarize the definitions of the
various nodes of BET_VH and describe how we take into account the effect of
the sea with respect to the tephra production at node 6:
Nodes 1–3 represent the probability of experiencing an
eruption in the time window Δt, that here we set to 50 years
as typical for long-term hazard. As regards the probability density function
(pdf) for nodes 1–3, we assume a Poissonian process with annual rate
12 times the monthly probability of eruption computed by ,
obtaining a best guess probability for an eruption at CFc in 50 years
of about 40 %.
Nodes 4 and 5 represent the conditional probability to
experience a specific eruptive scenario – that is, an eruption from a given
vent position (Node 4) and of a given size (Node 5). For the spatial
probability distribution (Node 4) of vent opening we rely on results by
and shown in Fig. (bottom panel). For the
probability of eruptive sizes (Node 5), as in we consider
four different size classes based on the geological history of CFc
: (i) a lava dome eruption (not producing tephra
fallout), (ii) a small explosive size similar to Averno 2 eruption,
(iii) a medium explosive size similar to Astroni 6 eruption, and (iv) a large
explosive size similar to Agnano–Monte Spina eruption .
Nodes 6–8 represent the impact due to a specific eruptive
scenario. At Node 6 we assess the probability of tephra production given an
eruption of a given size from a given vent. Such probability is parameterized
according to different possible hypotheses, as explained in
Sect. . In particular, we assume P0= 1 for all the
explosive sizes and P0= 0 for the lava dome eruptions. Nodes 7 and 8
represent the conditional probability (given a specific eruptive scenario)
that tephra covers different points (Node 7) in the target domain and
overcomes a given intensity of tephra load (Node 8), here set at
3 kPa as a representative threshold for potential roof damage
e.g.. For each explosive size, such conditional
probabilities are estimated as in , using 1000 HAZMAP
simulations of tephra deposits randomly sampled from the 13 149 ones
performed by . The HAZMAP input parameters of the three
reference explosive eruptive sizes are the same as listed in Table 3 in .
Results and discussion
By modelling with BET_VH the water effect at CFc under the four different
hypotheses H1, H2, H3 and H4, we obtain four different PVHAs for the target
region, respectively labelled in the following as CF1, CF2, CF3 and CF4. CF3
is the statistical mixing of CF1 and CF2, giving equal weight to the two, as
we have no evidence that one of the two hypotheses H1 and H2 could be more
reliable than the other. In Fig. we report the results for the
four PVHAs, displayed as maps showing the best guess (mean) probability of
experiencing a tephra load larger than 3 kPa in 50 years. We
also report, for each of these maps, the 10th and 90th percentile maps, in
order to provide an idea of the epistemic uncertainty associated with the best
guess maps.
As mentioned above, for CF4 we set Dmax= 300 m. However,
in order to check the effect of this assumption we used different values for
Dmax (from 200 to 400 m). Results using such different
values show no significant changes in CF4 results; the same applies if we
consider increasing values of Dmax as the size increases (200,
300 and 400 m for respectively small, medium and large explosive
eruptions). However this insensitivity to Dmax might be due to
the shallow bathymetry in the Gulf of Pozzuoli, where maximum water depth is
about 150 m, and may not be generalizable to volcanic areas
characterized by deeper bathymetry.
The overall feature resulting from a comparison of the four PVHAs is that the
maximum of difference is to the southeast of the submerged part of the
caldera. This is due to a combination of factors very peculiar to CFc: the
submerged part of the caldera has a much lower probability of vent opening
, compared to the subaerial part (see Fig. ,
bottom panel), and the prevalent wind direction is towards the SE
, away from the coastline. This implies that the influence
of the sea on the tephra fallout hazard posed by CFc eruptions is mostly
relevant offshore, while on land it may be relevant only within the caldera and
in the western part of the municipality of Naples. This can be better
visualized in Fig. , where, in the left column, the relative
differences in terms of PVHA are highlighted by showing residual probability
between CF1 and CF2, CF1 and CF3, and CF1 and CF4 respectively, all divided
by CF1 (which is by definition the model implying the largest hazard). More
specifically, the maximum relative differences are offshore, and respectively
around 50, 30 and 15 %, while inland they are about 30, 20 and 10 %.
Such differences are all well captured by our estimate of the epistemic
uncertainty in the most commonly used reference model CF1. However, this may
be different at other (partially or totally) submerged volcanic systems
. Similarly, we show the residual
probability between CF3 and CF2, and CF4 and CF2 respectively, all divided by
CF2 (see top and middle panels in the right column of Fig. ).
With CF4, we try to capture the main feature contained in the data by
, i.e. a decrease in explosivity as vent depth increases.
In this view, the latter results emphasize the evident underestimation of
model CF2 offshore and in the coastal areas of Pozzuoli and Posillipo.
Moreover, we calculate the relative variation between CF4 and CF3, which
shows similar results between the two, with a general underestimation of CF3
respect with CF4 (Fig. , bottom right panel). The maximum
variation on land, although within the uncertainty estimated in each model,
is below 15 % and never exceeds 25 % offshore. As a consequence of this
similarity, we can argue that both CF3 and CF4 can be used to estimate the
effect of the sea on the final PVHA applied to CFc.
As we have stressed above, the variations found in the hazard assessment
computed in this study making different hypotheses is due to the features of
CFc that are not general for other volcanoes, as for example in the case of
the Auckland Volcanic Field . In such cases,
the sensitivity of PVHA to the effect of the sea might be important also at
inland locations. Furthermore, the value of the differences in PVHA obtained
here for CFc might not be negligible when using the hazard assessment to take
rational decisions for risk mitigation based for example on cost/benefit
analysis e.g., as in general they might change
significantly the areas where an action should or should not be taken.
Conclusions
We have explored the effect of potentially offshore eruptions on the PVHA for
tephra fallout, by comparing four different hypotheses for tephra production
from submerged vents. The proposed models H3 and H4 seem to be a reasonable
way to account for submerged vent locations, at least in our application at
CFc. In such application, the differences among the four proposed PVHAs are
within the epistemic uncertainty attached to the most commonly used H1 model,
and are mostly confined to offshore areas. However, this might be
a consequence of two peculiarities of CFc (i.e. the low probability of
offshore vent opening and the SE direction of prevalent winds). In addition,
such differences might not be negligible in terms of risk mitigation
strategies and the effects could be completely different for other volcanoes
worldwide. Both H3 and H4 models can, in principle, be applied to any other
(partially or totally) submerged volcanic system. However, while for CFc they
provide similar results, this might not be generalized to other volcanoes,
since their results depend on local features of the considered volcano
(i.e. bathymetry, spatial probability of vent opening, prevailing wind field
compared to coastline direction, etc). In conclusion, we argue that a
comparison with PVHAs based on H3 and H4 assumption might be a simple and
computationally cheap strategy to quantify the effect of submerged vents on
subaerial tephra production and related hazard.
The proposed contribution neglects possible efficient magma–water
interaction at very shallow waters, that should be considered in future works
on more comprehensive PVHA for tephra fallout and other phenomena, to further
explore the sensitivity of hazards to such effect.
Acknowledgements
The work has been developed in the framework of “ByMuR”
(http://bymur.bo.ingv.it), a project funded by the Italian Ministry of
Education, Universities and Research, “V1: Probabilistic Volcanic Hazard
Assessments”, a project funded by the Italian Civil Protection Department, and
“MEDSUV: MEDiterranean SUpersite Volcanoes”, a project funded by EU (FP7).
A. Costa acknowledges also the EU Research Project (FP7) “VERTIGO”.
Edited by: G. Macedonio
Reviewed by: A. Felpeto and one anonymous referee
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