In the present study, we analyze ground-motion hazard
maps and hazard disaggregation in order to define areas in Italy where
liquefaction triggering due to seismic activity can not be excluded. To this
end, we refer to the triggering criteria (not to be confused with
liquefaction susceptibility criteria, which essentially take into account
soil type and depth to groundwater) proposed by the Italian Guidelines for
Seismic Microzonation, which are described in the main body of the
paper. However, the study can be replicated in other countries that
adopt different criteria. The final goal is the definition of a screening
map for all of Italy that classifies sites in terms of liquefaction
triggering potential according to their seismic hazard level. The map, which
is referred to with the Italian acronym MILQ – Mappa del potenziale d'Innesco
della LiQuefazione (i.e., map of liquefaction triggering potential), and the
associated data are freely accessible at the following web address:
https://distav.unige.it/rsni/milq.php (last access: 28 April 2023). Our results can be useful to guide
land-use planners in deciding whether liquefaction is a hazard that needs to
be considered within the planning processes or not. Furthermore, they can
serve as a guide for recommending geological and geotechnical investigations
aimed at the evaluation of liquefaction hazards or, conversely, rule out
further studies with consequent savings in efforts and money.
Introduction
Liquefaction is one of the most complex earthquake-induced phenomena in
geotechnical earthquake engineering. Commonly the term liquefaction, though
used to indicate a variety of different but related phenomena (e.g., flow
slide failures, lateral spreading, sand boils), is associated with soil
permanent deformations produced by a rapid excess pore pressure increase
(and a concurrent reduction in effective stresses) in saturated cohesionless
soils subjected to earthquake loading (e.g., Kramer, 1996). Under these
conditions, the pore water pressure in the soil mass increases in response
to the seismic shaking, and the soil loses its shear strength and behaves
temporarily as a viscous liquid.
The assessment of liquefaction hazards at a site always involves two
questions: (1) are the geological conditions prone to liquefaction and (2) will
liquefaction be triggered by future earthquakes? The latter is usually asked
in case of a positive answer to the first question, namely the following: if the site is
susceptible to soil liquefaction, will liquefaction be triggered by future
seismicity? While answering the first question may involve costly in situ
investigations mainly aimed at defining the depth of the water table and
soil granulometry, the answer to the second question can be simply obtained
by querying national hazard maps via online web services, at least at the
screening level. The scope is to get the value of the two ground-motion
parameters commonly used in liquefaction hazard assessments: peak horizontal
acceleration (in the present work, the terms “peak horizontal
acceleration” and “peak ground acceleration”, as well as the respective
acronyms PHA and PGA, are used interchangeably) at the ground surface for
the return period of interest and earthquake magnitude (e.g., maximum
magnitude, mean, or modal magnitude from hazard disaggregation). These
parameters are related to the duration and intensity of ground shaking and
are key factors of the well-known simplified approach of Seed and Idriss (1971) for evaluating the liquefaction resistance of soils (see also Youd et
al., 2001). Thus, it may be convenient to reverse the question above: if
liquefaction can be triggered by seismic activity at the site of interest,
is the site susceptible to soil liquefaction? Indeed, if the seismic hazard
is deemed too low to trigger liquefaction (i.e., if the values of the
surface PGA and earthquake magnitude for that site are below some threshold
values), then one can avoid recommending specific investigations aimed at
the geological and geotechnical characterization of the subsoil. This
reversed decision scheme can be useful to guide land-use planners in
deciding whether liquefaction is a hazard that needs to be considered within
the planning processes or not.
With the increased availability of data in recent years, substantial
research has been carried out to establish thresholds of magnitude and PGA
at the ground surface below which the possibility of triggering liquefaction
can be discounted. Although the use of a minimum threshold magnitude is
controversial (Musson, 2020; Green and Bommer, 2019), available studies
converge to a value of 5.0 (e.g., Atkinson et al., 1984; Green and Bommer,
2019), while a threshold of about 0.1 g has been suggested for the PGA at the
ground surface (e.g., Santucci de Magistris et al., 2013; Santucci de
Magistris, 2015). However, exceptional liquefaction phenomena induced by
minor events (i.e., with magnitude less than 5.0) have been observed in
extremely susceptible soils (e.g., Martino et al., 2014; Santucci de
Magistris, 2015, and references therein; Zimmaro et al., 2019; Brandenberg
et al., 2020; Bozzoni et al., 2021). Such values are adopted by the Italian
building code (Ministero delle Infrastrutture e dei Trasporti,
2018
It is worth noting that, compared to the original version
released in 2008 (Ministero delle Infrastrutture e dei Trasporti, 2008), the
2018 update of the Italian building code does not specify a lower-bound
magnitude for liquefaction triggering.
), as well as by the Italian
Guidelines for Seismic Microzonation released by the SM Working Group (2008
and 2015
This is the English edition of the original document
released by the SM Working Group in 2008.
) (hereinafter, we will use the
ICMS acronym, commonly used in Italy).
The scope of the present work is to define a liquefaction triggering map of
Italy that can be used for seismic microzonation purposes and, therefore,
can guide land-use planning within the framework of risk mitigation
programs. According to the Italian Catalogue of Earthquake-Induced Ground
Failures – CEDIT (Martino et al., 2014; Caprari et al., 2018), more than
300 liquefaction events have occurred in Italy since 1000 CE, the last of
which are associated with the 2012 Emilia seismic sequence (e.g.,
Scognamiglio et al., 2012; Minarelli et al., 2022). Therefore, it is of
paramount importance to define a map that classifies sites as a function of
their liquefaction triggering potential. To this end, we combine data from
the reference Italian seismic hazard maps (MPS Working Group, 2004; Stucchi
et al., 2011) and hazard disaggregation (Barani et al., 2009), which is here
expressed in terms of magnitude contributions to the hazard. Note that the
results presented in this work do not address the issue of liquefaction
assessments for structural design, for which the hazard corresponding to
multiple return periods should be taken into account. In this work, we focus
on the 475-year return period according to the ICMS. The reliability of the
results is checked by comparing them with observations of past liquefaction
events in Italy (Martino et al., 2014; Caprari et al., 2018), as well as in
light of recent case studies associated with the 2012 Emilia seismic
sequence.
Although we are aware that liquefaction hazard requires site-specific
geological and geotechnical investigations (liquefaction is indeed a highly
localized phenomenon), our study may serve as a reference guide for
identifying sites where the possibility of triggering liquefaction can be
discounted. Moreover, it provides basic ground-motion data for evaluating
liquefaction resistance of soils at specific sites and for defining
liquefaction susceptibility maps (e.g., Zumpano et al., 2022). Such data are
freely available through the web service at
https://distav.unige.it/rsni/milq.php (last access: 28 April 2023).
Criteria for liquefaction triggering
According to the ICMS, liquefaction is expected to occur if the following
conditions are met (SM Working Group, 2008, 2015).
The lithological sequence presents layers of non-cohesive, saturated soil
(sandy silts, sands, silty sands, gravely sands, clayey sands, and sandy
gravels) down to 20 m depth.
The average depth of the water table is within 15 m of the ground surface.
Expected seismic events must be characterized by moment magnitude values.
Mw≥5 and must produce a surface peak ground acceleration amax≥0.1 g.
If even one of the conditions above is not verified, we can assume that the
area under study is not susceptible to liquefaction.
In this study, our focus is on the third condition. Contrary to the first
two ones, it does not require local data, at least at a screening level
(corresponding to the so-called level 1 microzonation in the ICMS). We use
national hazard maps along with the corresponding hazard disaggregation
results to define those sites in Italy where this condition is not met –
that is, where the triggering of liquefaction phenomena is unlikely and can
be excluded. If the condition is not met, then we can avoid recommending
specific geotechnical investigations aimed at verifying the first two
criteria, with consequent savings in efforts and money.
Methodology
In order to define those areas in Italy where seismic activity could trigger
liquefaction (according to the third condition listed in the previous
section), we analyze the reference seismic hazard data for the national
territory. First, the PGA hazard for rock conditions and flat topography
associated with a return period of 475 years (MPS Working Group, 2004;
Stucchi et al., 2011) is modified to incorporate site amplification due to
local geology and irregular topography (recall that the 475-year return
period is assumed as the single, reference return period by the ICMS).
Essentially, the resulting amax map allows us to define areas with high
(i.e., amax≥0.1 g) and low (i.e., amax<0.1 g)
local seismic hazard. Then, 1D disaggregation results (i.e., magnitude
disaggregation) are produced based on the reference work of Barani et al. (2009) and analyzed to define those sites where the hazard is controlled by
scenarios with magnitude Mw≥5. As sites respond at specific
characteristic frequencies (depending on local geological characteristics)
and disaggregation results may vary significantly with response period
(T), disaggregation analysis is carried out for different values of T.
Particular attention is paid to the choice of mean or modal magnitude values
as reference scenarios in order to avoid both non-conservative and
over-conservative scenarios. A cross analysis of hazard and disaggregation
data is finally performed via the QGIS software (https://www.qgis.org/en/site/, last access: 28 April 2023; QGIS.org, 2021) to
define the sites where liquefaction triggering is expected to occur.
(a) Soil amplification factor (SS) map; (b) topographic
amplification factor (ST) map (topographic classes, T1–T4, are
indicated in the legend).
Seismic hazard map
In order to define those sites with surface peak horizontal acceleration
amax≥0.1 g for a return period of 475 years, we refer to the
Italian seismic hazard map adopted by the national building code to define
the design seismic action (MPS Working Group, 2004; Stucchi et al., 2011).
We recall here that this map was developed assuming flat topography and rock
conditions. Hence, following the definition of surface peak horizontal
acceleration, where the word “surface” is used to indicate that amax
refers to site-specific conditions, we first need to amend the reference
hazard in order to incorporate possible ground-motion amplification effects
related to sub-surface lithological conditions and topographic
irregularities. According to the updated Italian building code (Ministero
delle Infrastrutture e dei Trasporti, 2018), the rock seismic action at a
given site is corrected by a site term (S) that accounts for both soil and
topographic conditions (i.e., S=SS×ST where
SS and ST are the stratigraphic and topographic
amplification factors, respectively). For that site, the values of
SS and ST can be defined according to simple
site classification criteria, which are not reported here for the sake of
brevity. While the soil classification is mainly based on the values of the
VS,eq parameter
Note that, in Italy,
VS,eq has replaced the well-known VS,30
parameter, which assumes a fixed depth of 30 m for the calculation of the
time-averaged shear-wave velocity in the subsoil.
(i.e., time-averaged
shear-wave velocity above the seismic bedrock, the latter being defined as
the rock formation or rigid soil with VS≥800 m s-1), the topographic
classification is based on site acclivity. Recently, Forte et al. (2019)
developed a soil classification map for all of Italy in compliance with the
ground types (also referred to as “soil types” or “subsoil classes”)
defined by the updated Italian building code
For the sake of
completeness, Forte et al. (2019) also developed a similar map based on the
VS,30 parameter, the parameter which was adopted by the older
Italian norms (Ministero delle Infrastrutture e dei Trasporti, 2008) and
Eurocode 8 (European Committee for Standardization, 2004) for the purpose of
site classification. Almost concurrently, a VS,30 map was
developed by Mori et al. (2020).
. Mascandola et al. (2021a) did the same
for topography, providing a topographic classification map of Italy. We
refer to these works to amend the reference PGA hazard map for rock
conditions and flat topography associated with a return period of 475 years
(i.e., ag hazard map). In compliance with the Italian building code,
amax=ag×S. The maps in Fig. 1 show the geographic
distributions of SS and ST using the same grid
of Forte et al. (2019), which is adopted as reference to produce the final
results. It is worth noting that the highest values of SS are
concentrated in the Po Plain sedimentary basin where, as mentioned
previously, several liquefaction phenomena were triggered during the 2012
Emilia seismic sequence (we refer the reader to the Discussion section for
details on this area). The comparison between the original ag hazard map
for a return period of 475 years and the corresponding amax map is
presented in Fig. 2. As expected, incorporating the site term into the
hazard increases the number of sites where liquefaction is likely to occur.
The number of sites with amax≥0.1 g grows from 753 820 to 883 363.
(a) Peak horizontal acceleration hazard map for rock conditions
and flat topography corresponding to a return period of 475 years (ag
hazard map); (b) peak horizontal acceleration hazard map at the ground
surface for a return period of 475 years (amax hazard map).
Hazard disaggregation
In order to define the sites where the seismic hazard is controlled by
Mw≥5 scenarios, we have disaggregated the hazard corresponding to a
return period of 475 years for all computation nodes considered in the
hazard assessment of Italy (MPS Working Group, 2004; Stucchi et al., 2011).
Specifically, the contributions from different magnitude scenarios are
determined (by summation) from the joint M–R–ε distributions
originally computed by Barani et al. (2009), where R is the source-to-site
distance and ε indicates the ground-motion error term. That
work still represents the reference disaggregation study for Italy and, as
such, is taken as reference by the ICMS. Accordingly, we consider bins of
0.5 magnitude units. Note that, according to Iervolino (2016), there is no
need to consider the site correction term S in the disaggregation process. In
our application, indeed, disaggregation results are invariant with soil
category.
As the ICMS does not specify how to handle response periods in the
disaggregation process (i.e., it is not specified if the controlling
magnitude should result from the disaggregation of the hazard for a specific
value of T), we have disaggregated the spectral acceleration hazard
associated with periods of 0.01 (i.e., PGA), 0.2, and 1 s. While PGA hazard
disaggregation may be appropriate for rock sites (which are known to
resonate at high frequencies and where one can exclude the possibility of
liquefaction), it can provide non-conservative results for most soil sites,
which generally have resonance periods in the 0.1–1 s range (this range
may extend up to longer periods in the case of deep alluvial valleys). It is
known, indeed, that the contribution from larger magnitudes increases as T
increases. Therefore, disaggregating the hazard for different spectral
periods allows us to define the controlling magnitude in relation to
geological conditions (through site classification). We expect that
with decreasing VS,eq and/or increasing soil thickness the site
resonance period increases (e.g., Kramer, 1996). Therefore, for those sites
characterized by lower values of VS,eq and/or thicker soils
(i.e., sites classified as ground type C, D, or E in the Italian building
code), it seems reasonable to define the controlling earthquake based on the
disaggregation of the hazard at longer periods.
Maps of mean (a, c, e) and modal (b, d, f) magnitude values obtained from the 1D disaggregation of the 475-year
spectral acceleration hazard corresponding to a response period (T) of 0.01,
0.2, and 1 s.
The maps in Fig. 3 show the geographic distribution of mean and modal
magnitudes (M‾ and M∗, respectively) for the three periods
considered. Comparing the maps for the same period immediately reveals
significant differences between mean and modal scenarios. On average, larger
magnitudes control the hazard in areas characterized by either a higher
seismic activity (e.g., Central and Southern Apennines) or very low
seismicity (e.g., southeastern edge of the Italian peninsula where the
hazard is controlled by large-magnitude, distant events), while lower
magnitudes dominate in areas of mild-to-moderate but relatively frequent
seismic activity (e.g., some areas in the Po Plain and western Alps).
However, with the same area, the values of M‾ and M∗ may
vary substantially. We observe that the maps of M‾ are generally
more conservative than those of M∗, especially in areas where the
hazard is lower (e.g., northern Italy). Which earthquake scenario should be
considered as representative of the site hazard? Should the mean or the mode be considered? Moreover, which mode should be considered in the case of bimodal
distributions? These questions are still open in the scientific community,
in the sense that there is no common opinion about the choice of the mean or
the mode. It is well known that the mode has the clear advantage of
representing the most likely scenario but is sensitive to the binning scheme
adopted. On the other hand, the mean is not sensitive to the bin size, but
it might not represent the most likely scenario or, in some cases (not so
infrequent, particularly in the case of bimodal distributions), it could
represent an unlikely one. In this work, we solve the crucial issue of the
controlling earthquake by adopting the conservative view that stronger is
safer. Specifically, for each period considered, we assume the following.
In the case of unimodal distributions with no skew (i.e., symmetric or
nearly symmetric about the mean) (Fig. 4a), either the mean or modal
magnitude can be indiscriminately taken as the preferred magnitude
(M^T=M‾T=M∗T).
In the case of unimodal, negatively skewed distributions (Fig. 4b), the
modal magnitude is assumed as the preferred magnitude (M^T=M∗T).
In the case of unimodal, positively skewed distributions (Fig. 4c), the mean
magnitude is assumed as the preferred magnitude (M^T=M‾T).
In the case of bimodal distributions, the second mode (M2∗) is
taken as the preferred magnitude (M^T=M2∗T) if its contribution to the hazard is greater than the contribution
associated with the mean magnitude (Fig. 4d). Conversely, we assume that its
contribution to the hazard is negligible, and the mean, which reflects that
contribution to some extent, is assumed as the preferred magnitude
(M^T=M‾T) (Fig. 4e). Thus, this
latter case resembles the case of a unimodal, positively skewed distribution
(see the previous bullet point) and avoids selecting over-conservative
scenarios with very small contributions (i.e., unlikely scenarios).
The maps resulting from the application of the criteria above are shown in
the left column of Fig. 5 for the three spectral periods considered. For
each of them, the maps in the right column show the geographic distribution
of the contributions associated with M^T. It can be
observed that such contributions are generally larger than 10 %. As
expected, the maps in the left column show that the values of M^
increase with increasing spectral period, reaching the largest values in
Southern Italy for T=1.0 s, where M^ is between 7.0 and 7.5. Note
the similarity of the maps corresponding to T=0.01 s (i.e., PGA) and T=0.2 s, with the latter being slightly more conservative. Despite this
similarity, we prefer to consider both these maps in order to distinguish
between rock sites, for which PGA hazard disaggregation is geologically
consistent, and sites characterized by deposits of dense soil, which are
expected to resonate at longer periods.
Example probability mass functions (PMFs) of magnitude: (a)
unimodal PMF with no skew; (b) negatively skewed PMF; (c) positively skewed
PMF; (d) bimodal PMF with the second mode contributing to the hazard more
than the mean scenario; and (e) bimodal PMF with the second mode contributing to
the hazard less than the mean magnitude. Mean (M‾), modal
(M∗), and preferred magnitude (M^) are
displayed in each panel. In the case of bimodal distributions (panels d
and e), the first and second mode are indicated as
M1∗ and
M2∗, respectively.
Maps of preferred magnitude M^T(a, c, e) and related contribution to the hazard (b, d, f) for spectral periods of 0.01, 0.2, and 1 s. The contribution to the
hazard for each value of M^T is expressed by
the probability mass function (PMF) of magnitude in that point (i.e.,
magnitude bin).
Results
The three maps shown in Fig. 5 are used in conjunction with the map of site
classification (Fig. 9b in Forte et al., 2019) to define the final map of
M^ (from here on, we drop the dependence of M^ on T to indicate
the preferred magnitude) according to the following criteria:
M^=M^T=0.01s for ground type A sites
(i.e., rock sites or stiff soils with VS≥800 m s-1);
M^=M^T=0.2s for ground type B sites (i.e.,
soft rock or deposits of dense soil characterized by a gradual increase in
the mechanical properties with depth and 360 ≤VS,eq<800 m s-1);
M^=M^T=1.0s for ground type C, D, and E
sites (i.e., sites characterized by deposits of loose-to-medium cohesionless
soil with thickness either greater (C and D sites) or less than 30 m (E
sites) and 100 ≤VS,eq<360 m s-1).
The resulting map of M^ is shown in Fig. 6.
Maps of preferred magnitude M^:
M^=M^T=0.01s
for ground type A; M^=M^T=0.2s for ground type B; and
M^=M^T=1.0s
for ground types C, D, and E (the reader may refer to the main body of the
article for an explanation of the ground types). Note that the value of
M^ assigned to the entire island of Sardinia, for which hazard and
hazard disaggregation are not available (MPS Working Group, 2004), is based
on historical seismicity (characterized by rare events with magnitude less
than 5; Rovida et al., 2020, 2022).
Combining the maps in Figs. 6 and 2b leads to the liquefaction
triggering map shown in Fig. 7, which is referred to with the Italian acronym
MILQ – Mappa del potenziale d'Innesco della LiQuefazione (map of
liquefaction triggering potential). The color scale has been chosen to
divide sites into classes of increasing liquefaction triggering potential
(LTP), from no potential (i.e., amax<0.1 g and M^<5.0)
to very high potential (i.e., amax≥0.2 g and M^≥6.0). Analyzing the map, we observe the following:
9.8 % of nodes fall in areas with no liquefaction triggering potential
(class LTP-0: amax<0.1 g and M^<5.0); they are mainly
concentrated in very few areas in the northwest and in almost all of the island of Sardinia.
20.4 % of nodes have very low liquefaction triggering potential (class
LTP-1: amax<0.1 g and 5.0≤M^<6.0 or 0.1g≤amax<0.2 g and M^<5.0); most of them are concentrated in
areas characterized by low-to-moderate ground-motion hazard, controlled by
small-to-moderate magnitude events (e.g., northern Italy, areas along the
northern Tyrrhenian coast).
8.8 % of nodes have low liquefaction triggering potential (class LTP-2:
amax<0.1 g and M^≥6.0 or amax≥0.2 g
and M^<5.0); most of these nodes are in low-seismicity areas (e.g.,
northeastern Alps and southeastern Italy) where the hazard tends to be
controlled by stronger, generally distant events or, conversely, in areas
of higher hazard but controlled by low magnitudes (e.g., northern Sicily).
10.5 % of nodes have moderate liquefaction triggering potential (class
LTP-3: 0.1 g ≤amax<0.2 g and 5.0≤M^<6.0);
most of them are concentrated in northern Italy, especially in the western
sector and in the Po Plain, and in central Sicily, where both the
ground-motion hazard and M^ are moderate.
31.6 % of nodes have high liquefaction triggering potential (class LTP-4:
0.1g≤amax<0.2g and M^≥6.0 or amax≥0.2g and 5.0≤M^<6.0). Most of them are concentrated in
areas of increased seismic activity in central and southern Italy and in
the northeast; the nodes with the highest values of M^ are located in
areas characterized by lower ground-motion hazard (e.g., some areas along
the southern Tyrrhenian and Ionian coasts and in central-eastern Sicily).
18.9 % of nodes have very high liquefaction triggering potential (class
LTP-5: amax≥0.2g and M^≥6.0); most of these nodes
are concentrated in central and southern Italy, following the Apennine arc
down to the volcanic area in eastern Sicily, and in the northeast, where the
ground-motion hazard reaches the highest levels and is dominated by (local)
moderate-to-high-magnitude seismicity.
Liquefaction triggering map of Italy (MILQ) for a return period of
475 years. Liquefaction triggering potential (LTP) classes are indicated in
the legend: no potential (LTP-0), very low potential (LTP-1), low potential
(LTP-2), moderate potential (LTP-3), high potential (LTP-4), and very high
potential (LTP-5).
Same as Fig. 7 but in red and white. Red is used when both the
value of surface peak horizontal acceleration for a return period of 475 years, amax, and the preferred magnitude, M^, are equal to or
greater than the thresholds of 0.1 g and 5.0, respectively. White is used
otherwise. The historical liquefaction phenomena that occurred in Italy from 1117 CE to 2018 (from the Italian Catalogue of Earthquake-Induced Ground
Failures – CEDIT; Caprari et al., 2018) are superimposed. The term “other
effects” in the legend refers to other earthquake-induced phenomena, such
as landslides, ground cracks, and surface faulting.
Online application for data retrieval
To make our results available to land-use planners and practitioners, we
have developed a web service, freely accessible at
https://distav.unige.it/rsni/milq.php (last access: 28 April 2023). The web service data are stored in a
“PostgreSQL” database. All the online components were developed in “PHP”
and “HTML5” languages to ensure adherence to current web standards.
Online maps are based on “Leaflet”, an open-source JavaScript library for
mobile-friendly interactive maps.
For a specified location (defined by a pair of geographic coordinates), the
web service provides the values of amax and M^ computed according
to the site classifications adopted in the present study. Specifically, the
values associated with the nearest node are returned (no interpolation is
performed). In addition, as the actual ground type and topographic class at
a site (e.g., resulting from site-specific data) can differ from those
considered here, the service allows the user to change them through a
user-friendly interface and returns the updated values of amax and
M^ as output. Alternatively, in line with the ICMS document (level 2
and 3 seismic microzonation) and the Italian building code (for sites that
can not be classified in one of the subsoil classes mentioned in the previous
section, and/or in the case of complex topography), the web service allows the amplification factor value (S) obtained from specific studies
(e.g., ground response analysis) to be entered. In this case, the user is also requested
to select the value of the spectral period T of interest among those
considered in the disaggregation analysis (e.g., as a function of site
response) to obtain the value of M^.
Distribution of liquefaction events in Italy (from the Italian
Catalogue of Earthquake-Induced Ground Failures – CEDIT; Caprari et al.,
2018) for each class of liquefaction triggering potential (LTP).
Discussion
Given the practical importance of liquefaction triggering potential maps,
they should be subjected to testing before application. In the present
study, we examine the reliability of the map in Fig. 7 by analyzing the
geographic distribution of past liquefaction phenomena reported in the
Italian Catalogue of Earthquake-Induced Ground Failures – CEDIT (Martino et
al., 2014; Caprari et al., 2018). These events have been superimposed on
that map (blue and light blue dots), which is now displayed in Fig. 8 using
a more intuitive red and white scale, with the red color indicating the
sites where liquefaction triggering is expected to occur (see the caption
for further details). As is evident, most observations fall in red areas,
thus indicating the consistency of our results. The matched observations are
314 out of 328. A more quantitative analysis of the results is provided by
the histogram in Fig. 9, which shows the percentage of CEDIT observations
for each class of liquefaction triggering potential defined above (from
LTP-0 to LTP-5). Among the observations that fall in the red areas of the
map in Fig. 8, 92 % of them correspond to nodes with high or very high
liquefaction triggering potential (classes LTP-4 and LTP-5), while 4 %
have a moderate potential (class LTP-3). Only 4 % of observations
correspond to nodes with very low or low triggering potential (classes LTP-1
and LTP-2). No observations fall in regions with no triggering potential
(class LTP-0).
Despite the good agreement between our map and past observations, a few
questions need to be answered. To what extent does this matching derive from
the criterion adopted to define M^ in relation to the soil resonance? In
other words, what is the sensitivity of our results to the choice of
M^T for different ground types? Can one obtain similar
conservative results just assuming the disaggregation results for a single
spectral period (e.g., T=0.01 s in line with the definition of
amax) and increasing the return time (e.g., to 2475 years)? To answer
these questions, we focus on the Po Plain area, which experienced
liquefaction in several locations during the 2012 Emilia seismic sequence
(Scognamiglio et al., 2012; Minarelli et al., 2022). The Quaternary
deposits, consisting mainly of an alternation of sands, silts, and clays, are
rather homogeneous throughout this area. Indeed, according to the soil
classification of Forte et al. (2019) (see Fig. 9b therein), most sites in
the Po Plain basin can be classified as ground type C or D (i.e., sites
characterized by deposits of loose-to-medium cohesionless soil with
thickness greater than 30 m and 100 ≤VS,eq<360 m s-1). However, deep stratigraphic discontinuities can be identified
(e.g., Mascandola et al., 2019). Such discontinuities are responsible for
significant ground-motion amplification at long periods (>1 s)
(e.g., Luzi et al., 2013; Abraham et al., 2015; Mascandola et al., 2021b).
For the Po Plain area, Fig. 10 compares alternative liquefaction triggering
scenarios by adopting different assumptions: precisely, Fig. 10a is just a
zoom of Fig. 8; Fig. 10b is the same as Fig. 10a but is obtained by assuming
M^=M^T=0.01s regardless of the ground
type; finally, Fig. 10c is the same as Fig. 10b but for a return period of
2475 years. As expected, comparing the map in Fig. 10b with the reference
one in Fig. 10a reveals that disaggregating only the PGA hazard can lead to
an underestimation of the liquefaction triggering potential at soil sites.
Such an underestimation can be significant at sites characterized by thick
and soft soil deposits, as they generally have resonance periods much larger
than 0.1 s. This justifies the use of the disaggregation of the hazard
associated with either short or long spectral periods depending on site
response. Especially for soft soil sites or sites characterized by thick
soil deposits, the use of hazard disaggregation corresponding to longer
periods prevents the adoption of non-conservative provisions by land-use
planners or practitioners. As for the map in Fig. 10c, which refers to a
return period of 2475 years, it still provides a conservative scenario like
the reference map in Fig. 10a, but that scenario is justified neither by the
local geology (see above) nor by the return period of a Mw=5.9
earthquake, such as the main shock of the 2012 Emilia seismic sequence with its
epicenter near the town of Ferrara. According to the MPS Working Group (2004), indeed, the return period of a Mw=5.9 earthquake in that
area is about 360 years.
Liquefaction triggering and historical liquefaction phenomena in
the Po Plain area: (a) zoom of Fig. 8; (b) same as panel (a) but obtained by
assuming M^=M^T=0.01s regardless of the ground type (see the map in top left corner of
Fig. 5); and (c) same as panel (b) but for a return period of 2475 years (i.e.,
the values of amax and M^=M^T=0.01s used as input refer to a return
period of 2475 years).
Conclusions
Besides ground-motion amplification effects, which are undoubtedly the most
important of all seismic (site) effects because of their impact on the
environment and society, instability phenomena induced by earthquake ground
shaking (i.e., surface faulting, ground failure, and soil liquefaction) also
play a key role in defining risk mitigation strategies and land-use
planning. In recent years, the identification of unstable areas and the
subsequent quantification of the instabilities have become primary
activities of seismic microzonation studies. In this context, our study has
focused on the identification of areas susceptible to soil liquefaction in
Italy. We have analyzed ground-motion hazard maps and the associated hazard
disaggregation to define areas where liquefaction triggering due to seismic
activity can not be excluded and where, therefore, further efforts are required to
evaluate liquefaction susceptibility. The final result is a liquefaction
triggering map showing areas with different triggering potential (Fig. 7).
The information contained therein (particularly the value of M^) can
be considered as an alternative to the results one can obtain by applying
other approaches, particularly those proposed by the SM Working Group (2008, 2015; interested readers may also refer to the note of Technical
Commission on Seismic Microzonation, 2018) for the determination of the
reference magnitude for liquefaction susceptibility assessment. It is worth
noting that, regardless of the approach used, soil liquefaction is a highly
localized phenomenon whose occurrence is intimately related to site-specific
geological and geotechnical conditions, which deserve focused
investigations. Therefore, studies like the one presented here primarily
serve as a basic guide to identifying sites where the possibility of
triggering liquefaction can be discounted within the land-use planning
process.
Despite the reliability of our results, which was examined by analyzing the
distribution of past liquefaction events over the Italian territory, they
can certainly be refined as soon as site-specific studies become available.
On the one hand, site-specific hazard analyses (e.g., Cramer et al., 2014; Barani et
al., 2020; Mascandola et al., 2023) allow the refinement of the values of
amax. On the other hand, in situ measurements provide a more accurate
definition of the ground type for the site of interest. At least as far as
this issue is concerned, practitioners and land-use planners can interrogate
our results through the web service at https://distav.unige.it/rsni/milq.php (last access: 28 April 2023) and
refine the level of liquefaction triggering potential of the site of
interest by changing the ground type and the topographic class through a
user-friendly interface or by entering the site-specific amplification
factor value at hand (e.g., determined from ground response analysis) along
with the value of the spectral period of interest. The service returns as
output the updated values of amax and M^, thus allowing the
refinement of the triggering potential level.
Data availability
The results presented in this paper are stored in a “PostgreSQL”
database and made available through the online web service at
https://distav.unige.it/rsni/milq.php (last access: 28 April 2023; Università di Genova, 2023). They can also be provided to the
editorial board members, referees, and readers upon request.
Author contributions
SB carried out most of the analyses (i.e., computation of amax and
hazard disaggregation), wrote the main body of the manuscript, and prepared
Figs. 4 and 9. GF contributed to the analyses (particularly to the
computation of amax), carried out an independent check of the values of
M^, and prepared all the figures (except for Figs. 4 and 9) via
the QGIS software. DS developed the web service
(https://distav.unige.it/rsni/milq.php, last access: 28 April 2023) for the dissemination of the results.
All authors reviewed the manuscript and contributed to the interpretation of
the results.
Competing interests
The contact author has declared that none of the authors has any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Special issue statement
This article is part of the special issue “Earthquake-induced hazards: ground motion amplification and ground failures”. It is not associated with a conference.
Acknowledgements
We are grateful to two anonymous reviewers for their suggestions that
brought significant improvements to the study. We are also thankful to Iunio Iervolino and Eugenio Chioccarelli for providing us with the VS,eq map and
the fruitful discussions.
We thank the staff of the “Seismic Section” of the Tuscany Region Administration, Massimo Baglione and Vittorio D'Intinosante, for the useful discussions and suggestions.
Review statement
This paper was edited by Giovanni Forte and reviewed by two anonymous referees.
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