SESolid EarthSESolid Earth1869-9529Copernicus PublicationsGöttingen, Germany10.5194/se-8-161-2017Defining a mid-Holocene earthquake through speleoseismological and
independent data: implications for the outer Central Apennines (Italy)
seismotectonic frameworkDi DomenicaAlessandraa.didomenica@unich.itPizziAlbertoDepartment of Engineering and Geology, University G. d'Annunzio,
Via dei Vestini 31, Chieti, 66013, ItalyAlessandra Di Domenica (a.didomenica@unich.it)10February20178116117616May201627June201612January201716January2017This 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://se.copernicus.org/articles/8/161/2017/se-8-161-2017.htmlThe full text article is available as a PDF file from https://se.copernicus.org/articles/8/161/2017/se-8-161-2017.pdf
A speleoseismological study has been conducted in the
Cavallone Cave, located in the easternmost carbonate sector of the Central
Apennines (Maiella Massif), in a seismically active region interposed
between the post-orogenic extensional domain, to the west, and the
contractional one, to the east. The occurrence of active “silent normal
faults”, to the west, close to blind thrusts, to the east, raises critical
questions about the seismic hazard for this transitional zone. Large
collapses of cave ceilings, fractures, broken speleothems with new
re-growing stalagmites on their top, preferential orientation of fallen
stalagmites and the absence of thin and long concretions have been observed
in many portions of the karst conduit. This may indicate that the cave
suffered sudden deformation events likely linked to the occurrence of past
strong earthquakes. Radiocarbon dating and, above all, the robust
correspondence with other coeval on-fault and off-fault geological data
collected in surrounding areas outside the cave, provide important
constraints for the individuation of a mid-Holocene paleoearthquake around
4.6–4.8 kyr BP. On the basis of the available paleoseismological data,
possible seismogenic sources can be identified with the Sulmona normal fault
and other active normal fault segments along its southern prosecution, which
recorded synchronous strong paleoevents. Although
the correlation between speleotectonic observations and quantitative
modeling is disputed, studies on possible effects of earthquake on karstic landforms and
features, when corroborated by independent data collected outside caves, can
provide a useful contribution in discovering past earthquakes.
Introduction
In highly seismically active regions characterized by karst environments,
where the identification and characterization of the seismogenic sources are
not strictly constrained by both geological and seismological data,
speleoseismology (i.e., the investigation of traces of past earthquakes in
caves) represents a potential tool for the improvement of the seismological
record and to better understand the seismotectonic framework and seismic
hazard of an area. Cave environments can contain a great amount of
well-preserved geological information and are ideal for paleoseismological
investigations because earthquake damages can be fossilized by
post-earthquake calcification and thus preserved from erosion.
Following some pioneering studies at the beginning of the 1990s (see Becker
et al., 2006, for a review), many works have focused on the significance of
cave damages and their possible correlations with active tectonics and, in
particular, with the effects of seismic shaking. This evidence includes
broken speleothems and fallen stalactites (e.g., Postpischl et al., 1991;
Ferranti et al., 1997; Lemeille et al., 1999; Delaby, 2001; Kagan et al.,
2005; Šebela, 2008; Panno et al., 2009; Bábek et al., 2015;
Méjean et al., 2015), blocks and ceiling collapses (e.g., Gilli, 1999;
Pérez-López et al., 2009), deformed cave sediments and fault
displacements (e.g., Gilli et al., 2010; Bábek et al., 2015), and speleothem
growth anomalies (e.g., Forti et al., 1981; Forti and Postpischl, 1984;
Akgöz and Eren, 2015; Rajendran et al., 2015). Although direct
observations of cave damages immediately after an earthquake have rarely
been observed, “seismothems” (i.e., speleothems potentially broken, or
deformed, by a seismic event; Delaby, 2001) have been increasingly
recognized in many caves worldwide, allowing researchers to discover past
earthquakes (see Becker et al., 2006, for a review). On the other hand,
robust evidence and correlations of specific paleoearthquakes in caves with
other independent paleoseismic records are very rare in the literature
(Becker et al., 2005).
(a) The three major arcs of the Apennine Neogene chain.
(b) Simplified structural map of the Central Apennines and related seismicity
(modified from Pizzi et al., 2010; de Nardis et al., 2011). Within the axial
zone of the chain major Quaternary/active post-orogenic normal faults,
superposed onto the Neogene thrusts, define the intra-Apennine extensional
domain (light-blue band). Contractional or strike-slip tectonics prevail in
the Adriatic foreland area, to the east. Epicenter location and
macroseismic intensity (Io), or equivalent moment magnitude (Maw) and moment
magnitude (Mw), of historical and recent earthquakes (Working Group CPTI,
2004; Rovida et al., 2011 and Ceccaroni et al., 2009 for the 2nd century AD
earthquake) indicate stronger seismicity within the inner extensional domain
and only moderate events in the outer contractional zone. Some events, such
as those in 1706 and 1933, localized in the Maiella Massif (i.e., at the
extension/contraction transitional domain), still have an uncertain source
and kinematics. Persistent structural barriers (dotted grey bands), oblique
to the main trend of the normal fault systems, control the propagation and
segmentation of the Quaternary/active normal faults (Pizzi and Galadini,
2009). AF: Assergi Fault; SF: Sulmona Fault; PWPF: Palena–western
Porrara fault system; RAFS: Rotella–Aremogna fault system.
The analysis of seismothems requires a careful and integrated study approach
aimed at the recognition of peculiar structures through the observation of
speleothem morphologies. Several typical features can be referred to
tectonic coseismic shaking (Forti, 2001). The most characteristic is
represented by stalagmites cut along sub-horizontal planes, with the upper
parts only translated from their original position but still standing
upright or toppled and lying on the floor close to their base, accompanied
with new-growing stalagmites covering the rupture surface of the stump
(e.g., Postpischl et al., 1991; Delaby, 2001; Kagan et al., 2005). The
geometries of the fallen part and the stump must match, allowing the
reconstruction of the original speleothem. Another piece of evidence of
earthquake-induced phenomena is the presence of specific spatial
distribution patterns of fallen concretions, resting on sub-horizontal or
slightly inclined cave ground. Collapsed stalagmites toward preferential
orientations may be consistent with the strike of the earthquake source
(e.g., Gilli et al., 1999; Ferranti et al., 2015) or its slip direction (e.g.,
Méjean et al., 2015). More suitable for this kind of observations are
fallen stalagmites cemented on the floor to ensure that broken speleothems
have maintained their original direction in time (Forti, 2001). These
observations have to be strengthened by dating in order to find correlations
with past earthquakes. To constrain the age of the damaging event it is
necessary to date the oldest layer at the base of the regrowth stalagmite
and the youngest layer at the tip of the fallen stalagmite (e.g., Postpischl et
al., 1991; Forti, 2001; Kagan et al., 2005). Statistical analyses and
comparison with other data (e.g., paleoseismological data collected along
neighboring seismogenic sources) are crucial to better constrain the age of
a paleoearthquake. On the other hand, seismothems-like features can be
induced also by non-seismic processes such as human and animal presence,
shocks due to mine blasting, cryogenic fracturing, gravitational collapses,
creep movements of sediments, glacial intrusion and catastrophic floods
(e.g., Gilli, 2005; Becker et al., 2012). Therefore, it is fundamental to
consider more factors that can interact and rule out all other possible
non-seismic causative processes.
Schematic geological map of the study area (a) (simplified from
Accotto et al., 2014; see Fig. 1 for the location) and geological
cross section along the Taranta Valley (c) showing SE-dipping Meso-Cenozoic
carbonates, passing to Mio-Pliocene gypsum and siliciclastic deposits,
describing the forelimb of the Maiella anticline. E–W-trending normal faults
represent pre-thrusting conjugate normal fault systems (b). The Cavallone
Cave is located along the cliffs of the Taranta Valley, in the southeastern
portion of the Maiella Massif; to the south, the village of Lettopalena is
situated on top of rock avalanche deposits. (d) Section and map views of the Cavallone
Cave showing the articulate morphology of the cave ground.
Finally, the correlation between field
observations and quantitative modeling is disputed. Some authors state that high
accelerations are needed to break speleothems and that only thin and long
concretions can be damaged following seismic shaking due to “reasonably”
strong earthquake (3 m s-2 < PGA < 10 m s-2) (e.g.,
Cadorin et al., 2001; Lacave et al., 2000, 2004, 2012). Nevertheless, field
evidence cannot be neglected and suggests that empirical mechanical
relations are possibly lacking for considering some significant parameters and conditions
(Delaby, 2001).
Due to the uncertainties affecting this method of analysis, it is
fundamental to combine speleoseismological data with independent
paleoseismic records in other geological archives outside caves, such as
liquefaction evidence within lake and flood-plain deposits, locations of
rock falls and coseismic fault displacements (Becker et al., 2005, 2006).
A few speleotectonic studies have been conducted in the high seismically
active region of the Apennines (Forti and Postpischl, 1984; Postpischl et
al., 1991; Ferranti et al., 1997, 2015). We performed a
geological–structural field study, with a paleoseismological approach, in the
Cavallone Cave located within the Maiella Massif, in the eastern Central
Apennines (Fig. 1). The aim was to find typical earthquake-related structures
and, then, to improve the paleoseismological record and characterization
of the seismogenic sources affecting a complex seismotectonic region such as
the easternmost sector of the Central Apennines. The evidence described in
this work suggests that the conduit underwent repeated earthquake damages in
the past. Radiocarbon dating provided important constraints for the
individuation of a paleoearthquake, and an exceptional geological record
available from surrounding areas allowed for robust correlation of the
recognized paleoearthquake with other independent data (i.e., paleoseismological
and geological data outside the cave), strengthening the reliability of
speleoseismological studies in discovering past earthquakes.
Geological and structural setting
The Apennines are a Neogene–Quaternary foreland-verging fold-and-thrust
belt, showing a complex structural arrangement derived from the interaction
between contractional structures and pre-existing extensional faults
(e.g., Coward et al., 1999; Scisciani et al., 2002; Tozer et al., 2002; Butler
et al., 2006; Calamita et al., 2011; Scisciani, 2009; Di Domenica et al.,
2014a, b; Pace et al., 2014; Cardello and Doglioni, 2015). The
orogenesis involved Triassic-to-Miocene sedimentary successions related to
different basin and platform paleogeographic domains of the Adria Mesozoic
paleomargin (e.g., Ciarapica and Passeri, 2002; Patacca and Scandone, 2007).
Post-orogenic extension affects the Apennine chain and is responsible for
present-day seismicity especially concentrated within the axial part of the
belt. Major seismicity is related to NW–SE-trending, 15 to 30 km long
Quaternary normal fault systems which are compartmentalized by Neogene
regional oblique NNE–SSW-trending thrust ramps acting as “persistent
structural barriers” and controlling the along-strike normal fault systems
propagation and segmentation (Fig. 1; e.g., Tavarnelli et al., 2004; Pizzi and
Galadini, 2009; Di Domenica et al., 2012). The NW–SE-trending active normal
fault systems are responsible for high-magnitude destructive earthquakes
(M≤ 7; e.g., Calamita and Pizzi, 1994; Ghisetti and Vezzani, 1999; Galadini
and Galli, 2000; Working Group CPTI, 2004; Rovida et al., 2011). On the
other hand, so-called “silent faults” characterize the easternmost
alignment of the Central Apennine normal fault systems (e.g., Sulmona Fault,
Assergi Fault; Fig. 1). No large-magnitude historical seismic events can be
strictly related to these structures which, however, show evidence of late
Pleistocene–Holocene activity (Barchi et al., 2000; Galadini and Galli,
2000; Papanikolaou et al., 2005, and references therein). Only recently has the
2nd century AD earthquake that occurred in central Italy been associated
with the Sulmona fault system through archaeoseismological and paleoseismological
evidence (Ceccaroni et al., 2009; Galli et al., 2015). This lack of data
could be due to long recurrence intervals (1400–2600 years; Galadini and
Galli, 2000) and/or to the incompleteness of the available historical
seismic catalogues (e.g., Stucchi et al., 2004), suggesting high seismic hazard
levels and raising critical questions about the identification of the
seismogenic structures and the true seismic potential of these areas. This
has unfortunately been confirmed by the recent August–October 2016 seismic sequence
that struck central Italy and that was related to the activation of
the Mt. Vettore–Mt. Bove normal fault system (Fig. 1), which had previously been considered “silent”
(Galadini and Galli, 2003), although already identified in the field and
mapped as an active fault (Pizzi, 1992; Pizzi et al., 2002; Calamita and Pizzi, 1992, 1994;
Calamita et al., 1992). The seismotectonic framework of
the Central–Northern Apennines is complicated by the occurrence of reverse
and strike-slip events delineating an active contractional domain along the
peri-Adriatic foredeep–foreland sector that exhibits a moderate and less
frequent seismicity with respect to the western extensional one and for
which paleoseismological and seismological data are very scarce (Fig. 1;
e.g., Frepoli and Amato, 1997; Pondrelli et al., 2006; Montone et al., 2012).
The Cavallone Cave entrance is located at 1475 m a.s.l., in the Taranta
Valley, a NNW–SSE/NW–SE-trending incision cutting through the Maiella Massif
forelimb (Figs. 1 and 2). The Maiella Massif represents the outermost
outcropping carbonate anticline of the Central Apennines, involving Early
Cretaceous–Miocene carbonate platform and slope–basin successions (e.g.,
Donzelli, 1969; Patacca and Scandone, 1989; Festa et al., 2014, and
references therein). The Maiella anticline is related to the emplacement of
a Pliocene–early Pleistocene NW–SE/NNE–SSW-trending curved frontal thrust,
buried beneath Mio-Pliocene siliciclastic deposits and Molise allochthonous
Units (e.g., Scisciani et al., 2002; Pizzi, 2003). Along the Taranta Valley the
Paleocene–upper Oligocene carbonate ramp succession is spectacularly exposed
and results affected by E–W pre-thrusting conjugate normal fault systems
(Fig. 2; Scisciani et al., 2000; Festa et al., 2014).
(a) Superposed multiple phases of large collapses and concretions
near the entrance of the cave. Meter-scale large stalagmites grew on top of
thick piles of chaotic materials, draped by flowstone deposits (sketch of b), within which large portions of
concretions tens of centimeters thick are involved (close-up of c). (d) Great collapses involving meter-scale
large stalagmites in the “Galleria della Devastazione”. Stalagmites still
standing upright show a clear tilting toward the axis of the conduit,
probably as a consequence of floor sinking (see sketch of e). See
Fig. 2d for sites location.
Given the seismotectonic framework described above, it is clear how the
Maiella Massif is interposed between two distinct domains: extensional, to
the west, and contractional, to the east (Fig. 1). Although the
Sulmona–Maiella area represents the epicentral zone of high-magnitude
historical earthquakes, such as those that occurred in the 2nd century AD (Maw= 6.3), in 1706 (Maw= 6.6) and in 1933 (Mw= 5.7; Working Group CPTI,
2004; Rovida et al., 2011), the identification of the seismogenic source(s)
responsible for this seismic activity is poorly constrained (by both
geological and seismological data) and still debated. The most hazardous
adjacent seismogenic structures are the Sulmona normal fault (or Mt. Morrone
fault) capable of producing M= 6.6–6.7 earthquakes (e.g., Vittori et al., 1995;
Galadini and Galli, 2000; Gori et al., 2011, 2014; Galli et al., 2015), the
Rotella–Aremogna normal fault system and the Palena–western Porrara normal
fault system (Fig. 1). Late Quaternary evidence of faulting have been
described for the latter fault system (Pizzi et al., 2010), while
paleoseismological studies allowed for four faulting events to be recognized in the
past ∼ 9 kyr along the Sulmona Fault (Galli et al., 2015).
Paleoearthquakes have also been identified along different segments of the
Rotella–Aremogna normal fault system (Frezzotti and Giraudi, 1989; Calderoni
et al., 1990; Brunamonte et al., 1991; D'Addezio et al., 2001; Tesson et
al., 2016; Fig. 1). Regarding the frontal area of this sector of the chain,
few seismological data are available and seismicity could be probably
associated with buried Apennine thrust fronts (e.g., Scisciani and Calamita, 2009;
de Nardis et al., 2011; DISS Working Group, 2015) such as those activated in
the Northern Apennines during the 2012 Emilia Romagna earthquake (e.g., Pizzi
and Scisciani, 2012; Govoni et al., 2014, and references therein).
(a) Typical setting of the cave ceilings where originally “long”
stalactites (probably more than 1 m) are all truncated at the same
point. Only short stalactites (tens of centimeters long) are entirely
preserved. (b) Centimeter-wide sub-vertical open fractures frequently
affecting flowstones, running at the base of the cave walls.
(c) Centimeter-
to decimeter-high stalagmites growing on top of collapsed materials: (1) ceiling rejuvenated by collapses, (2) blocks from the collapsed
ceiling, and (3) stalagmites growth after the ceiling collapse. (d) Speleothem cuts along a
sub-horizontal plane, with the upper part only translated from their
original position but still standing upright. See Fig. 2d for site
locations.
Method
We performed a careful structural–geological analysis along the main conduit
and some lateral branches of the Cavallone Cave (Fig. 2), for a total length
of ca. 1 km. The Cavallone Cave has been exposed to human presence since the
end of the 1600s and ca. 500 m of it is now open for touristic purposes. In the vicinity of the cave entrance and along the touristic path, broken
stalagmites may be related to human depredation. Nevertheless, there are
portions of the cave, far enough from the entrance and from the touristic
path, which allow for excluding the presence of humans and animals in the past
that might have altered the karstic landforms and features of the cave
(rooms “Sala dei Cristalli” and “Sala dei Merletti” in Fig. 2d). For the
speleotectonic analysis, considerable attention has been paid to distinguishing the
features described in the literature as earthquake-related structures
(e.g., stalagmites cut along sub-horizontal planes, with the upper part lying
close to their base and cemented on the floor), according to the indications
of Forti (2001). Measurements of fall directions, dimensions of
speleothems and distribution of speleothem fracturing were collected in the
sectors of the cave considered suitable for the study, far enough from
possible human disturbance. Only broken stalagmites lying and cemented on
sub-horizontal or slightly inclined cave ground have been considered for the
statistical analysis of the representative azimuths of fall directions.
Careful observations have also been made to eliminate other causes for
speleothem damage. For a detailed paleoseismic interpretation, accelerator mass
spectrometry (AMS)
radiocarbon dating of the damaged speleothems and of a fragment of a tree
involved in a rock avalanche adjacent to the cave have been performed (by
Beta Analytic Radiocarbon Dating Laboratory, Florida, USA).
Speleoseismological analysis
The Cavallone Cave mainly develops in a N–S direction within the
Paleocene–upper Oligocene calcarenite and cherty limestone of the Maiella
carbonate ramp succession (Fig. 2). Within the cave, patches of both fine-
and coarse-grained deposits are preserved. In the majority of cases, these
latter represent collapsed material over which new generations of
concretions have grown up. The cave shows many concretions, especially
stalagmites. Near the entrance, the largest stalagmites (more than 2 m high)
are preserved. An exposed longitudinal section of one of these speleothems
shows that the concretion grew onto a ca. 2 m thick pile of chaotic material
(likely originated from repeated collapse events), involving a large portion of
stalagmites and draped by flowstone deposits (Fig. 3a, b and c). Large
stalagmites (more than 3 m high, with diameter of 30–50 cm) are also
preserved within the first eastern branch of the cave named “Galleria
della Devastazione”, which cannot be visited by tourists (Fig. 2d). Here
stalagmites rest tilted converging toward the central axis of the conduit,
and some of them are broken (Fig. 3d). The stalagmites located at the
entrance of the cave and in the Galleria della Devastazione branch are
the largest observed in the cave, indicating that they probably represent the
oldest preserved speleothems. Evidence of falls and collapses, moreover,
shows that the cave underwent several sudden deformation events (probably
of different origin) during its development. Nevertheless, for the
Galleria della Devastazione branch earthquake-induced damages are
difficult to be invoked as the main cause of its present-day configuration,
since the strength of the seismic events would have to have been far greater to break such large
concretions (e.g., Cadorin et al., 2001; Lacave et al., 2000, 2004, 2012). A
strong influence, instead, could have been exerted by gravitational
processes that possibly caused the floor to sink (Fig. 3e), probably as a
consequence of the collapse of a deeper karst network. However, it cannot be
excluded that this collapse could have also been triggered by a strong
seismic event.
Apart from these first meters of the cave, stalagmites found in the conduit
do not exceed 1 or, exceptionally, 2 m in height and have diameters
ranging between 10 and 30 cm on average. Intact stalagmites showing
these dimensions, however, are very scarce and are preserved only in
restricted portions of the conduit, while the most abundant unbroken
stalagmites do not exceed 20 cm in height. Regarding the stalactites, those
appearing well developed (more than 1 m long and with a diameter of
about 10 cm) result as broken in the majority of cases. On the other
hand, where the stalactites are entirely preserved, they show uniform
lengths that do not exceed some tens of centimeters (Fig. 4a). From a general
point of view, therefore, the Cavallone Cave is lacking intact long and thin
speleothems. This raises questions because long and thin concretions are the
first to be destroyed during an earthquake and it is generally accepted in
the literature that soda straw damages are reliable indicators of coseismic
shacking (e.g., Lacave et al., 2004; Becker et al., 2006). Moreover, ceiling collapses
are widespread and corroborated by the presence of piles of
collapsed materials in many points along the conduit. On top of these piles,
small stalagmites, from a few centimeters up to 20 cm high at most, can be
observed (Fig. 4c). Often the floor of the conduit shows centimeter-wide
sub-vertical open fractures that affect concretions (Fig. 4b) and run at the
base of the cave walls, probably suggesting the interplay of different
processes, from gravitational to seismic.
Especially in the rooms “Sala dei Cristalli” and “Sala dei Merletti”
(see Fig. 2d for the location), typical features that are associated in the
literature with earthquake damages can be observed. These features are represented by
fractured stalagmites showing sub-horizontal cut planes located in the lower
third of their height. Where it was possible to observe both the stumps and
the fallen portion of the speleothems that underwent these kinds of damages,
the concretions show diameters of 10–15 cm and original lengths of 40–100 cm on average. The upper parts of these broken speleothems are standing
upright, only translated on the fracture plane (Fig. 4d) or have fallen and
are cemented on the floor (Fig. 5b), lying close to their corresponding stumps.
Most of the examples of these stalagmites were found to remain on
sub-horizontal or slightly inclined cave ground. The cut planes of the
broken speleothems are often covered by new-growing stalagmites, a few
centimeters high (2–5 cm), similar to those found on top of collapsed
materials. The analysis of the orientation of the fallen stalagmites
cemented on the floor reveals two preferential trends, NNW–SSE and ENE–WSW
(Fig. 5a), respectively roughly parallel and perpendicular to the strike of
the main seismogenic normal faults in the extensional domain to the west and
south of the Maiella Massif (e.g., Sulmona, Rotella–Aremogna and Palena–western
Porrara normal fault systems), whereas, to the east, the thrust planes
generally strike ca. N–S (i.e., parallel to the Sangro–Volturno thrust zone; see
Fig. 1). The preferential orientation of broken speleothems has been
considered as seismic-related, and has often been found as concordant with the
orientation of the seismogenic causative faults (e.g., Delaby, 2001; Forti,
2001; Postpischl et al., 1991; Kagan et al., 2005; Ferranti et al., 2015).
(a) Rose diagram of the measured orientations of fallen stalagmites
cemented on the floor. Two main trends can be recognized: ENE-WSW and
NNW–SSE, respectively orthogonal and parallel to the main active normal
faults of the area. (b) Seismothem-like feature chosen for radiocarbon
dating: stalagmite (originally ∼ 40 cm high) broken along a
sub-horizontal plane, fallen toward N190 and resting cemented on the floor
(see Fig. 2d for the location). On the stump surface a ca. 2 cm high new
stalagmite is growing. “GC/1” and “GC/2” are the codes of the dated
samples (see Fig. 6).
In the cave we found peculiar examples of features similar to that showed in
Fig. 5b. The two parts of the broken speleothem perfectly match and a new
generation of concretion, with dimension analogous to other stalagmites'
regrowth on broken speleothems and collapsed materials (2–5 cm high), can be
found on top of the cut plane. A peculiarity of the analyzed concretions is
the presence of so-called “macroholes”, i.e., millimeter- to centimeter-size
cavities which can determine significant secondary porosity of the
speleothem (Shtober-Zisu et al., 2012). These holes can develop parallel to
the growth axis of the stalagmite (“axial holes”), causing the
characteristic antiformal shape of the surrounding growth layers which are
bent downward towards the holes (Fig. 6a). The origin of the axial holes has
been linked to a syn-depositional slow rate of calcite accumulation and
falling-drop erosion. Also, “off-axis holes” can occur and they are related
to post-depositional calcite corrosion, possibly controlled by bacterial
activity (Shtober-Zisu et al., 2014).
According to the available literature (e.g., Forti, 2001), the speleothem of
Fig. 5b can be compared to a “seismothem” (Delaby, 2001); hence, we
chose it for our paleoseismological analysis. A sample of the tip of the
upper part of the broken stalagmite and a sample of the base of the small
stalagmite growing on top of the stump have been collected. The AMS analysis
has been performed on the youngest layer of the broken stalagmite (sample
“GC/1”; likely pre-seismic) and on the oldest layer of the new-growing
stalagmite, as close as possible to its mutual contact with the cut plane of
the stump (sample “GC/2”; likely post-seismic; Figs. 5b and 6a). The age
of the possible paleoearthquake should be more recent than the age of the
tip of the broken stalagmite and more ancient than the base of the new
stalagmite growing on the stump. The break and the start of the regrowth of
the stalagmites should be assumed as instantaneous and simultaneous events.
The results of both samples are in fact highly comparable and indicate a
time span that extends from 4840 to 4525 yr BP (2σ cal. age; Fig. 6b). As sample GC/1 belongs to a stalagmite older than sample
GC/2, we can strengthen our chronological constraint considering only
the intervals within which the dating of the two samples overlaps. In this
way, we obtain that the rupture of the speleothem occurred around
4730 ± 85 yr BP (Fig. 6c).
Results of the radiocarbon dating performed by the Beta
Analytic Radiocarbon Dating Laboratory (Florida, USA) on the wood fragment
buried within the Lettopalena rock avalanche.
SampleLaboratory14C age2σ cal. 95 %code(yr BP)LRABeta-4449814610 ± 305445–5385 yr BP5325–5300 yr BPCorrelation with independent geological records
In order to validate the possible seismic origin of the broken speleothem we
looked for other synchronous and independent evidence outside the cave, and
we found that the result of the radiocarbon dating matches with other
geological evidence and data collected in surrounding areas.
(a) Images of the analyzed samples GC/1 (tip of the
broken stalagmite, likely pre-seismic) and GC/2 (new stalagmite growing
on the stump, likely post-seismic); see sample locations in Fig. 5b. The
axial section of the two samples shows a characteristic internal porous
structure due to the presence of widespread syn-genetic “macroholes”
(Shtober-Zisu et al., 2012), which are also responsible for the
downward bending of the growth layers (marked with dotted lines in a).
White stars indicate the points where material was collected for
radiocarbon dating. (b) Results of the dating performed on the two samples
through AMS analysis (by Beta Analytic Radiocarbon Dating Laboratory,
Florida, USA). (c) Overlapping time intervals that allow for restricting the time
span within which the rupture of the speleothem occurred (4730 ± 85 yr BP).
A few kilometers south of the Cavallone Cave, the
Lettopalena rockslide avalanche deposits outcropping at the base of the
Maiella forelimb are of interest (Fig. 2a). The rock mass volume involved is on the order of
106 m3 and includes meter-scale angular-shaped rock blocks, with a
sandy matrix, arranged in a chaotic texture and an inverse grading (Paolucci
et al., 2001; Di Luzio et al., 2004; Bianchi Fasani et al., 2014). The
origin of the Lettopalena rock avalanche has been linked to deep-seated
gravitational slope deformation (Di Luzio et al., 2004; Bianchi Fasani et
al., 2014) or to earthquake triggering, as initially proposed by Paolucci et al. (2001), who reproduced the rockslide considering a M 5.5 event. Well-preserved buried wood was found within the landslide body. According to
specific analyses reported in Paolucci et al. (2001), the fragment belongs
to an inner-lateral portion of a Quercus ilex trunk that was dated around 4800 ± 60 yr BP
(can be considered a “radiocarbon age”, corresponding to
ca. 5.3–5.6 kyr BP 2σ calibrated age). Due to the importance of this
datum, we performed a new radiocarbon dating of the wood sample (preserved
at the geological museum in the village of Palena, Italy) obtaining a 2σ calibrated age of 5445–5300 yr BP (Table 1), which is hence slightly younger than
that of Paolucci et al. (2001). However, considering that the sample comes
from the inner part of a long-lived tree (i.e., Quercus ilex, which can often reach the age of
1000; e.g., Pignatti, 1982), it is clear that the obtained age is not
that of the rock slide; instead, it simply corresponds to the age of the
inner part of the tree. Therefore, it is possible that the rock avalanche
could have occurred some hundreds years later than 5445–5300 yr BP (that is,
when the tree was likely 600–700 years old), in a time span strictly
comparable with the age of the speleothem rupture in the Cavallone Cave
(Fig. 7).
Although we cannot assess for certain whether the two events were perfectly
coeval, the comparable age range of these exceptional and catastrophic
events suggests that both the cave damages and the Lettopalena rock
avalanche may represent the effects of a strong paleoearthquake. Therefore,
we searched for paleoseismological evidence along the major seismogenic
sources in the vicinity of the Maiella Massif. This evidence is represented by the
SW-dipping Sulmona Fault, to the west, and the thrust faults, to the east.
Regarding the extensional domain to the west of the Maiella Massif,
paleoseismological trenches excavated along the northwestern portion of the
Sulmona active normal fault allowed for four paleoearthquakes to be recognized (Galli
et al., 2015). One of them is corroborated in two trenches by the presence of a
faulted colluvium. The ages of the involved material are included within
5580–3370 yr BP. Combining the dating coming from the two
trenches, the authors defined the most probable earthquake interval to be around
4800–4300 yr BP (Galli et al., 2015). This age surprisingly matches with the
speleothem and rock avalanche dating, showing a strong correspondence
between speleoseismological and classical paleoseismological analyses,
identifying a mid-Holocene earthquake around 4.6–4.8 kyr BP that was likely
responsible for both the high cave damages found in the Cavallone Cave and
the Lettopalena rock avalanche (Fig. 7a).
Moreover, faulting events with a comparable mid-Holocene age have been
recognized along the Rotella–Aremogna normal fault system (composed of a set
of sub-parallel, closely spaced fault segments: Pizzalto, Rivisondoli,
Cinque Miglia and Aremogna faults; Fig. 7b; see Tesson et al., 2016, and
references therein), located along the southeastern prosecution of the
Sulmona Fault. Paleoseismological data collected along the Rivisondoli
normal fault allowed for defining an earthquake post-dating the deposition of a
paleosol dated 5415–5324/5585–5443 yr BP (2σ calibrated age interval
obtained from the conversion of the 4730 ± 50 yr BP 14C age
reported in Calderoni et al., 1990, and Brunamonte et al., 1991; calculations
were performed using the CALIB 7.0 software by Stuiver et al., 2016) and
then overlapping the Sulmona and Cavallone Cave dating (Fig. 7). A nearly
coeval paleoearthquake resulted from paleoseismological investigations along
the Aremogna normal fault (D'Addezio et al., 2001; Fig. 7), where two
paleosols pre-date (3735–3365 yr BC 2σ calibrated age, corresponding
to 5300–4890 yr BP) and post-date (3735–3365 yr BC 2σ calibrated
age, corresponding to 5685–5315 yr BP) the event, respectively (Fig. 7a).
Concerning the contractional domain to the east of the Maiella Massif,
although active structures are known (e.g., de Nardis et al., 2011, and
references therein), due to the scarcity of the available data we cannot
assess whether a strong earthquake related to thrust faulting occurred in the
same investigated time interval. Therefore, further studies are needed to
deepen this issue.
Discussion and conclusions
The use of speleotectonics for the recognition of paleoearthquakes is highly
debated in the literature both because there are many possible breakdown
causes that must be discounted (e.g., Gilli, 2005) and because the required
conditions to break speleothems, in terms of speleothems vulnerability and
seismic input, are still unclear (e.g., Cadorin et al., 2001; Lacave et al.,
2004). On the other hand, peculiar features, recognized in many caves
worldwide, have been often linked to earthquake damages (e.g., Postpischl et
al., 1991; Ferranti et al., 1997; Lemeille et al., 1999; Delaby, 2001;
Forti, 2001; Kagan et al., 2005; Šebela, 2008; Panno et al., 2009;
Bábek et al., 2015; Méjean et al., 2015), although specific
paleoearthquakes identified in caves have been rarely constrained by
independent data outside caves (Becker et al., 2005). The speleotectonic
analysis conducted in this study within the Cavallone Cave (in the most
external portion of the Central Apennines) allowed for recognizing a
mid-Holocene paleoearthquake and finding robust correlations with
independent on-fault and off-fault data in surrounding areas. The seismic history
of this region of the Apennines and the characterization of possible
seismogenic sources are still debated issues, even more so because the study
area remains interposed between two seismotectonic domains (extensional to
the west and contractional to the east).
(a) Comparison between the ages resulting from radiocarbon
dating of on-fault primary and off-fault secondary paleoseismic evidence
recorded in the Maiella surroundings. One of the colluvial wedges that faulted
along the Sulmona Fault reveals an earthquake within 4800–4300 yr BP (Galli
et al., 2015). Paleoevents with similar ages have also been recognized along
the Rivisondoli and Aremogna fault segments belonging to the
Rotella–Aremogna normal fault system (Calderoni et al., 1990; Brunamonte et
al., 1991; D'Addezio et al., 2001). Dating performed in this study in the
Cavallone Cave (light bar) on the broken stalagmite and the new re-growing
one, overlap in the range included between 4815 and 4645 yr BP (dark bar).
The wood found within the Lettopalena rock avalanche predates the rock slide
event which occurred after 5445–5300 yr BP. This dating matches with
around 4.6–4.8 kyr BP, constraining the age of a mid-Holocene
paleoearthquake. (b) Main SW-dipping active normal faults in the vicinity of Maiella. Symbols mark comparable ages (matching around 4.6–4.8 kyr BP)
resulting from the dating of coseismic faulting along the Sulmona Fault
(SF), the Rivisondoli Fault (RF) and Aremogna Fault (ArF) segments; the broken
speleothem within the Cavallone Cave; and the Lettopalena rock avalanche (LRA).
The synchronous activation of the Sulmona Fault with one or more normal
fault segments to the south, could justify the large off-fault secondary
effects recorded in the Maiella area (Cavallone Cave damages and Lettopalena
rock avalanche). See the text for further explanation. PF: Palena Fault; WPF: western Porrara Fault;
PiF: Pizzalto Fault; CMF: Cinque Miglia Fault; SV: Sangro–Volturno persistent structural barrier. PiF, RF, CMF and ArF all
belong to the Rotella–Aremogna normal fault system.
Many interesting features that can be associated with sudden and catastrophic
events have been recognized in the Cavallone Cave. In any case, because we cannot
select only one cause for the analyzed broken speleothem, we must eliminate
all other possibilities. Large glacial intrusions in the Cavallone Cave can
be excluded since the climate around 5500–4500 yr BP is considered a warm
period (e.g., Walker et al., 2012, and references therein), as also suggested by
the presence of Quercus ilex discovered within the Lettopalena rock avalanche deposit
dated by Paolucci et al. (2001), indicating the presence of an evergreen oak
forest and Mediterranean climate conditions in the cave surroundings.
Moreover, the dated stalagmite is located quite far from the entrance of the
cave, where the ice could not have acted.
Fracturing due to catastrophic floods can also be excluded because there is
no evidence of impacts on the concretions and related deposits. Moreover, falling
azimuths of speleothems are clustered in two directions and the fallen
tips of the stalagmites match with the stumps and lie uphill in their
vicinity, suggesting the absence of transport linked to hydrodynamic floods
or ice.
Most examples of the standing broken stalagmites were found on
sub-horizontal cave ground, suggesting that their deformation is not related
to slope instability or sediment creep. An exception can be found within the
first branch of the conduit (Galleria della Devastazione), where the presence
of meter-scale broken concretions, resting tilted toward the axis of the
conduit, suggests that gravitational processes may have caused the local
collapse of the floor (Fig. 3d and e), which in turn could also have been
triggered by a seismic event.
As the entrance of the cave is on a near-vertical
cliff, more than 100 m high, human and animal access seems to have been unlikely at
the time of the dating event. Even if it could have been possible, human and
animal disturbance can be excluded for the portions of the conduit where
paleoseismological analysis has been performed. These sectors are
too far from the entrance (∼ 1 km) and, in addition, they would have been
too hard to reach because of the articulate morphology of the cave
ground (Fig. 2d).
Although gravitational processes can play an important role in defining the
stability and configuration of the cave (as in the case of the Galleria
della Devastazione; Fig. 3d and e), ceiling collapses, broken
speleothems (both stalactites and stalagmites), preferential orientation of
fallen stalagmites, and the absence of thin and long concretions observed in
many portions of the conduit indicate that the cave also suffered from sudden
deformation events likely linked to the occurrence of earthquakes. Two
preferential trends of fallen tips of broken stalagmites emerge as
parallel and perpendicular to the main known NW–SE-trending normal faults
affecting this area of the Central Apennines. Moreover, a large number of stalagmites that resemble seismothems (Delaby, 2001; Forti, 2001) have been recognized.
They are cut along subhorizontal planes, with the upper part lying close to their
base and cemented on the floor and with a new speleothem generation re-growing onto the stumps.
Stalagmites a few centimeters high
are widespread in the cave, also growing on top of both collapsed deposits
and broken speleothems, likely suggesting the occurrence of a sudden event
after which concretions started to regrow. The seismothem chosen for dating
(Fig. 5b) showed a rupture age around 4730 ± 85 yr BP (Fig. 6).
Moreover, as the regrowth (and still growing) stalagmite is ca. 1.5 cm high
and its base has been dated at around 4.8–4.6 kyr BP (Fig. 6b), we can
estimate a growth rate of 0.003 mm yr-1 for the new stalagmite generation. The
obtained value is quite different from the mean rates recognized in the
literature that vary between 0.015 and 0.37 mm yr-1 (e.g., White, 2007; Akgöz
and Eren, 2015, and references therein). This may be explained by variations
in the growth rate of stalagmites, as well as by complexities in their
structure, texture, and chemical composition (e.g., Akgöz and Eren, 2015) or
by peculiar geochemical processes governing the cave environment. Moreover, the
observed evidence of surface erosion and internal cavities in several
stalagmites strongly suggests the occurrence of periods with a slow
rate of calcite accumulation and falling-drop erosion.
Empirical relations and quantitative modeling linking dimensions of
speleothems and peak ground accelerations are difficult to be applied for
the dated stalagmite, as too high accelerations (PGA > 10 m s-2) would have been needed to break it (e.g., Cadorin et al., 2001;
Lacave et al., 2004). The answer to this question can be found considering
that these correlations do not take into account some parameters and
conditions such as structural and chemical compositional variability,
presence of pre-existing discontinuities and/or anomalies within the
concretions, and seismic site response (e.g., Delaby, 2001; Kagan et al.,
2005), which can significantly influence the mechanic behavior and the
strength of the material. In our case the analyzed stalagmites show
a peculiar porous inner structure due to the presence of millimeter- to
centimeter-sized holes (macroholes in Fig. 6a; Shtober-Zisu et al., 2012).
We believe that the widespread distribution of these voids within a
concretion could make it weaker and thus more susceptible to rupture during
seismic shaking. Therefore, the ways through which speleothems convey
seismic shaking still need to be understood, and more in situ and laboratory studies
are needed to evaluate peak ground accelerations required to break porous
speleothems.
In summary, all the data collected within the Cavallone Cave allow for the occurrence of past earthquakes to be considered plausible.
In this kind of study, the correlation of speleoseismological data with
independent geological data collected outside the cave may represent the key
to constrain the results (Becker et al., 2005, 2006). In our case, the
dating of the seismothem analyzed in the Cavallone Cave finds very strong
correlations with other independent phenomena that occurred in surrounding areas.
Both the rock avalanche affecting the Lettopalena village (off-fault
secondary earthquake evidence) and the coseismic faulting along the Sulmona
normal fault (Galli et al., 2015; on-fault primary earthquake evidence) show
radiocarbon ages that are strictly comparable with the dating performed on
the broken stalagmite (off-fault secondary earthquake evidence; Fig. 7a).
Similar ages are also obtained from paleoseismological investigations along
the Rivisondoli (Calderoni et al., 1990; Brunamonte et al., 1991) and
Aremogna (D'Addezio et al., 2001) normal faults (Fig. 7). Strong earthquakes
associated with buried thrust fronts to the east may have also occurred in the
same time interval, causing similar high damages. Although these thrusts
have been proposed as alternative seismogenic sources for the 1706 (Maw= 6.6)
and 1933 (Mw= 5.7) Maiella earthquakes (de Nardis et al., 2011, and references
therein), at present, paleoseismological and seismological data are
inadequate to reliably relate them to strong earthquakes and further
investigations are needed to constrain their role within the seismotectonic
framework of the Maiella area.
Based on the available data we therefore suggest that the mid-Holocene
paleoearthquake that occurred along the Sulmona normal fault may represent the
best candidate for the causative event that produced the Cavallone Cave
damages and also triggered the Lettopalena rock avalanche. Thus, following
this hypothesis, the paleoearthquake can be constrained to within a time span of
4.6–4.8 kyr BP (Fig. 7a).
According to Galli et al. (2015), the 4.6–4.8 kyr BP earthquake can be
considered the penultimate event for the Sulmona seismogenic structure,
where the last event occurs in the 2nd century AD. Our study suggests
that the 4.6–4.8 kyr BP earthquake may have been stronger than the Maw= 6.3
2nd century earthquake. No other comparable large cave damages and rock
avalanches are known to have occurred after the 4.6–4.8 kyr BP
event at the Maiella Massif, even during the more recent strong earthquakes
recorded in 1706 (Maw= 6.6) and in 1933 (Mw= 5.7), although the Maiella
forelimb is characterized by a near-uniform morpho-structural setting
(i.e., ca. 30–40∘ southeastward dipping) that by itself can favor
gravitational slope instability. Furthermore, many studies indicate that,
globally, more than 40 % of rock avalanches that dammed streams (as in the
case of the Lettopalena landslide with respect to the Aventino River; Fig. 2a) were caused by earthquake shaking (Costa and Schuster, 1991; Jibson,
1996) and have been associated, in the New Zealand area, with M≥ 6.5
earthquakes (Perrin and Hancox, 1992). The Sulmona Fault is potentially able
by itself to produce M 6.6–6.7 earthquakes (e.g., Gori et al., 2011), large
enough to trigger rock avalanches such as that of Lettopalena. On the other
hand, the cave damages associated with the 4.6–4.8 kyr BP earthquake seem to
be the last of such an extent within the Cavallone Cave, even if the study area
was affected by other strong seismic events after the 4.6–4.8 kyr BP
earthquake. A larger event (M > 6.6–6.7) can therefore be
hypothesized considering a synchronous activation of the Sulmona Fault
together with one or more adjacent active fault segments (Figs. 1 and 7b).
This is supported by paleoseismological investigations in trenches that
allow for identification of the 4.6–4.8 kyr BP paleoearthquake along the Sulmona
normal fault, in a time span comparable with the Rivisondoli (Calderoni et
al., 1990; Brunamonte et al., 1991) and Aremogna paleoevents (D'Addezio et
al., 2001). Moreover, Tesson et al. (2016) recognized an event along the
Pizzalto normal fault segment with an age (around 1.7–1.9 kyr) similar to
the more recent 2nd century AD earthquake recorded along the Sulmona Fault
(Galli et al., 2015). These paleoseismological data suggest that distinct
fault segments of the Rotella–Aremogna fault system can be reactivated at
different times and in different ways and that they can be repeatedly activated
synchronously with the Sulmona normal fault (Fig. 7b). In this context, a
total cumulative rupture length of about 40–50 km (up to the intersection
with the Sangro–Volturno thrust zone; Figs. 1 and 7b) could have occurred
during the 4.6–4.8 kyr BP event, likely producing a M∼ 7 earthquake,
according to scaling relationships relating surface rupture length vs.
magnitude (e.g., Wells and Coppersmith, 1994). Therefore, the synchronous
activation of the Sulmona Fault together with adjacent fault segments to the
southeast may explain both the higher magnitude of the resulting earthquake
and why large off-fault secondary effects have been recorded in the southern
Maiella area (i.e., Lettopalena rock avalanche and broken stalagmites observed
at the Cavallone Cave). Nevertheless, further paleoseismological studies are
needed to constrain the exact length of the surface rupture trace, the
possible associated magnitude, the relations between adjacent seismogenic
faults, and the modes and times of their synchronous activation. The way
through which adjacent seismogenic structures interact is, at present, one
of the main issues for seismically active regions such as the Apennines,
where the August–October 2016 seismic sequence also showed complex
deformation and rupture patterns involving distinct active normal fault
segments (e.g., Pucci et al., 2016).
In conclusion, this study compares on-fault primary and off-fault secondary
effects related to a single paleoevent, showing a strong correspondence
between speleoseismological and classical paleoseismological analyses. Our
observations indicate that major coseismic off-fault secondary effects
recorded in the external zone of the Central Apennine chain (i.e., at the
transition zone between inner post-orogenic extension and outer
chain/foreland deformation) are likely ascribed to seismogenic normal faults
which are capable of producing M≤ 7 earthquakes, instead of
contractional structures capable of M≤ 6 events. This is corroborated by
the historical seismicity of the Central–Northern Apennines, which shows that the major
earthquake hazard in the Maiella region is linked to normal faulting
(e.g., Working Group CPTI, 2004; Rovida et al., 2011). Nevertheless, it should be noted that, at present, due to the scarcity of paleoseismological data, the
seismic potential associated with buried contractional seismogenic sources has still not been revealed, and further studies are needed to constrain both the
seismogenic structures responsible for other events, such as those of 1706 and 1933, and the seismic hazard associated with this transitional zone.
The authors declare that they have no conflict of interest.
Acknowledgements
We would like to thank the Speleo Club Chieti, the municipality of Taranta
Peligna and the Parco Nazionale della Maiella for supporting this study and
providing the map of the Cavallone Cave. We are grateful to the tourist
guides of the Cavallone Cave; to Raffaele Madonna for availability and logistics; to Pierpaolo Ciuffi and Simone D'Agostino
for their help in data collection; to Paolo Forti, Ezio Burri
and Jo De Waele for the useful discussions in the cave; and to
Gabriele Scarascia Mugnozza, Gianluca Bianchi Fasani, Silvano Agostini and the Museo di Palena, who helped us with recovering and dating the
wood sample. The constructive review by Alessandro Maria Michetti allowed
for improvements to the quality of the manuscript and is gratefully acknowledged. We
also thank Luigi Ferranti and an anonymous reviewer for their comments
and suggestions. We dedicate this work to the memory of our colleague Paolo Scandone,
one of the greatest experts in Apennines geology and a lover of Maiella. This
research was financially supported by the Italian Ministry for Education,
University and Research (MIUR) (ex 60 % grants to Alberto Pizzi).
Edited by: F. Rossetti
Reviewed by: A. M. Michetti, L. Ferranti, and one anonymous referee
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