NHESSNatural Hazards and Earth System SciencesNHESSNat. Hazards Earth Syst. Sci.1684-9981Copernicus PublicationsGöttingen, Germany10.5194/nhess-16-737-2016Boulder accumulations related to extreme wave events on the eastern coast of MaltaBiolchiSarahttps://orcid.org/0000-0003-2446-3627FurlaniStefanoAntonioliFabrizioBaldassiniNiccolóCauson DeguaraJoannaDevotoStefanosdevoto@units.itDi StefanoAgataEvansJulianGambinTimothyGauciRitiennehttps://orcid.org/0000-0002-4496-3138MastronuzziGiuseppeMonacoCarmeloScicchitanoGiovanniDipartimento di Matematica e Geoscienze, Università di Trieste, Via Weiss 2, 34127 Trieste, ItalyENEA, UTMEA, Casaccia, Rome, ItalyDipartimento di Scienze Biologiche, Geologiche e Ambientali, Sezione Scienze della Terra, Università di Catania, Corso Italia 57, 95129 Catania, ItalyDepartment of Geography, University of Malta, 2080 Msida, MaltaDepartment of Biology, University of Malta, 2080 Msida, MaltaDepartment of Classics and Archaeology, Archaeology Centre, University of Malta, 2080 Msida, MaltaDipartimento di Scienze della Terra e Geoambientali, Via Orabona 4, Università di Bari, 70125 Bari, ItalyStudio Geologi Associati T. S. T., Via Galliano 157, Misterbianco (Ct), ItalyStefano Devoto (sdevoto@units.it)16March20161637377562September20156October20152March20163March2016This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from https://nhess.copernicus.org/articles/16/737/2016/nhess-16-737-2016.htmlThe full text article is available as a PDF file from https://nhess.copernicus.org/articles/16/737/2016/nhess-16-737-2016.pdf
The accumulation of large boulders related to waves generated by either
tsunamis or extreme storm events have been observed in different areas of
the Mediterranean Sea. Along the eastern low-lying rocky coasts of Malta,
five sites with large boulder deposits have been investigated, measured and
mapped. These boulders have been detached and moved from the nearshore and
the lowest parts of the coast by sea wave action. In the Sicily–Malta
channel, heavy storms are common and originate from the NE and NW winds.
Conversely, few tsunamis have been recorded in historical documents to have
reached the Maltese archipelago.
We present a multi-disciplinary study, which aims to define the
characteristics of these boulder accumulations, in order to assess the
coastal geo-hazard implications triggered by the sheer ability of extreme
waves to detach and move large rocky blocks inland.
The wave heights required to transport 77 coastal boulders were calculated
using various hydrodynamic equations. Particular attention was given to the
quantification of the input parameters required in the workings of these
equations, such as size, density and distance from the coast. In addition,
accelerator mass spectrometry (AMS) 14C ages were determined from selected samples of marine organisms
encrusted on some of the coastal boulders. The combination of the results
obtained both by the hydrodynamic equations, which provided values
comparable with those observed and measured during the storms, and
radiocarbon dating suggests that the majority of the boulders have been
detached and moved by intense storm waves. These boulders testify to the
existence of a real hazard for the coasts of Malta, i.e. that of very high
storm waves, which, during exceptional storms, are able to detach large
blocks of volumes exceeding 10 m3 from the coastal edge and the
nearshore bottom, and also to transport them inland. Nevertheless, the
occurrence of one or more tsunami events cannot be ruled out, since
radiocarbon dating of some marine organisms did reveal ages which may be
related to historically known tsunamis in the Mediterranean region, such as
the ones in AD 963, 1329, 1693 and 1743.
Introduction
The central part of the Mediterranean Sea often experiences strong winds and
high waves related to marine storms, which are common during both the winter
and autumn seasons. In recent years they have been linked to an increase in
the occurrence of violent “tropical-like cyclone” events (Emanuel, 2005;
Fita et al., 2007; Lionello et al., 2006). Such a type of storm waves may
represent a severe geo-hazard for inshore facilities and related land use
development, as evidenced by the recent impact of severe storms on the
coasts of Apulia (southern Italy) which caused extensive flooding both on
Ionian and Adriatic sides. In historical and recent times, tsunamis of
impressive heights have been recorded to have hit some parts of the
Mediterranean coasts. The 1908 earthquake-generated tsunami that struck the
coasts of Calabria and Sicily in southern Italy developed waves up to 13 m
above sea level, contributing to heavy destruction and 1500 casualties
(Bertolaso et al., 2008). More recently, in 2006, the collapse of
Sciara del Fuoco along the flanks of the Stromboli volcano island generated waves that
destroyed harbour structures and other facilities on the island and
along the Tyrrhenian coasts of Calabria and Sicily (Mastronuzzi et al., 2013a).
In the Maltese archipelago the recorded tsunamis (Tinti et al., 2004) are
the ones of AD 1169, 1693 (described locally also by Agius de Soldanis in
1746) and 1908. The latter is well known to have affected the eastern
coast of Malta (Pino et al., 2008) and the southern coast of Gozo with a
wave height ranging between 0.72 and 1.50 m (Guidoboni and Mariotti (2008)
and references therein). These events have been triggered by earthquakes
which occurred in eastern Sicily. Moreover, according to the model provided
by Tinti el al. (2005), a potential tsunami impacting on the Maltese coasts
may be generated either by an earthquake of Mw= 7.4 along an
offshore fault located parallel to the Malta Escarpment (the former
considered as one of the possible sources for the AD 1693 earthquake) with a
wave height of 0.15–1 m, or by an earthquake from the western Hellenic Arc
(Mw= 8.3) with marginally higher wave heights (1–1.5 m).
One of the most impressive pieces of evidence of extreme wave impact on the
Mediterranean coasts is represented by the occurrence of mega-boulders,
sparse or in fields or berms whose accumulations have been attributed both to
tsunamis and storm events (Mastronuzzi and Sansò, 2000, 2004;
Mastronuzzi et al., 2007; Scicchitano et al., 2007, 2012; Barbano et al.,
2010; Vacchi et al., 2012; Raji et al., 2015). Notwithstanding the
impressive growth in the last 50 years of studies aimed to develop an
appropriate methodology, which may (in the absence of field witnesses) link
these boulder deposits to a well-defined process (Williams and Hall, 2004;
Hall et al., 2006; Mastronuzzi et al., 2006; Scheffers and Scheffers, 2006;
Pignatelli et al., 2009; Goto et al., 2010), no undisputed consensus has yet
been reached on how to differentiate between the boulders accumulated by a
sea storm from those deposited by a tsunami. Some studies point to the
presence of boulders and their size, to evaluate the characteristics of the
impacting waves (i.e. Mastronuzzi and Pignatelli (2012); Mastronuzzi et al. (2013b)
and references therein). An important degree of uncertainty lies in
this methodology due to the definition of the origin of the wave responsible
for the deposition of the boulders. The hydrodynamics of boulder emplacement
and transport to the shore platform have been dealt with, among others, by Nott (1997,
2003) and Noormets et al. (2004). Nott attributes the force required
to transport boulders to wave height and proposes a straightforward method
to determine if storm or tsunami waves are responsible for their
emplacement. In the equation developed by Noormets, hydrodynamic forces at
the low submerged shoreline cliff are computed using design wave
characteristics, based on linear wave theory and experimental results, which
include also the local wave climate, nearshore bottom topography and
initial fracturing of cliff rocks. More recently, research attention has
shifted its focus to the role of impacting wave height compared to the wave
length and to the wave period. Different theories have been proposed (Goto
et al., 2007, 2009, 2010; Hansom et al., 2008; Imamura et al., 2008;
Pignatelli et al., 2009; Nandasena et al., 2011), suggesting that in order to
evaluate the wave impact on a rocky coast, these parameters should be
considered all together.
The eastern coast of the island of Malta is characterised by the occurrence
of deposits of anomalous calcareous boulders (Furlani et al., 2011;
Mottershead et al., 2014; Causon Deguara, 2015). Their surface is frequently
covered by biogenic encrustations, which indicate without any doubt that
they were detached from the mid- or sublittoral zone.
The aims of this paper are to identify the physical processes responsible
for the accumulation of the coastal boulders and to understand whether these
boulders can be a potential geo-hazard for the low-lying rocky coasts of Malta.
The study area
The Maltese archipelago is located in the Sicily–Malta channel (central
Mediterranean Sea, Fig. 1a), 90 km south of Sicily and 290 km north of
Libyan coasts, and consists of three main islands, namely Malta (245.7 km2),
Gozo (67.1 km2) and Comino (2.8 km2). From a
geo-tectonic point of view, together with the Hyblean Plateau (SE Sicily),
the archipelago belongs to the Pelagian Block (Grasso and Pedley, 1985), the
northernmost sector of the African plate, mostly composed of foreland
Neogene carbonate successions (Patacca et al., 1979). Eastwards, the
Pelagian Block is bounded by the Malta Escarpment, a Mesozoic passive margin
separating the continental domain from the oceanic crust of the Ionian Basin
(Scandone et al., 1981; Makris et al., 1986). Since the middle Pleistocene,
it has been locally reactivated by normal faulting (Argnani and Bonazzi,
2005), related to a regional WNW–ESE-oriented extension (Monaco et al.,
1997; Bianca et al., 1999; Palano et al., 2012) (Fig. 1a). It is marked by a
high level of crustal seismicity, producing earthquakes with intensities of
up to XI–XII MCS and M∼ 7, such as the 1169, 1693 and 1908 events
(Baratta, 1901; Postpischl, 1985; Boschi et al., 1995).
(a) Geodynamical setting of the Maltese archipelago (redrawn from
Cultrera et al., 2015); (b) earthquakes felt on the island according to
historical records (Galea, 2007) and the attested tsunamis on Malta
according to De Soldanis (1746), Tinti et al. (2005) and Galea (2007),
Bertolaso et al. (2008). Source: http://pubs.er.usgs.gov/publication/ofr20101083Q.
The Sicily–Malta channel underwent transtensional processes during the
Neogene–Quaternary times, which led to the development of the Pantelleria,
Linosa and Malta grabens, partially filled by Pliocene–Pleistocene
sediments (Finetti, 1984). These structures are mostly bounded by NW–SE-trending subvertical conjugate normal faults (Fig. 1a) whose activity would
have reached the acme at approximately 5 Ma. Reactivation of the fault systems
accommodated SW–NE extension in the late Quaternary (Corti et al., 2006;
Catalano et al., 2009). As revealed by an available seismic database (INGV,
http://emidius.mi.ingv.it/DBMI11), the above-cited structural features are
a source of moderate seismicity, mostly located in the Linosa graben, with
shallow events (h< 25 km) and a magnitude usually from 2 to 4 (Civile
et al., 2008).
Several earthquake-generated tsunamis struck the Ionian coast of
south-eastern Sicily and the Maltese archipelago in historical times such as
in AD 1169, 1693 and 1908 (Tinti et al., 2004) (Fig. 1b).
According to published geological data and numerical modelling, the
seismogenic source of these events should be located in the Strait of Messina
and in the Ionian offshore (the Malta Escarpment) between the towns of
Catania and Siracusa (e.g. Piatanesi and Tinti, 1998; Bianca et al., 1999;
Monaco and Tortorici, 2000; Azzaro and Barbano, 2000). On the other hand, we
must consider that many tsunamis that have occurred in the Mediterranean Sea have been
generated in the Hellenic Arc area (i.e. Vött et al. (2010), Mastronuzzi
et al. (2014) and references therein).
From a geomorphological point of view, the southern and western sectors of
the Malta island are characterised by subvertical cliffs increasing in
height northwards (mean heights of 100–120 m in the southern tract, and
of 200–225 m along the western coast of the island, close to the Great
Fault). Low-lying coasts are dominant on the eastern and north-eastern parts
of Malta, showing a system of surf bench and wave-cut platforms, some of
which host boulder accumulations. This difference in the coastal morphology
is linked to the development of a northeastwards tilting in response to the
fault system activity, also responsible for the forcing of the surface
waters in a WSW–ENE direction and the formation of NE-oriented fluvial
valleys (Alexander, 1988; Biolchi et al., 2016). Wide sectors of NW coast of
Malta are characterised by the presence of extensive landslides, mainly rock
spreads and block slides (Devoto et al., 2012, 2013; Piacentini et al.,
2015). These slow-moving landslides detach and move hundreds of limestone
blocks from the karst plateaus towards the sea, forming peculiar coastal
landforms named “rdum” by locals. Conversely, slow-moving landslides and related
slope-failure accumulations are not common on the NE coast, although rock
spreads and block slides have been recognised and investigated on the
northern side of Xemxija Bay by Panzera et al. (2012).
The submerged landscape is mainly composed of flat to gently sloping terrain
and comprises coastal landforms, such as fault-related scarps, palaeo shore
platforms, palaeo shoreline deposits and slope-failure deposits, as well as
terrestrial landforms, such as river valleys, alluvial plains, karstified
limestone plateaus and sinkholes (Micallef et al., 2013).
The Maltese sedimentary succession mainly consists of pelagic limestones,
clayey terrains and marls (Pedley et al., 1976, 1978). As illustrated in
Fig. 2, it includes five formations: (1) Lower Coralline Limestone formation (LCL),
consisting of late Oligocene (Brandano et al., 2009) bioclastic
limestones; (2) Globigerina Limestone formation (GLO), late Oligocene to
middle Miocene in age (Baldassini et al. (2013), Baldassini and Di Stefano (2015)
and reference therein), consisting of pelagic marly limestones. It is
subdivided, based on the occurrence of phosphoritic conglomerate beds
(Baldassini and Di Stefano, 2015), into three members; (3) Blue Clay formation (BC),
middle to late Miocene in age (Giannelli and Salvatorini, 1975; Hilgen
et al., 2009), formed by silty marlstones; (4) Greensand formation, late
Miocene (Tortonian) in age (Giannelli and Salvatorini, 1975), consisting of
greenish marly/clayey glauconite sands and arenites; (5) Upper Coralline
Limestone formation (UCL), late Miocene in age (Giannelli and Salvatorini,
1975), consisting of shallow-water bioclastic limestone deposits.
Geological map of Malta and location of the boulder deposits
(redrawn from Oil Exploration Directorate, 1993; Devoto et al., 2012).
Hydrodynamic equations (a, b and c for major, medium and minor axis
respectively; ρb is the boulder density; ρw is the sea water
density; CL is the lift coefficient, which is 0.178; θ is the bed slope
angle; μ is the coefficient of static friction, which is 0.65; V is the boulder
volume; CD is the coefficient of drag, which is 1.95; q is the boulder area
coefficient, which is 0.73).
EquationJoint bounded scenarioSubmerged/subaerialscenario (saltation)Pignatelli et al. (2009) TsunamiHT>0.5c⋅ρbρw- 1CL–Pignatelli et al. (2009) StormHS>2c⋅ρbρw- 1CL–Nandasena et al. (2011) TsunamiHT>0.5c⋅ρbρw- 1⋅cosθ+μsenθCLHT≥0.5c⋅ρbρw- 1⋅cosθCLNandasena et al. (2011) StormHS>2c⋅ρbρw- 1⋅cosθ+μsenθCLHS≥2c⋅ρbρw- 1⋅cosθCLEngel and May (2012) TsunamiHT≥0.5μVρbCDa⋅c⋅qρw–Engel and May (2012) StormHS≥2μVρbCDa⋅c⋅qρw–
N-type Schmidt Hammer R values performed on boulder accumulations
situated along the eastern coast of Malta. The densities of boulders were
determined by the formula developed by Katz et al. (2000).
BoulderGeologicalLocationDateAveragedDensityno.formationHR(kg m-3)AB1UCLArmier Bay18 May 2014351780AB2UCLArmier Bay18 May 2014371850AB3UCLArmier Bay18 May 2014311620AB4UCLArmier Bay18 May 2014361810AB5UCLArmier Bay18 May 2014321660MAS newUCLArmier Bay18 May 2014301580AB7UCLArmier Bay18 May 2014321660AA1UCLAhrax Point30 Jan 2015261390AA9UCLAhrax Point30 Jan 2015311620AA11UCLAhrax Point30 Jan 2015321660AA12UCLAhrax Point30 Jan 2015432050AA14UCLAhrax Point30 Jan 2015321660B1LCLBahar iċ-Ċaghaq18 May 2014331700BICGLOBahar iċ-Ċaghaq30 Jan 2015261390QW1LCLQawra peninsula29 Jan 2015151670QW2LCLQawra peninsula29 Jan 2015341740QW3LCLQawra peninsula29 Jan 2015381880LB1LCLBuġibba29 Jan 2015331700LB2LCLBuġibba30 Jan 2015411980LB3LCLBuġibba30 Jan 2015442080LB4LCLBuġibba30 Jan 2015311620LB5LCLBuġibba30 Jan 2015371850LB6LCLBuġibba30 Jan 2015422020LB7LCLBuġibba30 Jan 2015411980LB8LCLBuġibba30 Jan 2015341740LB9LCLBuġibba30 Jan 2015331700LB10LCLBuġibba30 Jan 2015432050P1LCLPembroke30 Jan 2015502240P3LCLPembroke30 Jan 2015482190P4LCLPembroke30 Jan 2015442080P7LCLPembroke30 Jan 2015442080Z1LCLŻonqor30 Aug 2014291530Z2GLOŻonqor30 Aug 2014331700Z3GLOŻonqor30 Aug 2014331700Z4GLOŻonqor30 Aug 2014341739Z5GLOŻonqor30 Aug 2014341739Z6GLOŻonqor30 Aug 2014341739Z7GLOŻonqor30 Aug 2014331700Z8GLOŻonqor30 Aug 2014381884Z9GLOŻonqor30 Aug 2014341739Z10GLOŻonqor30 Aug 2014341739Z11GLOŻonqor30 Aug 2014341739Z12GLOŻonqor30 Aug 2014281485Z13GLOŻonqor30 Aug 2014371850Z14MixedŻonqor30 Aug 2014351777Z15MixedŻonqor30 Aug 2014401752Material and methods
To identify and map the boulder accumulations, field surveys were carried
out along the eastern low-lying coasts of Malta (Fig. 2). Some of these
sites had already been recognised and categorised by various authors: Armier
Bay by Furlani et al. (2011) and Biolchi et al. (2016); Ahrax Point,
Pembroke and Xghajra by Mottershead et al. (2014); and Żonqor by
Causon Deguara and Gauci (2014) and Causon Deguara (2015).
The most representative boulders, in terms of size, shape, distance from the
coastline and presence of encrusted marine bioforms, were chosen for further
analysis. The 77 selected boulders included the largest observed blocks,
slab-like, roughly cubic and rectangular, as well as assembled and isolated ones.
To verify whether the detachment of the boulders is due to the storm wave
regime of the area or a tsunami-related event, a hydrodynamic approach was
applied in this study. In particular, we applied the well-known and accepted
equations developed by Pignatelli et al. (2009), Nandasena et al. (2011) and
Engel and May (2012), in order to calculate the minimum tsunami and storm
wave heights required to detach a boulder from the coast edge or the
nearshore environment (Table 1).
Direct measurements on each boulder were carried out to determine size,
imbrication direction and distance from the shoreline, whilst the density
was determined by means of the N-type Schmidt hammer (SH). The latter is a
field instrument (Viles et al., 2011) to determine rock physical properties
(intact rock strength and densities) by means of non-destructive testing
(Yilmaz and Sendir, 2002). Katz et al. (2000) correlated an index named
Hammer Rebound (HR), which is a function of the resistance of surface material
hit by the SH to the density of the rocks. The boulder density was
associated to averaged HR value assigned to each block by means of Eq. (1):
ρ=1308.2ln(HR)-2873.9,
where ρ is the density unit expressed in kg m-3.
To avoid interferences due to the occurrence of discontinuities, fossils and
weathering processes, we followed recommendations by the International Society
for Rock Mechanics (ISRM). We took at least
10 single-impact readings for each block and averaged only the upper 50 %
for the determination of the boulder HR value, as suggested by the ISRM (1978).
Moreover, in order to minimise deviations that would arise from an oblique
impact (Aydin and Basu, 2005), we performed field tests, keeping the hammer
axis perpendicular to the boulder surface (Table 2).
Given that the hydrodynamic approach also depends on the pre-transport
environment, the most likely setting (submerged, subaerial, etc.) prior to
transportation has been determined. Moreover, detailed submerged profiles of
the five coastal sites have been constructed by carrying out direct underwater surveys,
in order to verify a correlation between the submerged limestone features in
the nearshore zones (such as fractures, rupture surfaces, detachment scarps,
holes) and the quantity and shape of the deposited boulders.
The onshore boulders at each site were inspected to identify the presence of
any biological encrustations, mainly of calcareous marine bioform type,
which may remain attached to the boulders after emergence above sea level.
When present on a boulder, such bioforms may serve as a strong indicator of
the original location of the boulder in a submerged environment and which
would die once the boulder is removed from its underwater environment. The
taxa of these bioforms were identified in order to create an identity list
of biota. The identity and ecology of the species served as a basis to draw
inferences on the origin and history of the boulders and thus be of help to
further corroborate the results obtained from hydrodynamic modelling.
Radiocarbon age dating of marine bioconcrections helped to reveal the
deposition time frame. They were performed by the CeDaD Laboratory (Centro
di Datazione e Diagnostica of the University of Salento, Italy). The
calibration is based on the data set available on the website
http://calib.qub.ac.uk/marine/index.html and provides a choice between
different procedures. We adopted the MMHM (i.e. mixed marine Northern Hemisphere)
equation, with a δR= 59 ± 40 and
δR= 71 ± 50, respectively obtained in the Tyrrhenian Sea
on Arca tetragona species and Cerastoderma genus. We preferred to use the first value because of the greater
ecological similarity, in particular for nourishment, with Vermetidae and
Chthamalus. Indeed, the genus Arca lives fixed on the rocky bottoms and is characterised
by suspension feeding behaviour. Conversely the genus Cerastoderma, although it shows
suspension feeding behaviour, occupies the infaunal niches. Furthermore, it was considered that
the percentage of carbonate origin besides continental starts from the value
of δ13C.
Finally, the collected field data were compared to the Maltese wave data and
to historical catalogues of earthquakes and tsunamis (Tinti et al., 2004;
Fago et al., 2014; Papadopoulos et al., 2014) in order to make a possible
correlation with known events. With regards to local wave height parameters,
the Malta Environment and Planning Authority (Malta Maritime Authority,
2003; Malta Environment and Planning Authority, 2007) provided a statistical
study of two different areas of Malta (close to Bahar iċ-Ċaghaq on the
NE coast and close to Żonqor on the SE coast) from data measured during
2007, through which the inshore wave extremes have been estimated as
5.1–5.6 and 5.3–5.9 m respectively for a return period of 50 years and as
5.3–5.8 and 5.4–6.0 for a return period of 100 years at 20 m depth. A wave height
of almost 7 m was recorded on 15 October 2007 during a very strong storm.
Drago et al. (2013) provided an analysis of the Maltese waves by taking into
account wave data over a span of 5 years (2007–2011), measured by a buoy
located 2 km offshore from the NW coast of Gozo. The highest wave was
registered on 6 January 2012 with a height of 7.46 m.
This value, associated with a wave period of 9 s, has been used to apply
the Sunamura and Horikawa (1974) equation which calculates the wave height
at breaking point (Hb) of a coastal area:
HbH0=(tanβ)0.2⋅H0L0-0.25,
where Hb is the breaking wave height, H0 is the wave height in
deep water, β is the slope of the sea bottom in the coastal area and
L0 the wave length in deep water (L0=gT22π;
Sarpkaya and Isaacson, 1981).
The resultant breaking wave height Hb for the five study areas were as
follows: 6.6 m at Armier Bay, 9.4 m at Ahrax Point, 8.7 m at
Buġibba, 7.1 m at Qawra peninsula, 8.0 m at Bahar iċ-Ċaghaq,
10.2 m at Pembroke and 8.4 m at Żonqor. These values have been
considered as thresholds to distinguish storm waves from tsunami waves.
ResultsArmier Bay and Ahrax Point
This boulder site was identified for the first time by Furlani et al. (2011)
and was recently studied by Mottershead et al. (2014) and Biolchi et al. (2016)
and is located in the north-eastern sector of the island (Fig. 2), which is
exposed to winds blowing from west to north–north-west.
From a geomorphological point of view, this part of the coast can be defined
as rocky low-lying coast, with an average slope of 5–6∘ (Said and
Schembri, 2010; Fig. 3b).
The coast is entirely composed of Upper Coralline Limestone. The bedding is
subhorizontal and has an average thickness of 50 cm. The boulders, ranging
in size from decimetric to metric, are clustered in the central part of the
deposit at a distance from the coastline varying from 10 to 30 m (Fig. 3a
and b). Away from the central outcrop, the boulders are more scattered in an
isolated manner and their size decreases with increasing distance from sea
level. The grain size distribution of boulders shows an exponential landward
fining trend.
The boulders reach inland limits of up to 50 m away from the shoreline,
at elevations of 8 m a.s.l. (above sea level). More than 100 boulders are
counted as total deposits (Mottershead et al., 2014; Fig. 3c). Some boulders are
imbricated toward NE and are indicative of the flow direction from which
they are deposited, with an orientation of the long axis toward WNW. With
regards to their shape, blocks in rectangular forms are more abundant as a
result of local discontinuities and quarrying activity on blocks along the
bedding planes, with the latter corresponding to the c axis of the boulders.
On the exposed surface of some boulders, small karst features, such as
solution pools, small pinnacles and microrills, were observed.
(a) Boulder AB1 at Armier Bay; (b) boulder deposit at Armier Bay;
(c) view of the Armier Bay boulder deposit from a UAV (unmanned aerial vehicle);
(d) submerged isolated boulders; (e) reconstruction of the submerged
environment at Armier Bay; (f) cliff top storm deposits at Ahrax Point.
Underwater surveys uncovered a submerged scenario characterised by
isolated boulders, both with fresh edges and/or covered by algae and
populated by marine organisms, niches and fresh detachment scarps (Fig. 3d).
The sea bottom is similar to the subaerial geomorphological setting, being
characterised by a gentle sloping platform, interrupted by small scarps
which correspond to the bedding planes (Fig. 3e).
A number of boulders at Armier Bay have remains of marine organisms. The
most common were shells of the vermetid molluscs Dendropoma petraeum
and Vermetus triquetrus (triqueter?), together with
calcareous Rhodophyta. These two vermetid species are typical of the lower
midlittoral to infralittoral transition, and thus indicate that the
boulders were at some point present at approximately mean sea level. This
was confirmed with the presence of barnacle shells belonging to the family
Chthamalidae on one of the boulders; such barnacles normally occur just
above mean sea level. Shells of the vermetid gastropod Thylacodes arenarius and the skeletal
remains of a coral belonging to the genus Caryophyllia were present on one of the
smaller boulders. Both organisms occur in the shallow infralittoral transition,
suggesting that this specific boulder was at one point fully submerged,
probably at a depth of ∼ 0.5–5.0 m. On the other hand, there were numerous
boulders at Armier Bay without any adherent bioencrustations; however, this
absence does not necessarily imply a non-marine origin, since only
encrusting organisms cemented to the surface of the boulders, are likely to
remain in place after emergence. Non-calcareous bioforms are very easily
eroded away, leaving no trace on the boulders. The proposed scenario is a
joint bounded submerged one.
The results of the hydrodynamic equations and those of radiocarbon dating
are listed in Tables 3 and 4, respectively. According to the equations
of Nandasena et al. (2011) and Pignatelli et al. (2009), only four boulders
seem to require storm waves with heights which exceed the local values,
given that they range from above 7 m to approximately 13 m. On the other
hand, with the Engel and May (2012) approach, the wave heights are markedly
lower (1.7–2.5 m). The tsunami wave heights are however comparable with
those observed both during the 1908 event (Guidoboni and Mariotti, 2008) and
also by Tinti et al. (2005). The rest of the values obtained with the
hydrodynamic equations are comparable with the Maltese storm waves:
2.27–6.34 m (with Nandasena et al., 2011), 2.15–5.99 m (with Pignatelli et
al., 2009) and 1.31–4.80 m (with Engel and May, 2012).
With regards to radiocarbon dating results, among the boulders that exceed
the estimated storm wave height of 6.6 m, three of them (AB4, AB7 and AB5)
recorded ages that may potentially align with historical tsunami events:
938 ± 70 (AB4), 869 ± 75 (AB5) and 122 ± 72 (AB7). The other
dated boulders, such as AB1 (514 ± 104 BC), AB2 (1298 ± 46),
AB6 (1290 ± 54) and Q2 (1384 ± 47), seem to have required wave
heights comparable both to the storm regime and the historical known tsunami waves.
On the other hand, the radiocarbon dating performed on the C16 boulder
confirms its origin to a storm-related event.
Moving eastwards, toward Ahrax Point, tens of boulders have been
deposited at relatively higher elevations. They actually represent the
boulder site with the highest elevation point across the island of Malta.
Some of them are scattered and isolated (Fig. 3f). Conversely, the majority
are gathered and disposed to forming a sort of storm berm, which is
aligned in the NW direction, at a distance from the coast varying from 10 to
40 m. Their maximum elevation is about 20 m. Locally, the boulders are
imbricated toward NE.
At this site, the boulders do not have any marine encrustations and seem to
have been detached from the top of the nearby cliff, which is deeply eroded
and indented. A detachment scarp, located at an elevation of 10 m a.s.l.,
seems to indicate a subaerial process-driven scenario. It is possible that
these blocks correspond to cliff-top storm deposits, very similar to
those characterised by some small karst pools, including sand with marine
shells, described by Hall et al. (2006). As a matter of fact, the underwater
surveys did not reveal scarps, pluck holes or fractured rocky outcrops.
Contrary to the hypothesis proposed by Mottershead et al. (2014), we suggest
a storm wave higher than 10 m, which was amplified by the topography of the
sea bottom, as the responsible mechanism for the boulders' detachment.
Qawra-Buġibba
The coast between Buġibba and Qawra stretches further east from
Mellieha Bay. This coastal area is exposed to strong winds and high
waves triggered by the north-easterly storms known locally as “Grigal”. The
cause of such large waves is the long fetch that stretches all the way to
the Ionian Sea of Greece. These violent storms generally last 24 h,
whilst in the successive days the sea conditions are characterised by large
swell conditions, which continue to pound on the coastline from the same
north-easterly direction.
The rocky coastline of Buġibba consists of Lower Coralline Limestone,
which outcrops at sea level as a subhorizontal terrace, and connects with
on overlying steep cliff, 3–5 m high, of Lower Globigerina Limestone.
The boulders originate from both lithologies and are scattered, locally
overlying each other, on the terrace, which in this area has an elevation of
about 10 m a.s.l. (Fig. 4a). They are mainly rectangular, as a result of the
orientation of three discontinuity sets, which act as lines of weakness on
the terrace surface. Their sizes vary from decimetric to metric, with an
a axis ranging from 1 to 2.5 m, while the c axis (which corresponds mainly
with the bed thicknesses) measures from 0.5 to 1 m. The average direction of
the long axis of the largest boulders is NW. The majority of these boulders
have collapsed from the top of the slope, leaving niches and detachment
scarps on the slope. Other boulders originate from the sea, as evidenced by
the presence of marine encrustations, including an aggregation of the
vermetid mollusc D. petraeum as well as serpulid tubes (i.e. boulder Qa2). The presence
of a vermetid crust at the surface, with a main discontinuity plane on the
opposite face of the boulders, indicates that these deposits originally
formed part of the coastline, with their surface at approximately mean sea
level. They were eventually detached through wave undercutting and
transported to their present location. Along the coastline, fresh detachment
surfaces are clearly visible, both above and below sea level, as evidenced
by direct underwater observations and unmanned aerial vehicle (UAV) images,
which show pluck holes and isolated submerged boulders (Fig. 4b). Fresh
impact marks, both on the rock surface and on boulders, can be observed. For
these boulders, a joint bounded submerged scenario is also being proposed.
Physical parameters of the boulders and results of the application
of the hydrodynamic equations provided by Nandasena et al. (2011),
Pignatelli et al. (2009) and Engel and May (2012). The dated boulders are
reported in bold characters.
SiteBoulderax aax bax cVolumeDensityMassNandasenaNandasenaPignatelliPignatelliEngelEngel(m)(m)(m)(m3)(g cm-3)(t)tsunamistormtsunamistormtsunamistorm(m)(m)(m)(m)(m)(m)Ahrax Point Armier BayAA14.12.41.110.821.3915.021.184.711.114.460.803.21AA22.81.21.13.701.706.282.188.712.068.240.491.97AA31.80.80.81.151.701.961.586.341.505.990.331.31AA432.20.654.291.707.291.295.151.224.870.903.61AA52.251.90.31.281.702.180.592.380.562.250.783.11AA71.710.81.361.702.311.586.341.505.990.411.64AA8210.51.001.701.700.993.960.943.750.411.64AA921.20.451.081.621.750.783.130.742.960.471.87AA109.521.324.701.741.992.619.702.439.740.823.28AA1131.50.94.051.666.722.466.321.596.350.602.40AA122.61.71.56.632.04713.573.9416.914.2416.970.843.36AA142.91.80.94.701.667.802.466.321.596.350.722.88AA151.110.30.331.70.562.612.240.562.250.411.64AB14.22.80.55.881.7810.451.104.411.044.171.204.80AB23.51.60.553.081.855.701.335.321.265.030.712.85AB321.60.82.561.624.141.395.571.325.270.622.50AB41.91.41.43.721.816.763.2412.953.0612.240.612.45AB61.61.20.50.961.701.630.993.960.943.750.491.97AB73.41.61.156.261.7010.642.289.112.158.610.662.62C160.90.80.250.181.800.320.572.270.542.150.351.39C82/AB52.561.060.922.501.704.241.827.291.726.890.431.74new2.391.690.823.311.585.221.335.311.265.020.642.57Q20.750.550.50.211.700.350.993.960.943.750.230.90Bahar iċ-ĊaghaqB12.31.850.62.551.704.341.144.551.124.490.763.03B24.353.650.46.351.8011.430.873.480.863.441.586.33B32.41.80.552.381.804.281.204.781.184.730.783.12B42.61.70.73.091.805.571.526.091.506.010.742.95B52.151.930.72.901.805.231.526.091.506.010.843.35B621.50.551.651.802.971.204.781.184.730.652.60B72.31.60.361.321.802.380.783.130.773.090.692.78B832.417.201.8012.962.178.702.158.591.044.17B93.31.650.63.271.394.530.662.620.612.430.552.21B103.11.60.62.981.394.130.662.620.612.430.542.14B113.31.80.694.101.395.690.753.010.702.800.602.41B123.12.350.53.641.395.060.552.180.512.030.793.15B134.33.40.710.231.3914.200.763.060.712.841.144.55B143.22.11.17.391.3910.261.204.811.114.460.702.81Buġibba and QawraLB1421.29.601.7016.322.449.782.258.990.823.28LB22.91.651.055.021.989.973.0312.132.7911.150.793.16LB32.61.81.15.152.0810.693.5414.173.2012.810.903.60LB43.32.80.65.541.628.971.094.370.993.951.094.37LB62.0161.120.350.791.851.460.893.540.803.200.502.00LB71.9841.81.13.932.027.923.3413.353.0212.070.873.50LB81.7391.60.852.371.744.111.867.451.686.730.672.68LB92.52.150.84.301.707.311.636.521.505.990.883.52LB102.42.30.52.762.055.651.505.981.415.661.134.54Qa11.81.41.33.281.805.902.9511.812.7911.170.612.43Qa22.21.20.651.721.803.091.546.181.405.580.522.08Qa31.51.50.71.581.852.911.777.081.606.400.672.68qawra_221.050.61.261.742.191.244.971.194.750.441.76qawra_32.31.51.13.801.887.152.7410.952.6210.470.682.72
According to the results of the hydrodynamic equations of Nandasena et al. (2011)
and Pignatelli et al. (2009) (Table 3), within the 10 measured
blocks, 4 of them have required waves higher than 8.7 m to be detached from
the coastal edge or from the nearshore bottom. On the contrary, according to
the Engel and May (2012) equation, the values are lower (2–4.5 m). Among
these boulders, the serpulid sampled from boulder Qa2 has been dated back to post 1954
(Table 4), confirming a storm origin.
On the Qawra peninsula, the coast is gentle sloping and tens of boulders are
distributed at an average elevation of 1 or 3 m (Fig. 4c). Their lithology consists of Lower Coralline Limestone. Their
distance from the coastline can reach up to 50 m and overall, the deposits
are imbricated towards north.
One radiocarbon dating test was performed on a serpulid polychaete, sampled from the most distant and representative boulder (Table 4).
This boulder also had cemented serpulid tubes, a skeleton of coral polyp
(likely Caryophyllia sp.) and several bores made by lithophage bivalves (such as the
date mussel Lithophaga lithophaga) with no shells visible in the holes. On the other hand, there
were no vermetid concretions. Taken together, these observations strongly
suggest that the sampled boulder was originally fully submerged, in a joint
bounded submerged scenario.
(a) Boulder deposit at Buġibba; (b) view of the deposit from
UAV; (c) view of Qawra peninsula from UAV; (d) vermetid shells;
(e) reconstruction of the submerged environment.
Accelerator mass spectrometry (AMS) 14C dating of marine organisms performed by the CeDaD
Laboratory (Centro di Datazione e Diagnostica) of the University of Salento,
Brindisi, Italy. The last column lists the historical tsunamis which occurred in
the Mediterranean Sea in the ranges of the radiocarbon ages (Tinti et al.,
2004; Papadopoulos et al., 2014). pCM denotes percent of modern carbon.
Splash and Fun Water Park: (a) scattered boulders belonging to LCL;
(b) UAV view of the deposit; (c) Bahar iċ-Ċagharaq:
boulders belonging to GLO; (d) isolated boulders at Pembroke (the underwater
profiles are very similar to Fig. 4e).
The results obtained by the hydrodynamic equations of Nandasena et al. (2011)
and Pignatelli et al. (2009) (Table 3) show that three out of four
sampled boulders require waves that exceed 7.1 m. On the contrary, according
to Engel and May's (2012) equation, the values are lower (1.7–2.7 m).
Among these boulders, the serpulid sampled from boulder Qa1 has been dated back to
post 1954 (Table 4). Given that only one accelerator mass spectrometry (AMS) age was obtained from these
sampled boulders, their original position may be questionable and needs to
be validated with more dating.
Bahar iċ-Ċaghaq
Bahar iċ-Ċaghaq is located on the central part of the
eastern coast, between Qawra and Pembroke (Fig. 2). The wind and wave
conditions that prevail in the Qawra-Buġibba area are also present here.
Relatively shallow waters in close proximity to the coastline create
high-energy areas with irregular and violent conditions.
Along the coast where the Splash and Fun Water Park is located, a wide flat
platform occurs and is composed of the highest unit of the Lower Coralline Limestone.
The platform is covered by tens of metric boulders (Fig. 5a and b), which are
imbricated toward NE.
Tens of boulders and several sections of the coast exhibit fresh detachment
surfaces and indented contours. Impact marks due to the dragging of boulders
on the platform are also still visible, suggesting recent movements. As seen
in Table 3, the results of the hydrodynamic equations provided values
comparable with storm waves.
Some of these boulders had dense clusters of Vermetidae (mainly D. petraeum) tubes
cemented together on the surface, and with spaces infilled by the calcareous
Rhodophyta Neogoniolithon brassica-florida, together with remnants of other biota (e.g. the bivalves
Cardita calyculata and Chama gryphoides) that are commonly associated with vermetid aggregations. These
vermetid crusts are typical of the midlittoral to infralittoral
transition, indicating that the surface of these boulders was originally at
approximately mean sea level. One of these boulders also contained bores
made by the bivalve Lithophaga lithophaga, an upper infralittoral species. Also in this case, we
propose a joint bounded submerged scenario.
Moving towards south, the lithology of the coast, as well as that of
boulders, changes into Globigerina Limestone formation. The boulder deposits
extend for about 700 m and consist of hundreds of blocks, all metric in
size, which have been deposited up to 30 m away from the coastline (Fig. 5c)
and are imbricated mainly toward NE.
At sea level, a 2 m high scarp is present and is connected to a wide
low-lying platform, with an average slope of 5∘. The bedding is
gently inclined toward the sea and its thickness is of about 0.5 m. The
scarp contour is indented and fresh detachment surfaces and fractures are
clearly visible. The boulders are all scattered on the low-lying platform,
where the bedding favoured the fracturing and the detachment of rock masses.
Some of them are covered by marine encrustations, often very recent. These
bioforms are similar to those observed on the LCL boulders slightly further
north, and include vermetid crusts and associated biota. One specific
small-sized boulder contained numerous bores with Lithophaga lithophaga, as well as serpulid and
spirorbid polychaete tubes, the coralline alga Ellisolandia elongata and remains of the green alga
Cystoseira amentacea, indicating that the boulder originated from the upper infralittoral region
(joint bounded submerged scenario). Given that green algae are not
encrusting species and rapidly erode away, this specific boulder must have
been transported out of the water during a very recent storm event.
The application of the hydrodynamic equations (Table 3) was tested on
14 sampled boulders, and only one result required a wave higher than 8 m
according to Nandasena et al. (2011) and Pignatelli et al. (2009), while for
Engel and May (2012), the result is completely different: 4.17 m. In any
case the values are comparable with the Maltese wave regime. The dated
vermetid crust on boulder B1, AD 1672 ± 45 (Table 4), may be linked to
two different historical tsunami events which occurred in the vicinity of
eastern Sicily: 1693 and 1743. However, the storm wave height obtained by
the hydrodynamic equations for this boulder was lower than 5 m. Even here,
such a result is inconclusive, given that only one AMS age was calculated.
Pembroke
The surveyed area is located along the eastern coast of Malta, a few hundred
metres east of the coastal town of Pembroke. The same conditions of winds
and waves which prevail in the Qawra-Buġibba area are also present here.
Relatively shallow waters in close proximity to the coastline create
high-energy areas with irregular and violent conditions.
From a geomorphological point of view, the outcrop consists of a low-lying
rocky area of Lower Coralline Limestone formation.
Numerous boulders, most of which are imbricated toward NNE, have been
measured and described. Generally they show a roughly rectangular shape,
sometimes more rounded, with a more or less evident planar side
corresponding to the detachment surface. The boulders are from decimetric to
metric in dimension and are characterised by a longer axis on average from 1 m,
up to maximum values of 2.5 m and an overall thickness of less than 1 m (Fig. 5d).
Most of the boulders identified in the Pembroke area are located more
than 20 m inland and are partly covered by a vermetid crust made by D. petraeum and the
coralline alga Neogoniolithon brassica-florida. These crusts occur at the transition between the lower
midlittoral and upper infralittoral transition, and therefore represent the evidence
of at least one submarine phase of these rocks with their upper surface
located at approximately mean sea level (i.e. joint bounded submerged
scenario). The absence of similar encrustations on the fracture planes of
the boulders suggests that these originally formed part of the rocky
coastline extending into the sea, and were subsequently detached and
transported to their present position on land.
With regards to the hydrodynamic equations (Table 3), for the 10 measured
boulders, the results show that only one of them required waves higher than
10.2 m to be detached from the coastal edge according to Nandasena et al. (2011)
and Pignatelli et al. (2009). However, according to Engel and May (2012),
all values are lower, between 2.6 and 5.7 m. The dated
organism from boulder P16 provided an age of AD 1723 ± 40, but the
hydrodynamic calculations seem to indicate more the likelihood of detachment
by ordinary storm waves.
Żonqor
Żonqor is the southernmost location on the NE-facing coast of the sites
investigated in this study. The area takes the shape of a headland formed by
the open coast to the left and the entrance to Marsascala Bay to the right.
It consists of a gently sloping rock coast where the slope is mainly
controlled by the dip of the bedding strata. The tip of the headland extends
below sea level for some 500 m in an ESE direction up to a depth of -10 m,
forming a long and narrow reef. The variation in water depth in the reef
area causes considerable wave refraction around the headland, whilst its
aspects render it susceptible to impact by waves approaching from a range of
directions between the N and the SE.
The local bedrock is composed of Lower Globigerina Limestone and Lower
Coralline Limestone. In this area the contact between these two layers is
marked by a phosphatic nodule conglomerate bed. The exposed Globigerina is
generally smooth in appearance and thickly bedded; however on the headland
it is highly weathered, exhibiting a number of fissures and fractures which
have been filled and hardened with a red-brown caliche crust.
The Lower Coralline Limestone layer is exposed in some tracts, where the
Globigerina layer above has been stripped off along lines of discontinuity
in the bedrock.
The Żonqor area is marked by a high quantity of boulders, many of which
are angular and cuboidal in form (Fig. 6a). Their shape and size are
determined by the joint patterns within the rock body from where they
originate and range from less than 1 m to more than 8 m in length. Their
average thickness varies between 40 and 80 cm depending on location and
lithology. On the headland, the boulders form a number of distinct clusters
and ridges. The two largest ridges measure 24 and 20 m in length and are
aligned WNW–ESE (Fig. 6b). The majority of boulders in these ridges are
either imbricated or aligned (a axis) towards the NE (Fig. 6c). Other
smaller ridges and clusters show a prevalence of boulders imbricated or
oriented towards the E, the ESE and the SE, corresponding to the aspect of
the headland in relation to their position. These boulder accumulations are
found approximately between 40 and 85 m from the shoreline.
Moving alongshore towards the NW, the boulder distribution changes. A fault
trending WNW–ESE has created a depression up to 6 m wide in which several
tens of boulders have been entrapped about 30 m from the shoreline. Some
isolated boulders or clusters composed of a few clasts were observed
landward of this fault. Further towards the NW, the coast is dotted with more
boulder clusters, the majority of which show a NE imbrication or orientation.
On the landward edge of the platform, a number of boulders form a berm that
merges with a vegetated soft sediment slope, originating mainly from
anthropogenic infill. This is located approximately 50 m inland.
Żonqor: (a, b, c) boulder deposits; (d) underwater fresh
detachment surfaces along bedding and fracture planes; (e) submerged
rectangular and rounded metric boulders; (f) reconstruction of the submerged environment.
The origin of the clasts at Żonqor seems to be principally from the
supralittoral zone as evidenced from the number of detachment scarps and exposed
joint facies in the backshore. However a small number of boulders with
encrusting algae and a variety of other marine organisms (including the
vermetid molluscs D. petraeum and T. arenarius, bivalves such
as C. calyculata and C. gryphoides and the lithophage Petricola lithophaga and several
serpulid polychaetes) indicate a sublittoral origin.
Storm wave impact on this site is considerable, especially when the wave approach
is from a NE direction and wave inundation can reach several metres inland.
This can be inferred from observed boulder movement following storms during
which wind speeds exceeded 45 km/h. One such boulder measuring 2.4 m × 1.3 m × 0.6 m
was detached from the sublittoral zone and carried 15 m from the
shoreline. The same boulder was moved a further 10 m inland and split into
two parts, and the smaller part was transported once again some 14 m inland
during subsequent storms.
According to the results obtained from the hydrodynamic equations (Table 3),
within the 15 measured blocks, 6 of them required waves higher than 8.4 m to
be detached from the coastal edge or from the sea bottom. The boulder Z1
provided a 14C dating of post AD 1954 (Table 4) and may have been
affected by a storm wave of 3.94 m according to Pignatelli et al. (2009),
4.32 m according to Nandasena et al. (2011) or 2.51 m according to Engel and
May's (2012) approach.
Discussion
From a morphological point of view, the occurrence of low-lying rocky coasts
makes the eastern coast of Malta better predisposed to the accumulation of
large boulder deposits derived from the impact of extreme waves. Moreover,
the horizontal bedding, the presence of subvertical discontinuities and the
poor geo-mechanical properties of the rocks play a crucial role in the
rupture and detachment of large blocks from the coastline.
Concerning the pre-dislodgement setting of the boulders, a joint bounded
submerged scenario is the most frequent, while for the boulders at Ahrax
Point and locally at Żonqor, Buġibba and Bahar
iċ-Ċaghaq, a subaerial scenario is suggested. These settings
were validated by underwater surveys carried out in all the investigated
sites; in all locations, the shapes of the boulders correspond to the shapes of the
pluck holes and detachment scarps. Originally, most of the large boulders
investigated in this study must have been part of the coastline edge, since they
comprise rock pools from the most seaward surface, as well as vermetid colonies.
Furthermore, according to local eyewitness accounts, several boulders
recently deposited by swell waves were dislodged, moved and transported
landward. Mechanical quarrying of the boulders requires the presence of
initial cracks. As a matter of fact, most of the measured boulder c and
b axes correspond respectively to bed thickness and bed planes, which are
smooth at the base and karstified at the top. These discontinuities favoured
the detachment of regular slabs. Especially at Żonqor, the quarrying of
regularly shaped boulders is encouraged by the presence of subvertical faults
and fractures, which are clearly visible also underwater.
On the other hand, at Ahrax Point, the boulders seem to have been
detached from the top part of the cliff face, as they have not been
colonised by marine organisms and the geomorphological setting includes a
steep cliff very close to the deposits. These boulders are referred to as
cliff-top storm deposits.
The application of the hydrodynamic equations (Table 3) has highlighted the
lack of correlation between density and volume values and the obtained
results, meaning larger boulders do not necessarily require higher waves to
be detached from the coastline edge. When comparing results, it can be
observed that the results of Nandasena et al. (2011) and Pignatelli et al. (2009) are very
similar: the highest values obtained reach 14 and 13.35 m for the
equations of Nandasena and 12.8 and 12.7 m for Pignatelli, thus
recording a marginal difference of slightly more than 1 m. For all the
other values, the decrease of the storm wave height values also decreases
the difference between the obtained results. Out of the 77 selected
boulders, 22 boulders recorded storm wave heights exceeding the estimated
breaking wave heights. Conversely, the calculated tsunami wave heights are
very low and range between 3.5 m (3.2 for Pignatelli) and 0.55 m (0.51 for
Pignatelli). Engel and May's (2012) equations obtained values
much lower relatively, with the storm wave heights ranging from 0.9 to 6.5 m and
tsunami wave height from 0.2 to 1.6 m.
Equally, the obtained values for tsunami wave heights are comparable to
those observed during the 1908 earthquake (Pino et al., 2008; Guidoboni and
Mariotti, 2008), as well with those obtained by Tinti et al. (2005) for
tsunamis generated by earthquakes both in eastern Sicily and in the Western
Hellenic Arc.
In comparing their results, these equations provide values which are too
different from each other, even though they take into account different
parameters and sometimes consider scenarios which are distant from the real
geomorphological setting. It was noticed that bulk and volume values do not
influence the results in the same way when using different equations.
Moreover, parameters such as the distance from the coastline, the elevation
position of a boulder and the local topographical characteristics of the sea
bottom are not taken into consideration. These are the reasons why it can
be concluded that the hydrodynamic approach as a stand-alone method is not
sufficient to distinguish between storm and tsunami waves.
With regards to the position of the boulders, we are not able to establish
whether the current position of the boulders is the original one when the
first depositional event occurred. There is the possibility that some
boulders may have been deposited on the coast by a tsunami event and later
transported and deposited by storm waves to their present position. As a
matter of fact, comparing the distance from the coastline with the mass of
each boulder for each site (Fig. 7), two different distributions can be
observed (the Ahrax Point case is not representative because of the poor
number of data).
At Armier Bay and Żonqor, the distribution is regular, with the lighter
boulders as the ones further way from the coastline. This could be
indicative of a storm event or of a “perfect tsunami”, but the radiocarbon
dating results on different boulders at Armier Bay are varied. In this case,
even if the diagram in Fig. 7 is typical of a storm, we suggest the combined action of
storms (which may occasionally be severe) and one or more tsunamis. For the Żonqor site, the hypothesis of extreme storms is confirmed by the
hydrodynamic approach as well as by radiocarbon dating and the
geomorphological and biological characteristics.
Relationship between the distance from the coast (x) and the boulder mass (y).
At Bahar iċ-Ċaghaq, Buġibba and Pembroke, the
boulders are scattered and their distribution is highly irregular,
indicating a chaotic event or the succession of several events. However, such
a distribution of the boulders and their dating (post 1954 for Buġibba,
AD 1672 ± 45 for Bahar iċ-Ċaghaq and AD 1723 ± 40
for Pembroke) may indicate the succession of several events
(storm and storms, or storm and tsunami or tsunami and tsunami).
Radiocarbon dating was performed on marine organisms sampled from
10 representative boulders in all the sites. Moreover, three dating samples provided by
Biolchi et al. (2016) were recalibrated (Table 4; Fig. 8). Four samples
support the hypothesis of recent strong storm events, dating back to post AD 1954. This occurred in particular at Żonqor Point, which is exposed to
storms blowing from more than one direction. The north-easterly storms
batter this stretch of coastline just as they do in areas such as Qawra.
However, Żonqor Point is also exposed to storms that originate from the
south-east – known locally as “Xlokk”. Such storms can blow both in winter
and in summer, and their strength, though not as powerful as storms from the
north-east, can create very rough conditions in the area.
Location of the investigated boulders, with a zoom of the Armier
Bay Ahrax Point site, with their AMS dating, size, distance from the
coastline and elevation above sea level.
Additional proof of recent extreme waves is provided by tracks on
freshly damaged karst surfaces, generated by rolling/saltating boulder
transport, which lead directly from the fresh scarp at the terrace edge to
the boulder's current position.
Other radiocarbon dating seems to be related to events (extreme storms
and/or tsunamis) occurring in a time span ranging from 514 ± 104 BC
to AD 1723 ± 40. They have been compared with historical
events (Papadopoulos et al., 2014; Tinti et al., 2004). Our results could be
tentatively linked to some of these tsunami events which have occurred in
the Mediterranean Sea and the Aegean Sea (Table 4). Amongst them, the most
ancient are the 373 BC (west Corinthian Gulf) and the 426 BC (island of Crete) events for
the boulder AB1 at Armier Bay; but this boulder, despite its significant
size (4.2 × 2.8 × 0.5 m), is located very close to the the coast (15 m) and is
placed above other boulders.
The event of AD 963, which occurred in eastern Sicily, which is reported in the tsunami
catalogue provided by Tinti et al. (2004) as a “false event”, could instead be tentatively proven by two nearby boulders at Armier Bay (AB4
and C82). The more recent event of AD 1303 (Crete Island) and, more
probably, the AD 1329 (eastern Sicily) event, could be related to three nearby boulders
at Armier Bay (AB2, AB6 and Q2). Finally, two other events, which
occurred in the early modern period and have been reported in historical
accounts of Malta (De Soldanis, 1746; Galea, 2007) are the AD 1693 (eastern
Sicily) and the AD 1743 (Apulia, Lower Ionian Sea) events. These strong earthquakes
triggered tsunamis which, according to Scicchitano et al. (2007) and
Mastronuzzi et al. (2007), are responsible for similar boulder deposits in
eastern Sicily and the Apulia region. These two tsunamis could be related to the
boulders located at Bahar iċ-Ċaghaq (B1) and Pembroke (16).
Gozitan historian Agius de Soldanis writes the following
description of the 1693 event in his 1746 account. “On 11 January of this
year, the earth trembled and everyone was scared. The earthquake damaged the
Collegiate Church and many other churches. The sea at Xlendi receded instantly
and returned back with great fury like a tidal wave and with a thundering sound.
At Sannat a part of the land measuring round a wejba crumbled down
into the sea” (De Soldanis, 1746, p. 149).
Unfortunately, these claims could not be further verified since
the radiocarbon age is limited due to the limited number of
samples and to the calibration;
the hydrodynamic approach does not seem to confirm the hypothesis of ancient
tsunamis, as the estimated values for storm wave heights are acceptable for
the Maltese regime.
Conclusions
Along the eastern and north-eastern Maltese coasts, about 20 boulder
deposits occurred. Reconstructing the history of these blocks and
distinguishing events, such as storm waves or tsunamis, play a crucial role
in assessing this type of coastal geo-hazard. A detailed field survey has
been carried out along the Maltese coasts in order to identify and map all
the sites in which these kinds of deposits occur, to analyse their
characteristics in detail, to determine their provenance and study the processes
responsible for their transport from the sea to the coast.
Data suggest that these boulders testify to the existence of a real hazard for
the eastern and north-eastern coasts, considering the high land use
development and coastal infrastructures present in proximity to the Maltese
coastline on this part of the island. The frequent storms affecting the
Maltese coasts are able to detach large boulders both from the coast edge
and the sea bottom, and to transport them onshore. Very high waves are
common. They can detach and move blocks whose volume can exceed 10 m3.
These blocks can be detached and moved inshore, or boulders can be initially
overwhelmed and brought inshore only at a later time. The occurrence of
recent extreme storm waves is supported by radiocarbon dating performed on marine organisms. Such events are likely to increase in
frequency and intensity due to climate change, whilst sea level rise, even of a
temporary nature, such as that brought about by a storm surge, could shift coastal
processes landward and impinge on urban areas.
However, the possibility that one or more tsunami events may have affected
these coasts cannot be ruled out, since radiocarbon dating of some marine
organisms encrusted on the boulders surfaces has revealed ages that can be
related to historical known tsunamis. In particular, at the Armier Bay site
(north-eastern coast), there could be geomorphological evidence of the AD 963
and 1329 tsunami events, which occurred in eastern Sicily
(southern Italy). Conversely, at Bahar iċ-Ċaghaq and
Pembroke (eastern coast), two boulders could be related to one of the two
most important tsunamis (AD 1693, eastern Sicily, or AD 1743, Apulia), which have
also been reported in the historical accounts of Malta.
Thus a national risk assessment of extreme wave events will need to consider
both an ongoing monitoring system of storm wave events and related impacts
on the low-lying urban coasts, as well as the inclusion of the
Maltese islands in a Mediterranean-based tsunami early warning
system, as part of a long-term strategic hazard management plan.
Acknowledgements
This work has been partially funded by the SIMIT Project “Integrated Civil
Protection System for the Italo-Maltese Cross-Border Area” (Italia-Malta
Programme – Cohesion Policy), the Research Project COFIN MIUR 2010–2011
“Response of morphoclimatic system dynamics to global changes and related
geomorphological hazard” and by the Flagship Project RITMARE – The Italian
Research for the Sea – coordinated by the Italian National Research Council
and funded by the Italian Ministry of Education, University and Research
within the National Research Program 2011–2013.
The paper is an Italian contribution to IGCP project no. 588 – International
Geological Correlation Programme by UNESCO-IUGS.
Finally, the authors are grateful to the Falck family for the partial
funding of research activities.
The authors wish to thank the editor S. Tinti and the two anonymus reviewers
for useful suggestions and corrections that significantly contributed
to the improvement of the paper.
Edited by: S. Tinti
Reviewed by: two anonymous referees
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