TITLE: Sand volcanoes induced by the April 6th 2009 Mw 6.3 L’Aquila
earthquake: a case study from the Fossa area
TITOLO: Vulcani di sabbia indotti dal terremoto dell’Aquila del 6 Aprile
2009, Mw 6.3: un caso di studio dall’area di Fossa
De Martini P.M., F.R. Cinti, L. Cucci, A. Smedile, S. Pinzi, C. A. Brunori, F.
Molisso.
Abstract
This paper presents the study of some liquefaction features occurred near the
Fossa village due to the April 6, 2009, Mw 6.3 L’Aquila earthquake (Central
Italy). Our investigation is based on trenching and coring campaigns as well
as sedimentological analyses and datings. The geometrical elements of the
sand volcanoes on the surface, of the dike used to rise up and of the probable
sandy source at depth are presented. A sandy pockets level found at less
than 1 m of depth, interpreted as possible evidence for a paleo-liquefaction
event is discussed. Sedimentologic and morphoscopic analyses both provided
the necessary elements and parameters to link the ca. 4 m deep sandy layers
to the 2009 sand blows on the ground surface as well as to the paleoliquefaction layer and defined the main characteristics of the deposits sealing
the sands that experience liquefaction at depth.
Finally a tentative correlation between the paleo-liquefaction layer and the
1461 AD or the 1703 AD local earthquakes is suggested based on the
available age constraints.
RIASSUNTO: Questo lavoro presenta lo studio di alcuni fenomeni di
liquefazione avvenuti vicino al villaggio di Fossa a causa del terremoto di Mw
6.3 dell’Aquila (Italia centrale). Le nostre ricerche si basano su campagne di
scavi e sondaggi cosi come su analisi sedimentologiche e datazioni. Vengono
presentati gli elementi geometrici dei vulcani di sabbia in superficie, della
frattura usata e della probabile sorgente di sabbia in profondità. Viene
discusso un livello di sacchette di sabbia trovato a meno di 1 m di profondità
ed interpretato come la probabile traccia di un paleo-evento di liquefazione.
Analisi sedimentologiche e morfoscopiche hanno fornito gli elementi ed i
parametri necessari a mettere in relazione i livelli sabbiosi trovati a 4 m di
profondità con i vulcani di sabbia del 2009 in superficie cosi come con il livello
di paleo liquefazione ed hanno definito le caratteristiche principali dei depositi
che sigillano le sabbie che hanno subito liquefazione.
Infine, basandosi sui vincoli cronologici disponibili, si suggerisce una
correlazione preliminare tra il livello di paleo-liquefazione e i terremoti storici
del 1461 o del 1703.
Keywords: liquefaction, sand volcanoes, 2009 L’Aquila earthquake
PAROLE CHIAVE: liquefazione, vulcani di sabbia, terremoto aquilano del
2009
1. INTRODUCTION
On April 6, 2009 a Mw6.3 earthquake struck the medieval town of L’Aquila
(Central Italy) and its surroundings, and it represented the apex of an intense
seismic activity started in December 2008. This seismic sequence included a
ML4.1 foreshock, as well as seven Mw5+ and about thirty Mw4+ aftershocks
recorded in the two weeks following the mainshock (Figure 1). During the
whole sequence the Italian Seismic Network recorded about 3,500 ML2+
shocks and more than 20,000 total events (as of end 2010). The seismicity
distribution (Chiarabba et alii, 2009), the focal mechanism of the mainshock
(Pondrelli et alii, 2010) and the GPS and DinSar modelling (Anzidei et alii,
2009; Atzori et alii, 2009; Walters et alii, 2009; Papanikolaou et alii, 2010)
consistently define a ~15-18 km-long, 50°SW-dipping NW-trending normal
fault as the seismogenic source responsible for the mainshock. The most
remarkable and continuous coseismic surface ruptures, observed for a length
of ~2.5-3 km along to the Paganica normal fault, are located along the updip
projection of the deep seismogenic source (Falcucci et alii, 2009; Boncio et
alii, 2010; Emergeo Working Group, 2010), and hence these features are
considered as a subtle but unquestionable evidence for primary surface
faulting associated with the event. Other coseismic surface effects were
recorded throughout the epicentral area nearby synthetic and antithetic
normal faults and differently interpreted (Emergeo Working Group, 2010; Galli
et al. 2010).
Figure 1: Epicentral area of the April 6, 2009 L’Aquila earthquake. Yellow
circles and stars are the epicenters of the L'Aquila 2009 seismic events
(recorded by the INGV National Seismic Network from April 6 to 23, 2009)
with M>4.0, the focal mechanism of the April 6 main shock (Mw=6.3) is after
Pondrelli et alii, 2009. The black and white dashed box is the projection on the
earth surface of the GPS and strong motion data derived fault model (Cirella
et alii, 2009). The studied site, located near Fossa village, is represented by
an orange small box. Red line is the envelope of 2009 primary surface
ruptures (EMERGEO Working Group, 2010). The elevation map is colorcoded DEM overlaid to the topography (shaded relief). The Abruzzi region is
shown in pink in the upper right inset, while the black box locates the Middle
Aterno Valley.
Several investigators have mapped the numerous active faults of the region
(Galadini & Galli, 2000 and reference therein; Foglio CARG 2009); however,
for some of these structures still no consensus exists concerning their rate of
activity and this prevented the univocal recognition of the causative faults of
several historical earthquakes that repeatedly hit the Abruzzi region in the
past (CPTI Working Group, 2004). Large magnitude, highly destructive events
occurred in 1349, 1703, and 1915, all having M≥6.5 and occurring within ~50
km from the 2009 epicenter. The historical earthquakes closest to the town of
L’Aquila have M<6.5 and are the 1461 M6.4, 1762 M5.9 and 1791 M5.4
events (Figure 2). In particular, the 1461 and 1762 earthquakes occurred in
the same epicentral area of the 2009 event (Tertulliani et alii, 2009; Cinti et
alii, 2011) or slightly towards SE.
Based on historical seismicity, the presence of active faults and the evidence
for large surface-faulting paleo-earthquakes (Pantosti et alii, 1996; Galli et alii,
2002; Galadini et alii, 2003; Salvi et alii, 2003) the L’Aquila region was
considered a high hazard seismic zone (Cinti et alii, 2004; Gruppo di Lavoro
M.P.S, 2004; Pace et alii, 2006; Akinci et alii, 2009).
Also, the macroseismic survey of the 2009 seismic event showed that the
largest damage was noticeably located SE of the instrumental epicenter (Galli
et alii, 2009), thus providing evidence for a combination of rupture directivity
and litostratigraphic amplification effects (Cirella et alii, 2009; Pino & Di
Luccio, 2009; Bergamaschi et alii, 2011; Cucci & Tertulliani, 2011).
Other than the case-history described in this paper, two more effects of
liquefaction induced by the 2009 event have been already reported: sand
blows, NE-trending fractures and lateral spreading along the left bank of the
Aterno river (Figure 2) 1 km SE of downtown L’Aquila (Aydan et alii, 2009),
and sand volcanoes nearby the village of Vittorito, ~40 km ESE of L’Aquila
(Monaco et alii, 2011).
It is well established in the scientific literature the fact that the studies on
liquefaction are of great importance in the seismic hazard evaluation of an
area. Since liquefaction itself can create great damage to man-made
structures, those studies provide a basic help to the proper design of buildings
located in liquefaction prone areas. Moreover, the evidence for
paleoliquefaction events can be used as a comparative tool in evaluating the
size of prehistoric earthquakes and to build up the seismic history of the site
(Obermeier, 1996). To this aim we show the study carried out on the
liquefaction phenomena occurred close to the village of Fossa (~10 kilometers
SE of the 2009 epicenter), presenting the investigation of the sand volcanoes
by means of drilling and paleoseismological trenching.
Figure 2: Shaded relief map of the April 6, 2009 L’Aquila earthquake.
White boxes locate historical events (CPTI, http://emidius.mi.ingv.it/CPTI08/);
red line is the envelope of 2009 primary surface ruptures (EMERGEO
Working Group, 2010). Recent deposits filling intramountain basins are here
represented by light blue areas. The orange box represent the studied site,
whilst the red box, SE of downtown of L’Aquila, is the liquefaction site
described by Aydan et alii (2009). The blue line is the Aterno River. White
arrows indicate active NE-SW extension strike (Mariucci et alii, 2010). The
Abruzzi region is shown in pink in the upper right inset, while the black box
locates the Middle Aterno Valley.
2. LIQUEFACTION PHENOMENA
The liquefaction of sediments is one of the most outstanding hydrogeologic
processes that can be originated by earthquakes. Seismic shaking during
earthquakes can cause saturated and unconsolidated sediments to transfer
pressure from grain-to-grain contacts to interstitial pore water. As the time
required to dissipate pore pressure in the sediment is much longer than the
duration of seismic shaking, the pore pressure increases, while the effective
stress, supported by the sediments, decreases correspondingly. If this
process continues, the effective stress eventually tapers to zero and the
sediments become fluid-like, i.e. liquefy. The most common surface features
induced by liquefaction are sand blows, lateral spread of sediments and mud
volcanoes.
Some of the most spectacular liquefaction effects worldwide were originated
by two seismic events occurred in 1964: widespread liquefaction phenomena
were documented along the Alaska coast following the M9.2 Alaska
earthquake (Waller, 1966; Seed, 1968), and in the low-lying areas of Nigata
City (Japan) following the M7.5 Nigata earthquake (Seed & Idriss, 1967).
Other noteworthy examples come from the New Madrid Seismic Zone in the
Central United States, where extensive liquefaction was induced by the 18101811 M8 New Madrid earthquakes (Figure 3) as well as by other prehistoric
events (Obermeier, 1989; Tuttle & Schweig, 1996), but also by the 1995 M6.9
Kobe (Japan), the 1999 M7.5 Chi-Chi (Taiwan) and the 1999 M7.4 Izmit
(Turkey) earthquakes (Elgamal et alii, 1996; Wang et alii, 2003; Wong &
Wang, 2007; Aydan et alii, 2008).
Though well graded gravels or gravel-sised granules have been witnessed to
liquefy during recent earthquakes (1983 M7.3 Borah Peak, Youd et alii 1985;
1995 M6.9 Kobe, Kokusho 2007), in the great majority of the cases the
process of liquefaction affects sandy or silty sandy deposits. Actually, most of
the examples reported above refer to earthquakes whose epicentral areas are
located close to regions highly susceptible to liquefaction such as wide alluvial
and coastal plains and alluvial fans.
Figure 3: Examples of liquefaction features in the field and of liquefactioninduced damage to man-made buildings. a) white spots correspond to areas
where sand was vented onto dark-colored clayey cap after the 1810-1811 M8
New Madrid earthquakes (photo S.F. Obermeier); b) field affected by sand
blows following the 1979 M6.5 Imperial Valley earthquake; c) weak ground
characteristics and inadequate foundation caused the leaning of this slender
building onto its neighbour in Adapazari during the 1999 M7.4 Kocaeli
(Turkey) earthquake (photo A. Tertulliani); d) apartment buildings founded on
top of loose, saturated soil deposits severely tilted after the 1964 M7.5 Niigata
earthquake (photo K.V. Steinbrugge Collection).
In the Italian Peninsula, the catalogue of liquefaction features by Galli (2000)
reports the historically known liquefaction cases, among which the effects
originated by the seismic events occurred in 1783 (M6.9) and 1905 (M6.8) in
the coastal plains of Calabria (southern Italy) (for details see Tertulliani &
Cucci 2009). Moreover, widespread liquefaction was also reported following
the 1915 M7.0 Fucino earthquake, ~45 kilometers S of the 2009 event (for
details see Galadini et alii, 1995), and the 1980 M6.9 Irpinia event (for details
see Porfido et alii, 2002), as well as following many other ~M≥6 events, all
located along the Apennines. Therefore, many intramountain basins that
straddle the Appennines and are bordered by active seismogenic faults bear a
not neglectable potential for liquefaction. In this regard, the 2009 L’Aquila
event in the Middle Aterno Valley, already identified as a liquefaction
susceptible zone by Galli & Meloni, 1993, even with its limited effects can be
considered as paradigmatic.
3. TRENCHING AND CORING ANALYSIS OF THE 2009 SAND BLOWS
We investigated the April 2009 liquefaction features occurring on agricultural
fields close to the Fossa village, located about 10 km SE from the
instrumental mainshock epicenter (Figures 1 and 2). We consider the April 6
Mw6.3 event responsible for the observed liquefaction being most if not all
fractures formed in the Fossa area (Figure 4 a) noticed immediately after the
quake by the local people. However, lacking any on-time observation of the
sand volcanoes, we can not definitely rule out as potential candidate the April
7 Mw5.5 aftershock (Pondrelli et alii, 2009) occurred, at a depth of ~15 km,
few km N of the site investigated (Chiarabba et alii, 2009). Severe damage
occurred in the village of Fossa as well as in the nearby villages (among them
S. Eusanio Forconese and Villa S. Angelo) suggesting that the area was
strongly shacked by the earthquake (Galli et alii, 2009). In April 2009, 8 to 10
smooth sand volcanoes (blows) were observed by the land owners in the area
between Fossa and Monticchio villages. Three months later, during our filed
survey, these features were still clear and well preserved on the ground
(Figure 4), appearing as sand blows with circular shape (diameter 1-2 m) and
undoubtedly related to liquefaction process.
Figure 4: The study area and the sand volcanoes. a) aerial photograph of the
liquefaction zone nearby Fossa village (Servizio per l'Informazione Territoriale
e la Telematica-Ufficio Sistema Informativo Geografico Regione Abruzzo
http://cartanet.regione.abruzzo.it), continous red line locate the Fossa Fault
while dashed red lines indicate observed coseismic fractures; b) detail of the
studied area showing cores (C1-C4 and P1) and trench (black dashed line)
position; c) & d) photos of two sand blows taken in July 2009.
Eight closely spaced volcanoes of fine yellowish sand are displayed in a flat
area of ~500m2 (max relative distance ~15 m). Blows occur across two
ploughed fields, near a cemented channel shown in Figure 4a, at the base of
the eastern slope of the Mt. Ocre ridge (Figure 1). The liquefaction area is
located on the downthrown side of the NW-oriented, NE-dipping, MonticchioFossa normal fault (antithetic to the Paganica fault; Cinti et alii, 2011 and
reference therein). In 2009, N125°-average striking minor surface ruptures
organised in a left-stepping pattern occurred on and close to the MonticchioFossa fault scarp at the base of the ridge, with a significant concentration near
the Fossa village (Figure 4a). Moreover, in the flat area located about 100
meters NE of the fault trace an alignment of continuous breaks with a mean
orientation of N110° affected paved road and manufacts (Emergeo Working
Group, 2010).
3.1 TRENCHING ACROSS THE APRIL 2009 LIQUEFACTION FEATURES
In order to investigate at depth the liquefaction features observed at the
surface and to obtain a 3D view of two coalescent sand blows we dug a zshaped trench across them (Figures 4, 5 and 6). We analysed in detail the SE
walls of the excavation that are identified as A (strike 230°), B (strike 320°),
and C (strike 230°) walls (Figure 5a). The trench reached a maximum depth of
~2 m and a total linear length of ~6.5 m.
The ejected sand has a maximum thickness of 10 cm, decreasing in the NESW direction (unit SB from meters 1.7A to 4.5C in Figure 5a), with a shape
resembling a smooth volcano that represent the venting of water and sand on
the pre-earthquake ground surface (Figures 5 and 6). Below the surface, the
stratigraphy is composed of fine, no-clast supported, alluvium of clay and silt
(1, 2 and 3 unit in Figure 5a). The differentiation between these deposits is
characterised by transition zones where the clayey or silty component
prevails.
Each of the differently oriented walls of the trench exhibits few cm-wide, sandfilled cracks vertically crossing the alluvial sequence (Figures 5a-d). The
cracks are quite rectilinear, stepping of ~0.1 m at about –1.1 m in B and C
walls. Except for wall A, where the top of the trace of the fracture terminates
at the depth of –1.1 m and does not lie directly below the sand blows (Figures
4a, d), the cracks on B and C walls clearly extented up to the sand volcanoes
lying on the surface (Figures 5a, b, c). Few vertical and horizontal sand sills
formed close to and are connected with the crack, particularly in the upper
portion of the wall, where intrusion is facilitated by roots voids and shallow
small fracturing.
The geometrical setting of the sand-filled cracks (location, strike and dip)
relative to the wall orientation indicates that the three cracks are connected
and form ca. 85°-striking, 70° to 75°-dipping, minimum 2 m-long, plane of
fracture (dike) through which water and sand flowed within non-liquefied
deposits, from depth to surface (Figures 6a, b, c). In order to expose part of
the plane that acted as a sand dike in 2009 we shoveled the junction of the B
and C walls backwards (trace B’ and C’ in Figure 6c). Figures 6a-b clearly
show that the cracks observed apart on walls B and C, actually correspond to
a single plane intersecting both walls. The source of the sand is not exposed
at the trench depth and either not reached by the -3.15 m-deep core FSV-C1
penetrating through the vented deposits at meter 4.1C (see Figures 4, 5a and
6a for location). However, the potential source bed was probably reached by
deeper cores through the nearby sand blows (see next section for core
analysis).
Figure 5: a) trench log showing the linear envelope of the walls A, B and C; b)
and c) view of the cracks filled by sand and 2009 sand blow at surface; d)
view of the buried crack and its deep connection with the crack in wall B.
Several closely spaced, discontinuous sandy lenses and pockets (SP in
Figure 5a, 0.03-0.06 m thick and 0.1-0.2 m wide) are soaked in the alluvial
non-liquefied deposit of unit 3 (Figures 5a and 6e, d) at mean depth of -0.75
m in all the walls. These features may represent the relicts of paleoliquefaction bodies; if this is the case, their position would coincide with the
paleo-ground surface at the time of the paleo-liquefaction event. The original
shape of these features (smooth sand volcanoes) would be incomplete and
discontinuous, due to weathering and superficial erosive processes with time.
In this context, the presence of the buried crack at wall A (Figures 5a, d, and
6a) could represent a portion of the fractured plane that was not re-occupied
by the liquefied sand in 2009 as it was in the past.
Figure 6: a) view of the trench and sketch showing cracks, blows, core and
fractured plane setting; b) plan view of the trench and liquefaction features; c)
view of the fractured plane with the vented sand at the junction on wall B’ and
C’; d and e) details of the lenses and pockets of sand (SP) within the clayey
and silty deposits.
3.2 CORING ACROSS THE APRIL 2009 LIQUEFACTION FEATURES
Cores FSV-C1 to FSV-C4 (C1 to C4 in Fig. 4b) were collected in the area
affected by the 2009 liquefaction phenomena. Differently, core PEPPE-C1 (P1
in Figure 4b) was sampled about 100 m towards SE, where the
geomorphological situation (almost flat floodplain) is very similar to that of the
liquefied area but no evidence for such phenomenon was noticed. We
collected 1 m long core sample sections (within pvc tubes) down to a 6.3 m
maximum depth, using a gasoline powered percussion hammer. The
stratigraphy of all the cores (Figure 7) is very similar and consists of fine,
hazel to brown alluvium made of clay and silt in the uppermost 3.5 m. Notably,
between 0.65 and 0.9 m of depth, this deposit is characterised by a number of
yellowish fine sand patches (absent only in core PEPPE-C1 and FSV-C3) that
delineate a distinct sandy pockets layer within clayey silt deposits (Figures 7
and 8). The origin of the fine deposits is likely related to the activity of the
Aterno River, with deposition of fine-grained suspended material on the
alluvial plain during overbank flooding events; while the peculiar sandy
pockets layer may represent the relicts of paleo-liquefaction deposit, similarly
to sandy lenses and pockets found in the trench at comparable depth.
Between 3.5 m and about 6.3 m (maximum depth reached only by core FSVC4), an alternation of grey to blackish clayey silt (low energy phases with
increment of organic rich deposition) and fine to medium sand layers (medium
energetic phases), likely deposited in a fluvial environment pertaining to a
paleo-meander of the Aterno River, dominate the sedimentary sequence.
We should mention that the sediments down to about 3.5 m were almost dry
while water was found when we reached the sand layers below 3.5 m (cores
PEPPE-C1 and FSV-C4). In core PEPPE-C1 the water was still visible in the
hole two days after the coring, while in core FSV-C4 the deepening of the
core below 5 m of depth made the water flowing away. These observations
suggest the existence of confined and saturated sandy deposits between ca.
3.5 and 5.0 m of depth.
Furthermore, two reddish brick fragments were found and collected, one in
core FSV-C3 at a depth of 1.85 m and the second in core FSV-C4 at a depth
of 5.05 m (Figure 7).
Figure 7: Simplified core logs, white boxes locate C14 samples while letters S
and M with numbers refer to samples collected for the sedimentological
analysis.
Because of the similarity of the cores stratigraphy we describe in detail (see
Appendix A at the end of this manuscript) only the longest (6.3 m) core
sampled, FSV-C4 (Figures 7 and 8), as representative of the whole
investigated area.
Figure 8: FSV-C4 core photo with roman numbers marking different sampling
pvc sections. The picture shows well the stratigraphy and the peculiar sand
patches found at the base of the first (I) meter. Note that at the base of the
fifth (V) section a small piece of reddish brick is visible.
3.3 Core FSV-C4 sedimentologic and microscopic analyses
In order to better understand the characteristics of the liquefied deposit and of
the investigated underlying layers, we conducted a detailed sedimentological
analysis on six selected samples (S1 to S6 between 0 and 5.5 m of depth,
Figure 7) from core FSV-C4. Textural analysis was conducted and the relative
grain size distributions are presented in Figure 9 and Table 1. In order to
separate the sandy fraction from the sediment bulk, the sampled sediment
was first wet-sieved using a 63-mm mesh, and then the coarser part was then
dry sieved, while the fine-grained fraction was delivered to the laser particle
sizer (Sympatec). Grain size statistical parameters (Table 1) have been
calculated following the classic graphical equations developed by Folk & Ward
(1957). Whereas samples M1 to M4 was only wet-sieved using a 63-mm
mesh and dried. Moreover, all samples (S1 to S6 and M1 to M4 in Figure 7)
were also examined under a binocular microscope for a detailed analysis of
the principal components (minerals and biogenic remnants), their shape and
degree of reworking.
Figure 9: Histograms showing the grain size distribution of the six samples
analysed with the laser particle sizer (Sympatec).
In the Appendix B at the end of this manuscript we briefly describe the results
of these analyses (for samples location see Figure 7).
Sample
code
S1
S2
S3
S4
S5
S6
Gravel %
0.00
0.50
0.00
0.00
0.00
0.00
Sand %
93.56
22.45
94.64
84.20
31.40
85.88
Silt %
5.14
60.17
4.23
12.46
51.22
11.11
Clay %
1.30
16.88
1.13
3.34
17.38
3.01
Mean
diameter
2.350
5.795
2.338
2.706
5.577
2.899
Sorting
index
0.751
2.255
0.684
1.222
2.454
0.934
Skewness
-0.054
0.058
-0.080
0.075
0.111
0.134
Kurtosis
1.554
0.861
1.440
1.446
0.840
1.750
Table 1. Grain size % of the S1 to S6 samples and other graphic parameters
(Folk & Ward, 1957).
Based on the results of our sedimentologic and microscopic analyses, we can
make some observations about the similarities and differences found among
the analysed samples that may help in better understanding the liquefaction
process occurred in 2009 and the potential to have found paleo-liquefaction
blow remnants.
In fact, samples FSV-C4 M4-S3 and FSV-C4 M3-S4 are very similar (more
then 80% of sand and almost identical graphic parametres), even if they
present two different degrees of weathering, more relevant for the latter, and
should share a common meander-like fluvial origin.
Samples FSV-C4 M2 (sand pockets) and FSV-C4 M1-S6 (sand blows at the
surface) have the same characteristics of the underlying (below 4 m of depth)
sample FSV-C4 M3-S4 even if they are more weathered and present an
important amount of vegetal remnants.
Samples FSV-C4 S2 and S5, collected below and above the main deep sandy
unit (Figure 7), show similar grain size distribution with an identical 17%
amount of clay (Table 1) and may be interpreted as related to low energy
phases of a meander-like fluvial environment.
3.4 CHRONOLOGICAL CONSTRAINTS
The age of the deposits investigated can be estimated by taking into account
the brick fragments found at different depth in the cores and 5 AMS
radiocarbon datings (Table 2).
As already shown, we found two reddish brick fragments, one in core FSV-C3
at a depth of 1.85 m and the second in core FSV-C4 at a depth of 5.05 m.
Considering the occupation history of this mountain area, these fragments
should fall in the historical period and likely in the last ca. 2500 yrs.
As for the radiocarbon datings, the samples collected in the trench walls,
dominated by overbank alluvial deposits, gave contrasting results with respect
to their relative stratigraphic position. In fact, two samples (C4 and A1, Fig.
5a) collected at different depths about 1 m apart, resulted to have a modern
age and this could be explained by contamination from living roots and/or due
to the intense plowing of the site. A third sample (B3b, Fig. 5a) located at the
same depth as sample A1 gave an anomalous old age, interpreted as the
result of old carbon inclusions transported during the over flooding events of
the Aterno River. Two samples (C4A and C4B, Table 2) were collected at a
depth of about 5 m (Figure 7) from core FSV-C4 within a dark grey silty clay
layer, representing a low-energy phase of the Aterno River, probably related
to a paleo-meander. Dating results from these two samples show a significant
age overlap and fit with the presence of a brick fragment of comparable age at
similar depth, providing the basis for a tentative sedimentation rate estimation.
Site /
depth
(m)
Sample / Lab
code
Type
Conventional
age B.P.
Calibrated age
2σ (95.4%)
Probably distribution
Trench
/ 0.7
C4
Poz-40396
charcoal
115 ± 30
1680 - 1955 AD
1.000
Trench
/ 1.7
A1
Poz-40395
charcoal
80 ± 30
1690 - 1955* AD
1.000
Trench
/ 1.7
B3b
Poz-40381
bulk
2550 ± 35
805 - 545 BC
1.000
Core4
/ 5.0
C4A
Beta-298826
bulk
2390 ± 30
730 – 395 BC
1.000
Core4
/ 5.2
C4B
Beta-298827
bulk
2320 ± 30
480 – 235 BC
1.000
Table 2. Measured and calibrated ages [according to Calib REV6.0.0, Stuiver
& Reimer, 1993; intcal09.14c, Reimer et alii, 2009] of the samples collected.
Please note that 1955* or 1960* denote influence of nuclear testing C-14.
Measurements were performed at the Beta Analytics Inc (Florida) and at the
Poznan Radiocarbon (Poland) laboratories, marked with Beta and Poz,
respectively.
We are aware of the fact that in a fluvial medium-low energy environment, as
the one investigated in this work, it is very difficult to assume a constant
sedimentation rate. In fact, it is well known that this environment implies
episodic deposition and erosive phases. Nevertheless, the two ca. 5 m deep
dated samples from core C4 give a first hand estimate of the average
sedimentation rate, comprehensive of all erosive and depositional episodes.
In fact, thanks to the fact that the dark organic rich clayey silt layer likely does
not experience any transport, samples C4A and C4B should attest the age of
sedimentation at the site, with very little external input. Following the previous
reasoning and taking samples C4A and C4B ages we obtain a rough average
sedimentation rate ranging between 1.8 and 2.3 mm/yr. These values suggest
that the entire 6.3 m long stratigraphic sequence of core FSV-C4 studied in
this work should be as old as 3500 yrs.
4. RESULTS AND INTERPRETATION
The geological investigation of the April 2009 liquefaction features occurred
on agricultural fields close to the Fossa village (ca. 10 km SE of L’Aquila)
furnished elements of discussion about the liquefied material, its potential
source and a probable paleo sand volcano, summarised in the following.
From the trench excavated across two coalescent sand blows (Figures 5 and
6), we were able to estimate the maximum thickness (10 cm) of the liquefied
sandy deposit on the surface. This information was confirmed by the cores
dug on other 3 sand blows (FSV-C2, -C3, -C4 in Figure 7) and it is in good
agreement with previous studies (Aydan et alii, 2009 and Monaco et alii,
2011) performed on different coseismic liquefaction features, as far as 40 km
from the mainshock epicenter. On the trench walls it was possible to observe
the ca. 2 m long, 85°-striking, 70° to 75°-dipping, fracture used by the
liquefied sand to reach the surface as well as a buried dike, inactive in 2009.
On both cores and trench walls, an interesting layer made by several sandy
lenses and pockets, within clayey silt deposits, was observed between 0.7
and 0.9 m of depth and interpreted as probable relict of paleo sand volcanoes.
Notably, core PEPPE-C1, performed about 100 m far from the liquefaction
area, showed no evidence for sand lenses at any depth even if it displayed a
sedimentary sequence similar if not identical to the other cores. Only the
cores reached the potential source (saturated sandy deposits) for the liquefied
material at a minimum depth of ca. 4 m.
Sedimentological and morphoscophic analyses (Figure 9 and Table 1) found
significant similarities in term of grain size distribution and graphic parameters
between the sand volcano on the surface and the sandy units below 4 m of
depth. An identical observation can be made for the sandy patches and
lenses of the peculiar layer observed at a maximum depth of 0.9 m,
reinforcing its interpretation as evidence for a paleo liquefaction event, initially
made on the trench walls thanks to stratigraphical considerations and upper
dike termination. Both sand layers are characterised by the presence of the
vegetal remains, potentially attesting their contamination with the grass when
they reached the ground surface. Moreover, the clayey silt layers above and
below the deep sandy units share similar sedimentological characteristics (silt
at 50-60% and clay at 17%, see samples S2 and S5 in Table 1) and in our
reconstruction acted as confining levels for the ca. 1.5 m thick saturated sand
under liquefaction.
Summarizing the data collected on the trench wall and cores, we can derive a
depth range varying from about 0.7 to 0.9 m for the base of the peculiar “sand
pockets” layer, interpreted as the remnants of a sandy paleo-liquefaction
blow. Unfortunately, the chronological constraints are not robust (particularly
those obtained from the trench within overbank deposits, see Table 2) and
thus the timing of deposition of the paleo sand blow will need supplementary
investigations and datings. However, we think that it is plausible to use the
age of the brick fragments (historical period, likely not older than 2500 yrs)
found at more than 5 m of depth to support and validate the ages of the
deepest samples collected within the organic rich clayey silt layer, which
experienced little if not null transport. If our reasoning is correct, combining
the 1.8 and 2.3 mm/yr average sedimentation rate values with the 0.9 m deep
“sand pockets” layer in core C4, we obtain an age range of 1510-1620 AD for
the deposition of this unit.
Even considering the uncertainties related to the latter time window
estimation, it is interesting to notice that both the historical 1461 AD
earthquake, likely occurred along the same Paganica Fault (Cinti et alii, 2011)
responsible for the 2009 mainshock, and the 1703 AD stronger earthquake
are close enough to be considered good candidates.
5. Conclusions
In this work we present the geometrical and sedimentological characteristics
of 4 sand blows formed close to the Fossa village following the Mw 6.3, 2009,
L’Aquila mainshock. Our approach involved trenching and coring as well as
sedimentological analyses and dating, performed with the aim to look for
paleo-liquefaction events as well as to better understand the deposits involved
in the liquefaction process and the relation between potential shallow sources
(6.3 m is the maximum depth reached by coring) and the sand volcanoes at
the surface.
We observed a maximum thickness of 10 cm for the sand blows at the
surface. This value is close to those observed in the broader epicentral area
by different research groups (Aydan et alii, 2009; Monaco et alii, 2011)
dealing with liquefaction features and fits well with the ca. 1 to 1.5 m thickness
of the potential source found at depth. The path used by the liquefied sand to
reach the surface in the 2009 is 2 m long, 70° to 75°-dipping, open fracture.
Sedimentological analyses highlighted strong similarities between the sand of
the blows and the sand units found below 4 m of depth suggesting the latter
being the “parent” material that was liquefied, even if we can not exclude a
different source located at greater depth than the 6.3 m reached by our
investigations.
A second layer made by discontinuous sand pockets, sharing most of the
sedimentological characteristics of the sand volcano at the surface, was found
on both trench and cores at the maximum depth of 0.9 m. The occurrence of
peculiar vegetal remnants mixed with the sand patches together with the
presence of a branch of the fracture ending just below this level on the trench
wall, suggests an interpretation as paleo liquefied sand volcano. The age of
this paleo event is not well constrained but should be very close to the ca.
1500-1600 AD, with the 1461 AD and the 1703 AD local events as the best
candidates. Finally, the deposits potentially sealing the sand units at depth are
made by clayey silts with an amount of clay not exceeding 20%, a value
apparently enough to confine a 1 to 1.5 m thick lens of saturated sand,
suspended within fine alluvial sediments.
Further studies are needed to confirm our findings and to better constrain the
age of the paleo event found during this research.
Acknowledgments: We are grateful to the Gran Sasso Acque S.p.A. for
supplying the backhoe for the trench and to G. Calvisi for helping us during
the excavation. Thanks to A. Coletti and C. Di Marco for their kindness and for
having allowed the trenching and coring of their private lands. We wish to
thank D. Pantosti, S. Pucci and R. Civico for the help in the field and for useful
discussions. We wish to thank two anonymous reviewers whose suggestions
contributed to improve the original manuscript.
This work was partially funded by the Italian Dipartimento della Protezione
Civile in the frame of the 2007–2009 Agreement with Istituto Nazionale di
Geofisica e Vulcanologia (INGV).
Appendix A: Core FSV-C4 log
Starting from the surface (Figure 7), a variable 1 to 10 cm thick medium to fine
yellowish to hazel sand layer of the liquefied volcanoes (with the exception of
core Peppe-C1, intentionally cored far from the sand blows area) lies on a
pale brown 15-20 cm thick (max 45 cm in FSV-C1) silty unit composed of
active soil and plowed horizon (equivalent to Unit 1 on the trench wall, Figure
5), with several roots and evidence for bioturbation. A ca. 40-50 cm thick layer
of hazel to pale brown silt, with some clay and few sparse small clasts,
interpreted as b-soil horizon, follows (same as Unit 2 on the trench wall). From
ca. 0.6 to ca. 1.8 m of depth, we found hazel to brown clayey silt layer,
generally massive (similar to Unit 3 on the trench wall). Similarly to what
observed on the trench walls, the separation between these units is
characterised by transition zones where the clayey or silty component slightly
prevails. Interestingly, between 0.65 and 0.9 m of depth, this deposit is
characterised by a number of yellowish fine sand patches (present also in
cores FSV-C1 and FSV-C2, at an average depth of 0.7-0.8 m, but absent in
core PEPPE-C1) that seem to delineate a peculiar sandy pockets layer within
clayey silt deposits (Figures 7 and 8). Below, from ca. 1.8 to ca. 3.4 m of
depth, the sequence is composed by brownish to hazel clay and silt deposits,
similar to the sediments above but enriched in clay. In the uppermost part of
this latter unit (at 1.85 m of depth in core FSV-C3) a small reddish brick
fragments was found (Figure 7).
A ca. 30 cm thick layer of pale brown to hazel fine sand (from 3.5 to 3.8 m of
depth) marks the change from overbank to meander-like deposition.
In fact from ca. 3.9 to 6.3 m of depth, we observed an alternation of decimetric
grey to light grey fine-medium sand layers and dark grey to blackish silty clay
to clayey silt layers. The change in color of the matrix from dominant hazel
(from the surface to ca. 3.9 m of depth) to prevailing grey (from ca. 3.9 to 6.3
m of depth) may suggest the occurrence of deposition under a thin water
sheet (pond-like), without important period of exposition to the atmosphere. In
particular, we should mention the presence of a black organic rich clayey silt
layer (from ca. 4.8 and 5.25 m of depth) attesting the occurrence of low
energy sedimentation, with abundance of organic material, for an unknown
period of time (abandoned meander?). Within this unit, at ca. 5.05 m of depth
(core FSV-C4), we also found a small brick fragment (Figures 7 and 8).
Appendix B: Sedimentologic and microscopic analyses of core FSV-C4
Sample (FVS-C4) M1-S6 (0.0 m) from the sand blow at the surface.
It is a yellowish fine to very fine sand, moderately sorted (Figure 9 and Table
1), with dark vegetal remnants often very crusted. Several see-through or
white angular grains of quartz, feldspar and calcite, sometime weathered,
were observed together with abundant white mica and rare fragments of mafic
minerals. The sample is also rich in reworked planktonic foraminifera and
sponge spiculas fragments.
Sample (FVS-C4) M2 (0.7 m) comes from the sandy pockets layer.
Unfortunately, the limited amount of this deposit did not allow us performing
quantitative sedimentological analysis.
It is a dark yellowish very fine sand, with dark vegetal remnants sometimes
very crusted. The abundant angular grains of quartz, feldspar and calcite,
often weathered, are well sorted. White mica and rare fragments of mafic
minerals are present. The sample shows sponge spiculas fragments and
several planktonic and some benthic foraminifera (mainly Cibicidoides and
Cibicides) all reworked.
Sample (FSV-C4) S5 (3.1 m) from fine grained unit.
Hazel to brownish sandy clayey silt (Figure 9 and Table 1), very poorly sorted.
It presents white angular grains of quartz, feldspar and calcite together with
abundant white mica and rare fragments of mafic minerals, often weathered.
Sample (FVS-C4) M3-S4 (4.3 m) from the upper portion of a sand unit.
It is a yellowish-grey fine to medium sand (Figure 9 and Table 1), poorly
sorted, with plenty of see-through or white angular grains of quartz, feldspar
and calcite, lightly weathered. Rare mafic minerals fragments and volcanic
glass were also observed together with abundant white mica.
It is enriched in reworked planktonic and benthic foraminifera (mainly
Cibicidoides, Bulimina, Pullenia and Siphonina) and sponge spiculas
fragments. It shares most of the characteristics of samples M1-S6 and M2.
Sample (FVS-C4) M4-S3 (4.5 m) from the lower portion of a sand unit.
Light grey-yellow fine to medium sand (Figure 9 and Table 1), moderately
sorted, with abundant see-through or white angular grains of quartz, feldspar
and calcite (few of them are slightly weathered) and white mica.
Fragments of mafic minerals and volcanic glass are frequent. It is rich in
sponge spiculas remnants and reworked planktonic and benthic foraminifera
(mainly Cibicidoides). In general it shows a lower degree of weathering with
respect to the samples above.
Sample (FSV-C4) S2 (4.9 m) from a organic rich fine deposit.
It is a grey sandy clayey silt (Figure 9 and Table 1), very poorly sorted, with
white angular grains of quartz, feldspar and calcite and white mica. It is
enriched in slightly weathered mafic minerals with respect to the samples
above. It also contains planktonic and benthic foraminifera, all reworked.
Sample (FSV-C4) S1 (5.4 m) from a sand unit.
It is a pale grey fine to medium sand (Figure 9 and Table 1), moderately
sorted, with abundant see-through or white angular grains of quartz, feldspar
and calcite and white mica. Fragments of slightly weathered mafic minerals
together with reworked planktonic and benthic foraminifera, are also present.
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