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Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
173
DOI: 10.4408/IJEGE.2013-06.B-14
MICROSEISMICITY RELATED TO
GRAVITY-INDUCED SLOPE DEFORMATIONS
FOR RISK MANAGEMENT
L
uca
LENTI
(*)
, S
aLvaTorE
MarTINo
(**)
, a
NToNELLa
PacIELLo
(***)
,
a
LbErTo
PrESTININZI
(**)
& S
TEfaNo
RIVELLINO
(**)
(*)
Institut Français des Sciences et Technologies des Transports, de l’Aménagement et des Réseaux (IFSTTAR-Paris)
(**)
Sapienza Università di Roma - Dipartimento di Scienze della Terra e Centro di Ricerca CERI - Rome, Italy
(***)
Agenzia Nazionale per le Nuove Tecnologie, l‘Energia e lo Sviluppo Economico Sostenibile (ENEA) - C.R. Casaccia -
Rome, Italy
logical risk associated with the deformational process
affecting the drainage plant.
K
ey
words
: gravity-induced slope deformations, displace-
ment monitoring, micro-seismicity, warning system
INTRODUCTION
This study used an accelerometric array to record
precursors, post-failure events and triggering im-
pulsive events within a karst rock mass hosting the
major drainage system of Rome’s aqueduct. Seismic
and microseismic recorded events were compared
with the data obtained by a stress-strain monitoring
system installed inside the slope, to analyse the de-
tected deformations in relation to both the ongoing
gravitational process and the external forcing (i.e.
earthquakes). In this regard, since the 70s’ the influ-
ence on entire slopes of the inertial forces due to the
interaction of long or very-long period dynamic ac-
tions (such as tides or teleseismic earthquakes) was
worldwide demonstrated in relation to volcano activ-
ity, volcano-tectonic flank deformations (J
ohNSToN
&
M
auk
, 1972; M
c
N
uTT
& b
EavaN
, 1984; r
ydELEk
et
alii, 1988; K
ASAHARA
, 2002; M
IYAZAWA
et alii, 2005;
S
OTTILI
et alii, 2007) and recently to interplate slow
slip event (I
TABA
& A
NDO
, 2011).
On the other hand, the pre-failure behaviour of
rock masses represents a complex geomechanical
problem because the stress and jointing conditions
as well as the joint setting can strongly constrain pre-
ABSTRACT
An accelerometric array installed on 4 September
2008, has been used to manage the geological risk in
the Peschiera Springs drainage plant of Rome’s aque-
duct, located in the Central Apennines approximately
80 km from Rome, Italy. The plant occupies a carbon-
atic slope that is extensively involved in gravitational
deformations, which are responsible for underground
failures such as cracks and collapses of karst caves. To
distinguish among different types of recorded events,
an automated procedure was implemented taking into
account duration, peak of ground acceleration (PGA)
and its variation within the accelerometric array. The
main sequence of underground failures so far recorded
was related to the L’Aquila seismic sequence occurred
in April 2009. Moreover, a very intense sequence of
underground failures occurred in September 2011
that was not related to seismic events, i.e. only due to
the gravitational processes affecting the slope. These
evidences prove that the ongoing gravitational slope
deformations have a key role in predisposing and/or
causing the underground failures within the karst rock
mass of the Peschiera Spring slope.
A control index (CI) is daily computed as a func-
tion of sub-indexes which are derived from the rate
of cumulative Arias intensity of underground failures,
the frequency of underground failures and the fre-
quency of earthquakes. The CI index identifies “ordi-
nary”, “alert” or “emergency” levels of attention and
represents a fundamental tool for managing the geo-
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L. LENTI, S. MARTINO, A. PACIELLO, A. PRESTININZI & S. RIVELLINO
174
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
THE PESCHIERA SPRING SLOPE
GEOLOGICAL SETTING
The Peschiera Springs slope corresponds to the
south-western flank of Mt. Nuria (Central Apennines,
Italy) and it is composed of Malm - Lower Cretaceous
limestones (C
APOTORTI
et alii, 1995; C
IOTOLI
et alii,
2001; B
IGI
& C
OSTA
P
ISANI
, 2002) (Fig. 1). The structur-
al setting of the slope is monoclinic, with EW-trending
and N-dipping (30°-40°) strata; many fault lines cross
the slope with roughly NS and N35E trends (Fig. 1).
The slope hosts a major karst aquifer which
represents the drainage system of the Nuria-Velino-
western Fucino and western Marsica (Velino-Si-
rente) mountains (total surface area: 1016 km
2
),
whose main springs are the Peschiera-Canetra ones
(measured total discharge: roughly 18 to 21 m
3
/s ac-
cording to B
ONI
et alii, 1986; B
ONI
et alii, 1995). The
Peschiera Springs drainage plant, which consists of
a complex system of drainage and collector tunnels
and related connection halls, is part of the Rome aq-
ueduct system managed by the ACEA-ATO2 S.p.A.
Italian company
failure effects, such as generation of cracks, opening
or closing of joints and readjustment of the stress field
within the rock mass. All of these failure precursors
can be monitored by specific devices. However, on-
going geological and geomechanical deformation,
which can proceed from a transitional phase to rock-
mass failure at yielding conditions, is indicative of
the proneness of a rock mass to further deformation
or failure (S
ZWEDZICKI
, 2003). Recognizing pre-failure
events by geological surveys as well as monitoring
natural and anthropogenic systems is a major goal
for the mitigation of risks due to the above mentioned
“unexpected” and “rapid” events. More complex
scenarios of failure involving rock masses can be as-
sociated with impulsive triggers (i.e. explosions, col-
lapses) or earthquakes. In these cases precursors do
not necessarily occur, while the events representing
possible triggers can be monitored.
Some experiments have been performed in mines
or in landslide areas that were aimed at monitoring
failure precursors by use of acoustic as well as seismo-
metric devices (M
ILLER
et alii, 1989; L
EI
et alii, 2004;
L
AI
et alii, 2006; P
ASKALEVA
et alii, 2006; D
EPARIS
et
alii, 2008). On the other hand hypogeous instabilities
triggered by impulsive events were recorded during
experiments of controlled explosions and collapses of
caves in mine areas (P
HILLIPS
et alii, 1997; Y
ANG
et
alii, 1998). Many authors (M
ILLER
et alii, 1989; P
hIL
-
LIPS
et alii, 1997; h
ENg
, 2009) have analysed spectral
features of microseismic emissions due to rock falls,
collapses and explosions, both natural and artificially
induced, distinguishing specific signal features for dif-
ferent types of events.
The analysis of sequences of precursors as well as
of post-failure events (i.e. underground instabilities in-
duced by impulsive triggers) can be considered a very
useful tool for managing early preventive interven-
tions, since monitoring the phases of failure propaga-
tion provides information on the changes which are in-
volving the rock mass, but also on possible occurrence
of more critical conditions (i.e. generalized collapse).
The monitoring system installed within the Pe-
schiera Spring slope is also devoted to evaluate possi-
ble trigger thresholds or to recognise different levels
of attention, based on the observed trends (specifi-
cally in terms of frequency and energy) of events; in
this way, it represents a remarkable risk management
tool for the drainage plant.
Fig. 1 - Location and geological sketch of the Peschiera
Springs slope: (1) Recent alluvia of the Velino
River; (2) Reddish soils; (3) Slope debris; (4)
Gravel and conglomerate (upper-Pliocene and
lower- Pleistocene parts); (5) Sandy-clayey fly-
sch (upper Miocene); (6) Marly limestone (upper
Cretaceous–lower Miocene); (7) Birdseye micritic
limestone (lower Cretaceous); (8) Coral limestone
(upper Malm); (9) Coral and echinoids limestone
(Malm part); (10) Fault (dashed if estimated):
FMF Fiamignano-Micciani Fault, CF Canalone
Fault, PF Pendenza Fault; (11) Strike and dip of
strata; (12) Springs; (13) accelerometric station
installed within the drainage plant; (14) moni-
tored joint within the drainage plant
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MICROSEISMICITY RELATED TO GRAVITY-INDUCED SLOPE DEFORMATIONS FOR RISK MANAGEMENT
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
Editrice
175
The geological-evolutionary model of the slope re-
flects a complex deep-seated gravitational deformation,
which initiates a ‘‘sackung’’ phenomenon (Z
ISCHINSKY
,
1969; S
AVAGE
& V
ARNES
, 1987) continuously evolv-
ing from rock-mass spreading (H
UTCHINSON
, 1988) to
rock-block mass deformation (M
arTINo
et alii, 2004).
In particular, the rock-mass spreading can be clearly
observed in the western portion of the slope, where it is
associated to a radial displacement field and it generates
continuous transversal scarps combined to longitudinal
trenches (i.e. multiple transversal trenches).
MONITORING SYSTEM
DISPLACEMENT MONITORING
A geotechnical monitoring system was installed,
from September 2007, inside the Peschiera Springs
slope to detect the continuous gravitative deforma-
tions which involve the mass rock. This monitoring
system consist in:
- 8 strain-gauge installed on structural reinforcements
- 7 line extensometers
- 14 uniaxial distantiometers
- 15 triaxial distantiometers
These devices were installed within the tunnels of
the Peschiera Spring drainage system to detect stress-
strain effects on structural elements and cracks on both
the exposed rock and the tunnel cover. All the sensors
have a displacement resolution of 0.1 μm and automat-
ically record at time intervals of 1 hour. The recorded
data are collected by a local data-logger which enables
a remote download; the downloaded data are automati-
cally processed to return a monitoring report.
ACCELEROMETRIC MONITORING
Starting from 4 September, 2008 four accelero-
metric stations (GA, C1, F1 and C6 of Fig.1) were
installed by ACEA-ATO2 S.p.A. within the drainage
plant of the Peschiera Springs in order to record both
seismic events and underground collapses. Each sta-
tion was instrumented by a triaxial accelerometer
(EPISENSOR KINEMETRICS) directly installed on
bedrock. The four accelerometers were connected via
cable to a digital data-logger (K2 KINEMETRICS) set
to the absolute local time by a GPS device.
The monitoring system was managed by the Re-
search Centre for Geological Risks (CERI) of the Uni-
versity of Rome “Sapienza” in order to properly set the
recording device, analyze the collected data and sug-
ONGOING GRAVITATIONAL SLOPE DEFOR-
MATION
Geomorphological surveys performed on the
slopes as well as a digital, high-resolution (2 m), eleva-
tion model of the slope (DEM) derived by a LIDAR
(Light Detection And Ranging) radar remote survey,
enabled to identify numerous gravity-induced mor-
phological elements (e.g. scarps, trenches, sinkholes
and tension cracks) (Fig. 2) (L
ENTI
et alii, 2012). These
landforms are indicative of slow, intense and pervasive
slope deformations, which affect the entire slope. These
deformations correspond to different evolutionary stag-
es ascribable to specific portions of the slope, as proved
by already published outputs of a stress-strain monitor-
ing system installed within the drainage plant (M
arTINo
et alii, 2004; M
affEI
et alii, 2005). More in particular, it
is possible to recognise three slope sectors with ongo-
ing gravity-induced processes (M
arTINo
et alii, 2004):
1) a sector, including the southern portion of the slope
and its top, with evidence of incipient and low deforma-
tions, i.e. in their early evolutionary stage; 2) a western
sector, with evidence of mature and not yet advanced
gravity-induced deformations, only concentrated close
to the main trenches or scarps and 3) an eastern sector,
with evidence of advanced gravity-induced slope defor-
mations, characterised by pronounced landforms, such
as scarps, trenches and sinkholes.
Fig. 2 - 3D views of the DEM showing the already occurred
landslide and the ongoing rock mass spreading
which involves the Peschiera Springs slope
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L. LENTI, S. MARTINO, A. PACIELLO, A. PRESTININZI & S. RIVELLINO
176
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
3. deformational effects which were not related to
seismic records.
Examples of the first typology of deformational
episodes were obtained: i) during the near field (epi-
central distance of about 30 km) L’Aquila seismic se-
quence (Fig. 4), started with the Mw 6.3 mainshock
occurred on 6 March 2009 at 3.32 a.m. and ii) during
the 11 March 2011 Japan earthquake (Mw 8.9) (Fig.
5), which was recorded as a teleseismic event by the
Peschiera Spring accelerometric array. In these cases
a maximum displacement up to 0.3 and 0.7 mm was
respectively recorded within the drainage plant along
the monitored joints; these displacements were reached
in about one week from the triggering seismic event. A
similar behaviour, was observed in the case of the 23
December 2008 Frignano (Mw 5.1) earthquake and in
the case of the 15 January 2009 Kuril Islands (Mw 7.3)
earthquake; nevertheless, in this case maximum dis-
placements up to 2 mm were instantaneously recorded
after the seismic shaking. Moreover, the July 2009 Mts
Reatini seismic sequence and the 12 July 2009 earth-
quake (Mw 4.0) from L’Aquila district (epicentral dis-
tance of about 25 km), triggered a deformational event
responsible for a maximum recorded displacement
along the monitored joints up to 3.5 mm (Fig. 6).
An example of the second typology of deforma-
tional episodes was obtained during the 13 September
2012 micro-earthquake sequence with a time duration
of ten of minutes and due to underground collapses of
karst caves within the slope; this event was responsi-
gest possible plans of management in case of a seis-
mic crisis or a sequence of underground events. Until
November 2012 more than 1300 events (about 950 far
and near earthquakes and 400 underground instabili-
ties) were recorded on the whole.
To distinguish among different kinds of recorded
events, a specific software was implemented through
SAC (Seismic Analysis Code) and Fortran codes on
Unix platform, taking into account the records ob-
tained from the accelerometric network (L
ENTI
et alii,
2012). The software allows to classify the events on
the basis of their physical properties (i.e. energy, time
duration, kinetic parameters and frequency content)
and, in particular, to recognise among earthquakes,
cracks (micro-earthquakes) and underground collaps-
es. The physical properties of the recorded waveforms
are analysed in the time domain, and the PGA variation
among the recording stations is also considered by the
use of a percentage PGA-variation index (VI) which
has been defined in the form:
VI=((PGAmax–PGAaverage)/PGAaverage))*100
The different types of events are actually distinguished
by using PGA, VI and time duration values according
to the scheme of Fig. 3.
OBSERVED DEFORMATIONAL EPISODES
Since 2009 several deformational episodes were
observed by the monitoring system within the Peschi-
era Spring slope; among these episodes it is possible
to distinguish the following typologies:
1. deformational effects clearly related to external
actions such as near field seismic sequences and
teleseismic earthquakes;
2. deformational effects related to local micro-seismic
sequences and corresponding to cracks or col-
lapses occurred within the slope;
Fig. 3 - Flowchart showing the detection procedure
adopted to distinguish different types of events
recorded by accelerometric network within the
Peschiera Springs slope
F
ig. 4 - Computed horizontal Arias intensity for the record-
ed seismic and micro-seismic events (up) and dis-
placement monitored within the Peschiera Spring
drainage plant (point J1 of
Fig. 1): the time win-
dow shows the L’Aquila seismic sequence start-
ed on April 2009 with the Mw 6.3 mainshock
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MICROSEISMICITY RELATED TO GRAVITY-INDUCED SLOPE DEFORMATIONS FOR RISK MANAGEMENT
Italian Journal of Engineering Geology and Environment - Book Series (6) www.ijege.uniroma1.it © 2013 Sapienza Università
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177
WARNING SYSTEM
The analysis of the recorded events is a useful
tool for managing the natural risk due to underground
failures at the Peschiera Springs drainage plant. For
this purpose, the frequencies of the earthquakes and
underground failures as well as the cumulative Arias
intensity of the underground failures are plotted as a
function of time.
To provide an alarm system for the plant in
the case of underground events, a frequency index
(FI(P,t)) was defined as the sum of an earthquake fre-
quency index (FI_er(P, t)) and a micro-earthquake fre-
ble for a maximum recorded displacements up to 0.3
mm in few hours (Fig. 7).
Many examples of the third typology of defor-
mational episodes are given by sudden displace-
ments (up to 2.8 mm) recorded along the monitored
joints which cannot be related to any seismic or mi-
cro-seismic sequence (Fig. 8).
It is worth noting that all the above reported de-
formational episodes, observed along the monitored
joints, represent significant strain effects if compared to
the average strain-rates recorded within the slope which
are in the order of few mm/year (i.e. up to 5 mm/year).
Fig. 5 - Computed horizontal Arias intensity (AI) for the
recorded seismic and micro-seismic events (up)
and displacement monitored within the Peschiera
Spring drainage plant (point J1 of Fig.1): the time
window shows the 11
th
March 2011 Japan ear-
thquake (Mw 8.9 - dashed envelope)
Fig. 6 - Computed horizontal Arias intensity (AI) for the
recorded seismic and micro-seismic events (up)
and displacement monitored within the Peschiera
Spring drainage plant (point J1 of Fig.1): the time
window shows the Mts Reatini seismic sequence on
July 2009 followed by the 12 July 2009 earthquake
from L’Aquila district (Mw 4.0 – dashed envelope)
Fig. 7 - Computed horizontal Arias intensity (AI) for the
recorded seismic and micro-seismic events (up)
and displacement monitored within the Peschiera
Spring drainage plant (point J1 of Fig.1): the time
window shows the micro-earthquake sequence due
to underground collapses (dashed envelope) oc-
curred on 13 September 2012
Fig. 8 - Computed horizontal Arias intensity (AI) for the
recorded seismic and micro-seismic events (up)
and displacement monitored within the Peschiera
Spring drainage plant (point J1 of Fig.1): the time
window shows a sudden displacement recorded on
16 May 2011 and not triggered by earthquake or
micro-earthquake sequences
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L. LENTI, S. MARTINO, A. PACIELLO, A. PRESTININZI & S. RIVELLINO
178
International Conference Vajont 1963-2013. Thoughts and analyses after 50 years since the catastrophic landslide Padua, Italy - 8-10 October 2013
quency index (FI_me(P, t)).
These indexes are daily assigned to each station ac-
cording to the daily frequency of recorded events (Tab.
1). Moreover, based on the previously described rate
of cumulative Arias intensity of the recorded micro-
earthquakes, an energy index (EI(P,t)) is attributed to
each station (Tab. 1).
A final control index (CI(P,t)) is daily computed for
each station of the network as a function of the sum
of the frequency and energy indexes (FI(P,t) + EI(P,t)).
This last index enables the association of each specific
sector of the plant (represented by the corresponding
accelerometric station) with three possible levels of
alarm: the “ordinary” level (OL) (CI(P,t)=1); the “alert”
level (AL) (CI(P,t)=2); and the “emergency” level (EL)
(CI(P,t)=3). To take into account additional individual
events of significant intensity, an alert threshold was
fixed at a PGA value of 10
-3
g based on the local PGA-
magnitude curve derived thus far (L
ENTI
et alii, 2012).
According to the above described warning system,
in the total period of monitoring, the “alert” level was
reached 7 times, due to seismic or micro-seismic se-
quences, while the “emergency” level was reached only
twice: i) during the near field L’Aquila seismic sequence
on April 2009 and ii) during a local the micro-seismic
sequence associated to collapses on September 2011.
CONCLUSIONS
The integrated stress-strain and accelerometric
monitoring systems installed within the drainage
plant of the Peschiera Spring slope recorded many in-
teresting deformational episodes which can be related
to the ongoing gravity-induced instability affecting
the slope sometimes associated to micro-earthquake
sequences due to underground failures (i.e. collapses
and cracks) as well as to the interaction of the slope
with external actions, such as seismic or teleseismic
events. Based on the monitoring data it is possible to
remark that significant displacements (i.e. up to few
millimetres) can occur suddenly or within a week-
time as an effect of external actions, if compared,
with the average recorded strain rate, i.e. up to 5 mm/
years. The installed accelerometric array is presently
devoted to a remote-controlled warning system which
returns, to the manager of the drainage plant, “alert”
or “emergency” levels, so providing a mitigation of
the geological risk related to the ongoing gravitation-
al instabilities of the slope.
ACKNOWLEDGEMENTS
The Authors wish to thank Dario Rinaldis and Car-
lo Romagnoli for the scientific discussions on the
findings. Thanks also to ACEA-ATO2 S.p.A. and
to Giorgio Martino for the technical support to this
study. The research was carried on in the frame of the
Convention between CERI and ACEA-ATO2 S.p.A.
for the study of the gravitational processes affecting
the Peschiera Spring slope, (Project leader: Prof. Al-
berto Prestininzi).
Tab. 1 - Values of the Indexes used for evaluating the
alarm levels within the Peschiera Spring
drainage plant
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