The Piano di Pezza fault (PPF) is the north-westernmost segment of the >20 km long Ovindoli-Pezza active normal
fault-system (central Italy). Although existing paleoseismic data document high vertical Holocene slip rates (~1 mm/yr) and a
remarkable seismogenic potential of this fault, its subsurface setting and Pleistocene cumulative displacement are still unknown.
We investigated the shallow subsurface of a key section of the PPF using seismic and electrical resistivity tomography coupled
with time-domain electromagnetic measurements (TDEM). We provide 2-D Vp and resistivity images showing details of the fault
structure and the geometry of the shallow basin infill down to 35-40 m depth. We can estimate the dip and the Holocene vertical
displacement of the master fault. TDEM measurements in the fault hangingwall indicate that the pre-Quaternary carbonate
basement may be found at ~90-100 m depth

Baccheschi, Paola

Civico, Riccardo

Di Giulio, Giuseppe

Fierro, Elisa

Pantosti, Daniela

Sapia, Vincenzo

Tulliani, Valerio

Vassallo, Maurizio

Villani, Fabio

Earth-prints Repository

6th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology,  
19-24 April 2015, Pescina, Fucino Basin, Italy 
 
INQUA Focus Group on Paleoseismology and Active Tectonics 
 
 
 
 
 
 
Shallow subsurface imaging of the Piano di Pezza active normal fault (central Italy) 
using high-resolution refraction and electrical resistivity tomography coupled with 
time-domain electromagnetic data 
 
Villani, F. (1), Tulliani, V. (2), Sapia, V. (3), Fierro, E. (4), Civico, R. (3), Baccheschi, P. (1), Di Giulio, G. (1), Vassallo, M. (1), Pantosti, D. (3) 
 
(1) Istituto Nazionale di Geofisica e Vulcanologia, via dell’Arcivescovado 8, 67100 L’Aquila, Italy. Email: fabio.villani@ingv.it 
(2) Anglia Cathodic Protection Services Ltd, Meadow View Industrial Estate, Reach Road, Burwell, Cambridgeshire, CB25 0GH, U.K. 
(3) Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, 00143 Roma, Italy 
(4) Dipartimento di Scienze e Tecnologie, Università degli Studi di Camerino, Via Gentile III da Varano, 62032 Camerino, Italy 
 
 
Abstract: The Piano di Pezza fault (PPF) is the north-westernmost segment of the >20 km long Ovindoli-Pezza active normal 
fault-system (central Italy). Although existing paleoseismic data document high vertical Holocene slip rates (~1 mm/yr) and a 
remarkable seismogenic potential of this fault, its subsurface setting and Pleistocene cumulative displacement are still unknown. 
We investigated the shallow subsurface of a key section of the PPF using seismic and electrical resistivity tomography coupled 
with time-domain electromagnetic measurements (TDEM). We provide 2-D Vp and resistivity images showing details of the fault 
structure and the geometry of the shallow basin infill down to 35-40 m depth. We can estimate the dip and the Holocene vertical 
displacement of the master fault. TDEM measurements in the fault hangingwall indicate that the pre-Quaternary carbonate 
basement may be found at ~90-100 m depth.  
 
Key words: Central Apennines, refraction tomography, electrical resistivity tomography, time-domain electromagnetic measurement, active fault. 
 
 
INTRODUCTION 
 
The inner sector of the central Apennines of Italy is 
characterized by a complex network of Pliocene-
Quaternary normal faults (Cavinato & de Celles, 1999; 
Ghisetti & Vezzani, 1999) that originated several 
intramontane continental basins (Bosi et al., 2003; Giaccio 
et al., 2012; Pucci et al., 2014). Under the current NE-
directed extensional stress regime, the high crustal 
seismicity rate is mainly released by normal-faulting 
earthquakes (Chiarabba et al., 2005; Chiaraluce, 2012 and 
references). Paleoseismic analyses have unravelled the 
late Holocene slip-history of many active normal faults in 
this sector of the chain (Galli et al., 2008 and reference 
therein; Cinti et al., 2011). Notwithstanding the wealth of 
geological data constraining the recent evolution of most 
normal faults, only in few cases detailed subsurface 
information is available to provide a proper structural 
model of the basins they originated. Shallow geophysical 
investigation of active faults may extend the limited 
investigation depth typical of paleoseismic trenching, 
moreover it can yield valuable information on long-term 
fault displacement and unravel minor structures (e.g.: 
Villani et al., 2014). 
As a case-study, we selected the Piano di Pezza fault 
(hereinafter PPF, Figure 1). The PPF is the north-
westernmost segment of the >20 km long active Ovindoli-
Pezza normal fault system, and according to paleoseismic 
data it exhibits one of the highest Holocene vertical slip 
rates in the Apennines (~1 mm/yr; Pantosti et al., 1996), with 
large inferred vertical displacement per event (> 2 m). This 
fault bounds to the north a 5 km long continental basin 
(Piano di Pezza; Figure 1) filled with glacial, lacustrine and 
fluvial deposits, and its footwall exposes Mesozoic-Tertiary 
marine limestones. The subsurface structure and infilling 
thickness of this basin are unknown, nor the shallow 
geometry of the bounding fault is well constrained. 
 
 
Figure 2: close up view of the survey area showing seismic, 
electrical resitivity profiles and TDEM measure points 
(orange); the PPF exhibits two splays: we investigated the 
shallow subsurface of the lower one, where it displaces late 
Holocene alluvial fans; tA is trench A after Pantosti et al., 
1996. 
 
Figure 1: geological sketch of the Piano di Pezza basin. The 
blue lines indicate normal faults, the red lines thrusts. 
511
6th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology,  
19-24 April 2015, Pescina, Fucino Basin, Italy 
 
 
INQUA Focus Group on Paleoseismology and Active Tectonics 
 
 
 
 
With the aim of extending information from previous 
paleoseismic analyses and very shallow GPR 
investigations (Jewell & Bristow, 2006), we performed an 
integrated geophysical survey focusing a key section of 
the PPF (Figure 2). We acquired 2 high-resolution seismic 
profiles, 1 electrical resistivity profile and 7 time-domain 
electromagnetic soundings (TDEM) crossing the fault 
where it displaces some small late Holocene alluvial fans, 
originating a ~5 m high fault scarp.  
We try to infer the subsurface fault geometry and 
recover the depth of the pre-Quaternary basement. 
 
METHODS AND DATA ACQUISITION 
 
Traveltime tomography proved successful in imaging 
complex media, such as fault zones (Morey & Schuster, 
1999; Sheley et al., 2003), so we conceived our seismic 
investigations to get tomographic images of the Piano di 
Pezza fault at depth. The 2 seismic profiles (named P1 
and P2) are 142 m long each, and they were acquired 
using a 72 vertical geophones array (40 Hz 
eigenfrequency) with 2 m spacing and a sledgehammer 
as energy source. We handpicked ~20,000 first-arrival 
traveltimes that were input to a non-linear tomographic 
code to produce 2-D Vp models of the shallow 
subsurface (details of the method in: Improta et al., 
2002).  
Electrical resistivity tomography (ERT) is a powerful tool 
to investigate shallow faults and complex structures as 
well (Griffits & Barker, 1993; Storz et al., 2000). We 
acquired 1 ERT profile paralleling seismic line P2. We 
deployed a 64 electrodes and 315 m long array with 
both Wenner-Schlumberger and dipole-dipole 
configurations of quadrupoles. The apparent resistivity 
data were inverted through a linearized least-squares 
algorithm to obtain true resistivity sections (Constable et 
al. 1987; Loke & Dahlin 2002).  
 
 
Figure 3: shallow electrical resistivity model across the 
Piano di Pezza fault. 
 
 
In order to infer the pre-Quaternary basement depth in 
the hangingwall of the PPF, we also performed 5 time 
domain electromagnetic measurements (TDEM). The 
method is based on the induction of a time-varying 
secondary magnetic field produced by decay current in 
the ground. This secondary magnetic field is measured 
by an induction receiver coil generally placed inside of a 
transmitter loop. As the current diffuses deeper into the 
ground (at later time), the signal measurements provide 
information on the conductivity of the lower layers. Our 
data were collected using a high frequency 1D receiver 
coil, placed at the centre and outside a transmitter 
square loop of 50 m size. We acquired very early time 
gates data starting from ~ 9 µs (after current turn off) up 
to 800 µs using a Geonics 47 transmitter. Random 
electromagnetic noise were also collected and checked 
against measured voltage data in order to discharge 
biased late time gates before data inversion. Data from 
field measurements were processed and inverted using 
SiTYEM\SEMDI software in order to obtain 1D resistivity 
models of the subsoil (Hydrogeophysics-Group, 2001)  
 
PRELIMINARY RESULTS AND DISCUSSION 
 
We show some preliminary results of our integrated 
geophysical surveys. The ERT model (Figure 3) with 
dipole-dipole configuration across the fault scarp clearly 
depicts the ~50° SW-dipping fault zone involving alluvial 
fan deposits with resistivity <500 m and their 
concurrent thickening in the hangingwall. Moreover, the 
top-surface of the underlying high-resistivity body 
(>1000 m) shows an evident dip towards the fault, 
suggesting back-tilting due to fault activity. We interpret 
the shallower layers as Holocene alluvial fan gravelly and 
sandy deposits (recovered in the trenches by Pantosti et 
al., 1996) laying over a > 20 thick Last Glacial Maximum 
till (~21-18 ka; Cassoli et al., 1986; Giraudi, 1989). Vp 
tomograms suggest this alluvial fan is affected by > 10 m 
vertical displacement (Figure 4). They also help defining 
the fault-zone down to 35-40 m depth suggesting it has 
a ~50° dip. 
 
  
Figure 4: detail of the Vp tomogram of seismic line P2 
across the PPF compared with trench A (after Pantosti et 
al., 1996). 
 
 
TDEM data inversion yielded 1-D vertical resistivity 
models (Fig. 5): they indicate that ~200 m far from the 
surface fault trace the pre-Quaternary carbonate 
basement is 90-100 m deep, thus giving a robust 
constraint to the evaluation of the cumulative offset of 
the PPF (~150 m). Moreover, for the uppermost part of 
the subsoil, TDEM and ERT model significantly agree in 
terms of depth and thicknesses of the shallower 
resistivity layers. 
 
512
6th International INQUA Meeting on Paleoseismology, Active Tectonics and Archaeoseismology,  
19-24 April 2015, Pescina, Fucino Basin, Italy 
 
 
INQUA Focus Group on Paleoseismology and Active Tectonics 
 
 
 
 
 
 
Figure 5: 1-D resistivity models obtained through inversion 
of TDEM data (position of soundings along the ERT profile 
is idicated by red stars). A deep resistive layer (>5000 m), 
interpreted as the carbonate basement, is evidenced at 90-
100 m depth (beneath soundings 2offB, 2c and 2offA) in the 
hangingwall of the PPF. Data from soundings 1off and 1c 
are less reliable in the deeper part due to strong 3D effects. 
 
 
Our geophysical data yield for the first time valuable 
information on the subsurface setting of an important 
active normal fault exhibiting one of the highest 
Holocene slip rates in the central Apennines. These data 
are crucial to understand the middle- to long-term 
behaviour of this fault, its overall displacement and 
possibly its inception age. 
 
Acknowledgements: This study was performed in the framework 
of the FIRB Abruzzo project, “High-resolution analyses for 
assessing the seismic hazard and risk of the areas affected by the 
6 April 2009 earthquake” (http://progettoabruzzo.rm.ingv.it/en; F. 
Villani, V. Sapia & R. Civico funding code: RBAP10ZC8K_005; P. 
Baccheschi funding code: RBAP10ZC8K_003; G. Di Giulio & M. 
Vassallo funding code: RBAP10ZC8K_007).  
 
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Shallow subsurface imaging of the Piano di Pezza active normal fault (central Italy)  using high-resolution refraction and electrical resistivity tomography coupled with  time-domain electromagnetic data

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Shallow subsurface imaging of the Piano di Pezza active normal fault (central Italy) using high-resolution refraction and electrical resistivity tomography coupled with time-domain electromagnetic data

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