GEOLOGICA CARPATHICA, AUGUST 2005, 56, 4, 297306
Effects of lithological features and tectonic structures in the
evaluation of local seismic response: an example from the
Hyblean Plateau (Eastern Sicily)
ROSARIA RIGANO and GIUSEPPE LOMBARDO
Dipartimento di Scienze Geologiche, University of Catania, C.so Italia 55, 95129 Catania, Italy; firstname.lastname@example.org; email@example.com
(Manuscript received July 1, 2004; accepted in revised form December 9, 2004)
Abstract: The influence of faults and different lithology in the evaluation of possible seismic energy amplification has
been studied. The area of Augusta, located in the Hyblean Plateau (south-eastern Sicily), was selected as test site and
several ambient noise measurements were carried out with the aim of investigating the local seismic response. Data were
gathered both on the main lithotypes outcropping in the study area and along some profiles crossing the Mt Tauro fault,
in order to estimate the site response connected to the different geo-lithological features and to investigate on the pos-
sible amplification effects caused by the presence of a tectonic structure. The horizontal to vertical (H/V) spectral ratios
calculated for the sites located near the fault showed a rough tendency towards amplifications in the frequency band 1.5
2.0 Hz, disappearing with increasing distances from the fault. Significant amplifications were observed at about 0.7 Hz
in the sites located on the clays and in all the lithotypes laying on them. Observed dominant spectral peaks have been
explained as the resonance of a soft sedimentary sequence overlaying an equivalent basement located inside the clayey
formation at a depth of 160290 m, or at the stratigraphic interface separating the clays from the calcareous basement.
Finally, the results of 1-D modelling allowed us to delimitate the trend of the Augusta graben at depth.
Key words: Hyblean Plateau, Sicily, Augusta, Mt Tauro fault, ambient noise, local seismic response, Nakamura technique.
South-eastern Sicily is one of the Italian regions with the high-
est seismic hazard. It is characterized by high-energy releases
with events (M ≈ 67), which have caused heavy damage over
a territory, which especially in recent times was highly urban-
ized. The study area is located in the north-eastern boundary
of the Hyblean Plateau, which is a Mesozoic-Cenozoic car-
bonatic massif with repeated volcanic levels laying on the Af-
rican continental crust, as shown in the top right inset in
A series of horst and graben structures characterizes the tec-
tonic setting of this area. They are oriented NWSE and
NNWSSE and are linked to the Malta-Hyblean escarpment.
It is along this tectonic system that the strongest earthquakes
historically affecting south-eastern Sicily took place. The
1169 and 1693 destructive earthquakes belong indeed to the
coastal sector of Hyblean Plateau and originated in such re-
gional scale tectonic structures. According to recent papers
(Barbano & Rigano 2001; Barbano et al. 2001a) the town of
Augusta is one of the localities with the highest probability of
occurrence of destructive events. The instrumental data refer
to the 19861995 time interval (Salvi et al. 1996) and describe
the seismicity in terms of small magnitude events that, in most
of the cases, are located along the eastern coastal sector. The
most recent large earthquake (M = 5.4) was located in the Ion-
ian Sea at a depth of about 10 km (Amato et al. 1995). It oc-
curred on December 13
1990 and it is the only event for
which instrumental parameters are available (Giardini et al.
1995). In spite of the moderate magnitude of the 1990 event,
the town was seriously damaged and the VIII degree of the
MCS scale was here found.
In the study area it is possible to distinguish a bedrock and
an overlaying lithostratigraphic sequence, whose geological
characteristics and geometry can potentially induce variations
in the seismic energy recorded at different shallow sites. Sig-
nificant differences in the site response of different lithotypes
laying on the bedrock were preliminarily observed by Barbano
et al. (2001b).
It is a well-documented phenomenon that earthquake
ground motion can be amplified by local site conditions (Aki
1988). Near-surface impedance contrasts between bedrock
and shallower deposits can significantly affect the frequency-
amplitude content and the duration of earthquake ground mo-
tion. A case history illustrating such a phenomenon occurred
during the 1985 Michoacan, Mexico, earthquake (M = 8).
This event, despite its significant epicentral distance, excited
the ~2 sec fundamental vibration mode of the Mexico City
lake bed zone, causing severe damage and thousands of deaths
(Anderson et al. 1986; Singh et al. 1989). Recently, strong ev-
idence that site response contributed to enhancing the damage
was observed in the town of Nocera Umbra, during the 1997
Umbria-Marche earthquake (Donati et al. 2001). The highest
damage grades were estimated for two zones, on the soft sedi-
ment of a river valley and on the terrains within a 200 m wide
fault zone crossing the town.
The aim of this paper is to determine the seismic site re-
sponse of the Augusta urbanized area, by investigating both
the local response of sites with different soil conditions and in
the neighbourhoods of a tectonic structure. The focusing of
298 RIGANO and LOMBARDO
Fig. 1. Geolithological map of the Augusta area. The upper right inset map shows a tectonic sketch of eastern Sicily.
seismic waves by tectonic discontinuities can indeed cause
spatial variations of ground motion in both amplitude and fre-
quency since stationary waves are generated by the impedance
contrast between the cataclastic area and the surroundings
rocks (Li & Leary 1990).
The methodology adopted involves the use of ambient noise
and the comparison of the spectral ratios with the results from
a 1-D modelling in order to evaluate the resonant frequency of
the soft cover layers.
Augusta urban area is located in the Hyblean Plateau, which
represents the emerged foreland elevated with respect to its
surroundings. It partially stands on a little island where ter-
rains of lower Pleistocene age, prevailingly formed by cal-
carenites and clays, outcrop. The whole lithostratigraphic se-
quence which characterizes this area consists of alternating
sedimentary deposits and pyroclastic layers laying on a Mi-
ocene carbonatic formation well known as the Syracuse Lime-
stone Member of the Mt Climiti Formation (Pedley 1981).
This last overlies unconformably on Cretaceous lavas mostly
represented by pyroclastics, and sometimes by highly altered
lavas. The geolithological features of the study area are shown
in the map of Fig. 1, redrawn from Lentini et al. (1986).
The distribution and the geometric features of the faults in
the north-eastern sector of the Hyblean Plateau are connected
with the lower Pleistocene alternating tectonic movements of
uplift and lowering, which imply a structural setting with horst
and graben. In this framework, the NNWSSE trending fault
structures of Mt Tauro horst are arranged in a typical en-eche-
lon pattern. They are linked, together with the Augusta gra-
ben, to the Malta Escarpment strike-slip component of motion
The geological features of the investigated area were also
identified by collecting all available information from existing
boreholes. Such data were used to draw the geological cross-
section shown in Fig. 2. It depicts the average thickness of the
main lithotypes outcropping in the study area. Down-hole and
SASW (spectral analysis of surface waves) measurements
were used as well in order to characterize the geophysical and
geotechnical properties of various lithotypes. In addition to
these data an electrical tomography profile (160 m) was per-
LITHOLOGICAL STRUCTURES IN THE EVALUATION OF SEISMIC RESPONSE (SICILY) 299
Fig. 2. Geological cross-section and results of electrical tomography in the Mt Tauro fault area.
formed in the area where the Cretaceous pyroclastics outcrop
in order to investigate the features at the depth of the Mt Tauro
fault. The results (see inset profile in Fig. 2) show that at about
20 meters depth there is a lateral electric discontinuity that
marks a sharp change of the resistivity values from 108 to
17.3 ohm·m. The sudden lateral discontinuity between clays
and limestones can be explained by postulating a step down
fault, which lowers the clayey formation at the same depth of
the carbonatic and volcanic pyroclastic levels.
Methodology, data acquisition and processing
Microzoning of an earthquake prone area, such as the study
one, can be tackled by analysing the available information
about tectonic, geological and geotechnical features of all ex-
isting formations and lithotypes. The best procedure for deter-
mining the site response is to observe the ground motion dur-
ing an actual event. This can be done, using strong or weak
motion records, by direct comparison of a sediment site to a
reference one located on competent ground. Another alterna-
tive approach, first introduced by Kanai (1957), involves the
use of microtremors or ambient seismic noise. Among the
most used methods, the spectral ratio technique is very popu-
lar using either a reference rock station or taking the horizon-
tal-to-vertical component ratio. In recent times many authors
have demonstrated that the horizontal to vertical (H/V) spectral
ratios from ambient noise measurements are consistent in shape
with the ones coming from earthquake recordings and with
those obtained by using a reference bedrock station (Bard
1999). Therefore the use of the H/V spectral ratio of ambient
noise (Nakamura 1989) is widely adopted to characterize earth-
quake site response. This technique has been successfully used
to estimate the natural frequency of resonance of sedimentary
layers and recently extended by Lermo & Chávez-García
(1993) to evaluate sediment-induced amplification of S waves.
The best performance of the method is achieved when there is a
strong velocity contrast between the bedrock and the soft upper
layers. Moreover, the features of the layering of sediments itself
strongly affect the frequency band and the shape of the domi-
nant peaks in the H/V spectra (Fäh et al. 2001).
Several seismological studies did provide evidence of non-
linear behaviour of soft soils. Non-linear effects have the ten-
dency to reduce the amplitudes of the spectral peaks and shift
them towards slightly longer periods. Because of possible
non-linearities, the choice of weak-motion signals to be used
in predicting strong-motion behaviour remains a problem, and
the answer may vary according to soil type. However, as dis-
cussed by Aki (1988), recent studies show good correlation
between weak- and strong-motion site response.
We calculated the H/V spectral ratios of ambient noise re-
corded at different sites in the area of Augusta in order to esti-
mate the resonance frequency of sedimentary layers and to in-
vestigate the local seismic response due to the presence of a
A series of three component microtremor measurements
were sampled in 51 sites with different lithology (Fig. 1).
Measurements were also performed along five profiles cross-
ing the Mt Tauro fault. A total number of 29 additional sam-
pling sites was therefore selected for each profile, starting
from the fault scarp and moving apart from both edges of the
structure with steps of about 50 and 100 meters. The sites lo-
cated along the profiles are marked with both number and star
in order to distinguish them from the numbering of the sites
located on different lithotypes (Fig. 4).
Seismic signals were recorded using a 1-Hz Mark L4C 3-D
seismometer connected to a 12 bit analog-to-digital converter
and a notebook. Horizontal components of the geophone were
oriented parallel to the NS and EW geographic directions.
Sampling frequency was 100 Hz and two antialiasing filters
cut higher frequencies with a 10 db/oct slope. At each site five
Fig. 3. Spectral ratio obtained by averaging the H/V ratios over 5
time histories of 120 s recorded on the calcareous basement. The
mean spectral ratio is the dark curve; the upper and lower thin
curves represent the ±1 standard deviation interval.
300 RIGANO and LOMBARDO
LITHOLOGICAL STRUCTURES IN THE EVALUATION OF SEISMIC RESPONSE (SICILY) 301
time series of 120-s length were recorded. The time series
were baseline corrected in order to remove spurious offsets
and low-frequency trends. After the application of a Hanning
window a Fast Fourier Transform algorithm was applied, to
obtain spectra in a frequency band 0.120 Hz. The lower limit
of 0.1 Hz was assessed through the low-frequency cut off
where the microtremor standard deviation becomes equal to
the natural noise fluctuation of the 12-bit digitizer.
For each time history the vector composition of horizontal-
components noise spectra were divided by the vertical one. Fi-
nally the arithmetic average operation provides the mean H/V
spectral ratio at each measurement site. A running smoothing
function of 0.1 Hz was also used, both on the spectra of each
single component and on the final ratio, to reduce spectral
Before describing the results it is worth specifying that am-
bient noise measurements were performed in some sites locat-
ed on the bedrock (limestones) in order to obtain a univocal
standard for defining the significant amplification spectral
peaks. The standard deviation evaluated for the H/V produces
spectral fluctuations of ±0.5 units, around mean amplitude
values not exceeding 3.0 units (Fig. 3). On the basis of these
Fig. 5. H/V spectral ratios computed from measurements performed in sites located on the limestones (a) and on the calcarenitic terraces
(Ms, Qs and Mvc) overlaying them (b).
fluctuations only spectral peaks higher than 3.5 units were tak-
en into account as statistically significant.
The schematic cross-sections shown in Fig. 4 refer to the
short profiles crossing the Mt Tauro fault. In each profile, H/V
spectral ratios obtained from noise measurements performed
in sites located at increasing distance from both edges of the
fault, are shown. It is possible to observe, in the frequency in-
terval 1.52.0 Hz, a tendency towards the amplification of the
spectral peaks in the sites located in proximity of the tectonic
structure. Although this is not a systematic behaviour, it is
more evident in the BB, CC, DD and EE profiles,
where the afore-mentioned spectral peaks tends to decrease
their amplitude at distance of about 3050 m from the fault.
Ambient noise was also sampled in 51 sites located on dif-
ferent soils outcropping in the whole territory of Augusta
(Fig. 1). The resulting H/V spectral ratios are grouped for sim-
ilar lithotypes and plotted in the Figs. 5, 6 and 7. The H/V at
sites located both on the limestones (Mc) (Fig. 5a) and on the
calcarenites (Ms, Qs, Mvc) (Fig. 5b) overlaying them show a
flat shape in the frequency range 0.515 Hz. Only at the sites
14, 15 and 17, amplification peaks at 1.52.0 Hz and 4.0
5.0 Hz, are more or less evident.
As regards the sampling sites located on the clays the H/V
spectral ratios show amplification at frequency values of about
0.7 and 1.5 Hz (Fig. 6a). Similar amplification peaks are ob-
served in almost all the spectral ratios obtained from data re-
302 RIGANO and LOMBARDO
corded in the lithotypes laying on the clays, such as the cal-
carenites Qm (Fig. 6b) and salt pits (Fig. 7a). These terrains
have a modest (not exceeding ten meters) thickness compared
to that of the clayey formation.
The H/V obtained from ambient noise measurements in
sites located on the alluvial deposits show amplification peaks
at values ranging between 1.5 and 2.8 Hz (Fig. 7b).
1-D modelling and evaluation of soft sediments thickness in
the Augusta graben
In order to quantify the effect of the local stratigraphy, spec-
tral peaks experimentally observed were compared with the
Fig. 6. H/V spectral ratios computed from measurements performed in sites located on the clays (a) and on the calcarenites (Qm) over-
laying them (b).
results of a 1-D modelling, using the Dobry et al. (1976) and
the Haskell-Thomson methods. The method of Dobry, well
known as the Successive use of two layers method was used
both to test the reliability of the obtained results and to inter-
pret them from a geological-structural point of view. The
physical model adopted is a resonant cavity closed at its lower
extremity, which generally coincides with the contact between
the basement and the overlying formations. The basement
has, in this case, a physical meaning rather than the classical
geological one. It refers indeed to the physical interface under-
neath which the seismic input can be considered constant.
This is, in other words, the so called equivalent basement
(SEAOC 1974) indicating the interface, existing in any mate-
LITHOLOGICAL STRUCTURES IN THE EVALUATION OF SEISMIC RESPONSE (SICILY) 303
Fig. 7. H/V spectral ratios computed from measurements performed in sites located on the salt-pits (a) and on the alluvial deposits (b).
The fairly good relationship observed between the comput-
ed theoretical transfer function and the experimental dominant
H/V spectral peaks suggested that the authors should attempt a
correlation of the resonant frequencies experimentally detect-
Fig. 8. Theoretical transfer functions (dark lines) and H/V spectral
ratios (thin lines) obtained from the sites 48, 20 and 19 located on
clays, calcarenites and salt pits, respectively.
Table 1: Input data and results of 1-D modelling for three sites lo-
cated on clays, calcarenites and salt pits.
rial, underneath which the shear wave velocity at low defor-
mation (0.0001 %) reaches the value of V
Using this method it is possible to calculate the fundamental
period of a stratigraphic sequence by dividing it into a series
of layers pair. The fundamental period T of a layers pair is ob-
tained solving the equation:
where and are the fundamental
periods of the layers a and b, Ha, Hb, Va, Vb,
ρρρρρa and ρρρρρb are
the thicknesses, the shear wave velocities and the densities of
the layers a and b respectively. A detailed description and
some examples concerning the use of this method are given in
Through the Haskell-Thomson method the theoretical trans-
fer function for SH vertically incident waves was also comput-
ed, using as input data the thickness of the layers, the density
values and the velocities of transversal waves taken from
down-hole tests and SASW measurements available for the
study area. The input data and the results obtained through the
1-D modelling are listed in Table 1. In Fig. 8 the comparison
between the theoretical transfer function and three experimen-
tal H/V spectral ratios, is shown. These last were selected as
examples since they refer to the sampling sites closer to the lo-
cation of down-hole (see Fig. 1).
The theoretical site response obtained by using this model-
ling, shows a tendency to underestimate real data amplifica-
tions, but it gives resonant frequencies lower than 1 Hz, as ex-
perimentally observed in the H/V spectra. The 1-D modelling
shows indeed that dominant spectral peaks can be explained as
the resonance of a soft layer, having thickness ranging between
160 and 290 m, overlying an equivalent basement located ei-
ther inside the clayey formation or at the stratigraphic interface
separating clays from the calcareous basement (Table 1).
304 RIGANO and LOMBARDO
In Fig. 9 a map obtained by plotting the values of thickness
of the sedimentary sequence filling the Augusta graben, esti-
mated through this equation, is shown. The map shows that
the clayey formation, widely outcropping in the Augusta gra-
ben, has a thickness ranging from 100 up to 300 meters going
from the edges to the central part of the basin.
Discussion and conclusions
The distribution of seismic energy depends on the complex-
ity of morphological and structural features of the investigated
area, as well as on its lithology. The different distribution of
earthquake damage in urban areas and, in general, of seismic
energy, depends indeed on the complexity of topographic,
structural and lithological features of the study areas. The fo-
cusing of seismic waves by tectonic discontinuities and the
trapping of the energy in basins can also cause spatial varia-
Fig. 9. Thickness contour map of the clayey formation in the Augusta graben.
ed with the thickness of the soft cover layers in the Augusta
graben. Such correlation is founded on the assumption that the
transfer function of a sediment site is connected to the geome-
try and physical parameters of the subsurface layers, namely,
their thickness and shear wave velocities (Ibs-von Seht &
Wohlenberg 1999). Theoretically the thickness (h) of a sedi-
mentary basin could be estimated from its resonant frequency
) and the shear wave velocity (V
) through the well known
relationship h = V
. According to Ibs-von Seht & Wohlen-
berg (1999) in order to take into account the gradient of ve-
locity with depth, the relationship can be used,
where m is the mean thickness and f
is the main peak in
the Nakamura spectral ratio. The above relationship was tested
in the Lower Rhine Embayment. We adopted it in the study
area since the geotechnical parameters of Rhine Embayment
= 303 m/s, ρ = 2.0 g/cm
) appear very similar to
those of the clayey sediments filling up the Augusta Basin
= 295 m/s, ρ = 1.9 g/cm
LITHOLOGICAL STRUCTURES IN THE EVALUATION OF SEISMIC RESPONSE (SICILY) 305
tions of ground motion in both amplitude and frequency. The
aim of this research was therefore to investigate the influence
of tectonic structures on the local seismic response and to de-
termine the resonant frequency of the soft cover layers.
Spectral ratios H/V have been computed for several sites in
the area of Augusta, located both on different soils and along
short profiles crossing the Mt Tauro fault. A tendency towards
site response amplification in the frequency range 1.52.0 Hz
was observed at the sites located close by the fault. Although
this is not a systematic behaviour, it clearly appears in some
profiles and such amplification tends to rapidly decrease for
distances ranging from about 30 to 50 meters from the struc-
tural discontinuity. Our methodological approach does not al-
low us to investigate the possible directional effects linked to
the strike of the fault. We rather directed our attention to rela-
tive differences between the amplitude of spectral peaks from
measurements close by the fault and at increasing distances
from it. Moreover, in order to simplify our observations we
deployed the profiles trying to locate them in portions of the
fault where it dislocates the same lithotype. The observed ten-
dency towards amplification effects in proximity of the Mt
Tauro fault appears therefore to be a significant consequence
of the impedance contrast between the gouge zone and the sur-
As regards the H/V obtained for the main lithotypes out-
cropping in the Augusta territory, at sites located both on the
limestones and on the calcarenites overlying them, a flat shape
of the spectral ratios in the frequency range 0.515 Hz, is ob-
served, therefore confirming the lack of amplification effects
in the lithotypes forming the basement. At sites located on the
clayey formation and on the thin sedimentary layers such as
salt-pits and quaternary calcarenites laying on them, signifi-
cant amplifications were observed at frequency values (0.7
1.5 Hz) that are compatible with the considerable thickness of
the clayey formation itself.
Comparison of experimental H/V dominant spectral peaks
with those obtained through the evaluation of the theoretical
transfer function, confirm that the resonance effect appears to
be related to the considerable thickness of the clayey forma-
tion, rather than to the thin shallow sediments. The equiva-
lent basement, detected through the 1-D modelling bounds
downwards the resonant soft sequence at a depth ranging from
160 up to 290 meters and it is located either among the clays
or at the stratigraphic interface separating them from the lime-
stones forming the basement.
The evaluation of the thickness of soft sediments infilling
the Augusta graben, shows that the maximum values of about
300 meters are reached in the central part of the basin. This re-
sult is in fairly good agreement with the thickness contour
map of the Pleistocene clayey formation, obtained from geo-
logical data by Carbone (1985).
Acknowledgment: This research was financially supported
by the Presidenza del Consiglio dei Ministri Dept. of Civil
Defence and the Presidenza della Regione Siciliana Sistema
Poseidon. The authors wish to thank Prof. A. Di Grande for
his support in describing and understanding the geological
features of the area. Special thanks are given to Prof. S. Gres-
ta for a series of useful discussions and suggestions both in the
planning and carrying out of the study. We are also grateful to
Prof. Riuscetti and to an anonymous reviewer for critical read-
ing of the manuscript and useful suggestions.
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