Open Journal of Earthquake Research, 2013, 2, 91-105
Published Online November 2013 (
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Analysis of the Low-Energy Seismic Activity
in the Southern Apulia (Italy)
Pierpaolo Pierri1, Salvatore de Lorenzo1, Gildo Calcagnile1,2
1Dipartimento di Scienze della Terra e Geoambientali, Università degli Studi di Bari “Aldo Moro”, Bari, Italy
2Osservatorio Sismologico, Università degli Studi di Bari “Aldo Moro”, Bari, Italy
Received September 20, 2013; revised October 25, 2013; accepted November 8, 2013
Copyright © 2013 Pierpaolo Pierri et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In this paper we analysed the historical and instrumental seismicity of the seismic district “Penisola Salentina” (Salento
peninsula) in the southern part of the Apulia region, making use of the most recent seismological database. Relocation
of available dataset points out that the events are spatially distributed over a belt of deformation that approximately
corresponds to Soglia Messapica (Taranto-Brindisi depression). Besides, computed source characteristics indicate dex-
tral strike-slip solutions with an approximately E-W orientation that seismologically confirm previous geodynamic
studies indicating a NE-SW extension in the Taranto-Brindisi depression. In particular, the tensional stress associated to
the present seismic activity could be the consequence of the relaxation of the buckling process following the extensional
re-arrangement of the Apenninic belt masses. Moreover attenuation QP values obtained in this study are much greater
than those inferred in other parts of Italian peninsula; this result agrees with previous macroseismic investigations and
indicates a greater efficiency of the studied area in the transmission of body waves.
Keywords: Salento Peninsula (Southern Italy); Seismicity; Focal Mechanisms; Source Parameters
1. Introduction
The Apulian region stretches for about 350 km in the
southern part of Italy, between the Adriatic and Ionian
Sea. It is constituted by an emerged sector of the Apulian
plate [1] or Adriatic subplate [2] or Adria [3], character-
ized by a relatively thick lithosphere [4] and by a weakly
deformed sedimentary cover [5].
This plate, which shows a marked elongation from
NW to SE, represents the Plio-Pleistocene foreland
(Apulian foreland) of the Southern Apennine orogenic
system to west and of the Dinaric and Hellenic chains to
east, generated along its boundaries by the interaction
with more deformable lithospheric structures. These re-
gions are affected by diffuse seismicity whose character-
istics are correlated to a general counter-clockwise mo-
tion of Adria [6,7] (Figure 1).
The internal part of Adria shows a minor, but not neg-
ligible, seismic activity [8,9]; in particular this region is
near to different areas in which seismicity is frequent and
intense; for example the Salento peninsula is less than
100 km far from Albanian and Greek coasts where many
energetic earthquakes occurred. Besides the propagation
characteristics of the lithosphere of the foreland permit to
the energy irradiated by hypocenters distant a few hun-
dreds of kilometres of arriving into the Salento peninsula
only weakly attenuated, as demonstrated from felts of
Albanian and Greek earthquakes [10,11] and from mac-
roseismic field of recent earthquakes (e.g. 8 January
2006, 3 February 2007, 25 March 2007).
The southern part of Apulia has been generally con-
sidered practically aseismic [12]. The seismic history of
Salento peninsula shows that only one event of magni-
tude higher than 6.0 is reported in the historical cata-
logues, killing hundreds of people, in particular in the
town of Nardò and Francavilla Fontana: this earthquake,
which occurred on 20 February 1743, caused damage
maximum effects of IX-X degree on the Mercalli-Canca-
ni-Sieberg scale (MCS) and also a tsunami [13], with
boulder accumulations along the Otranto-Leuca coast
[14]; it has been felt in an abnormally wide area, from
Greece, Albanian, Malta to northern Italy; its magnitude
was in the range 7.13 ± 0.19 [15]. It is affected by remark-
able uncertainty in the location, but most likely occur-
red south of the Salento Peninsula (Apulian Ridge), in the
Figure 1. Structural sketch of Italy and surrounding areas [6]. Black arrows indicate the slip vectors of Africa vs. Europe and
of Adria vs. Europe obtained from geodetic data. Adria RP is the Adria rotation pole.
same area where there have been several other recent
earthquakes (e.g. 20 October 1974, 23 November 1974).
According to Argnani et al. [16] the main triggering fac-
tor is the local stress accumulation due to the small ra-
dius of curvature of the Adriatic-Apulian plate under the
double load of the Hellenides and Apennines-Calabrian
arc. This event has been studied in detail by Galli and
Naso [11]: it seems that the highest intensities in Salento
peninsula have been controlled by local amplification
(double resonance) that occurred in all localities charac-
terized by thin Pleistocene basins filled with soft sedi-
ments, such as Nardò, Francavilla Fontana and Leverano.
The depth of the seismogenic source (30 - 40 km), its
directivity effects (toward Salento, in NW-SE direction)
and the strong site amplification were considered the
reasons of both the large areal distribution of effects and
the locally high gravity of damages causing the devas-
tating shaking [11].
The occurrence of strong earthquakes in this area is
also suggested by Pieri et al. [17], considering the sparse
occurrence of seismites in the Tyrrhenian deposits along
the Adriatic-Apulian coast; besides additional data on the
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possible presence of a strong seismic activity that would
affect the Salento peninsula have been put forward on the
basis of geomorphological (e.g. through the study of
speleothems and tsunami traces) and archaeological evi-
dence (recent excavations have revealed the possible
origin of collapse of walls).
The seismic activity inside Salento peninsula is almost
absent or of low-energy and therefore it is rarely re-
corded, due to the lack of seismic stations (until about 15
years ago the “Istituto Nazionale di Geofisica e Vulca-
nologia” (INGV—Rome) managed only 2 stations in an
area more than 5000 km2). In the last years, four seismic
stations were implemented in the southern-central part of
Apulia region and added to the network managed by the
“Osservatorio Sismologico dell’Università di Bari”
(OSUB). The recordings of this network, in conjunction
with those of the Italian National Seismic Network
(RSNC), operated by the INGV, allowed detecting sev-
eral low-energy events, such as those occurred on 5 May,
2012 near Ostuni and felt by many inhabitants, of local
magnitude ML 2.8.
In this paper we present an analysis carried out on this
earthquake and on other events located in the seismic
district named “Penisola Salentina” (Figure 2). The main
goals of this work are: 1) relocation of the recorded
events with Vp/Vs computation using a modified Wadati
method; 2) focal mechanisms computation; 3) determina-
tion of source (corner frequency, source dimension,
seismic moment and stress drop) and attenuation (quality
factor) parameters.
The geographical position of the most important lo-
calities of the studied area and the seismic stations (be-
longing to OSUB and INGV network), considered in the
revision of instrumental seismicity, are shown in Figure
2. Geological Setting
The studied area is located in the southern part of the
Apulian region, stretching between the Ionian and the
Adriatic Sea (Figure 2). This region is the emerged part
of the foreland domain of both Apenninic and Dinaric
orogens; it constitutes a Variscan basement covered by a
3 - 5 km thick Mesozoic carbonate sequence and is over-
lain by thin deposits of Tertiary and Quaternary age. The
Apulian foreland is weakly deformed and affected by
0 50
16˚E 15˚E 18˚E 19˚E 20˚E
Figure 2. Geographic location of the study area with the seismic stations used in the relocation. Red triangles represent the
stations of OSUB network, while blue circles indicate the stations of INGV network. The yellow rectangle is the area selected
for the extraction of seismic events shown in Figure 4; the fuchsia line encompasses the “Penisola Salentina” seismic district.
eographical position of some localities mentioned in the paper is also shown (F.F. indicates Francavilla Fontana). G
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Apenninic and anti-Apenninic trending faults which sub-
divide it into five main structural blocks with different
uplift rates (from NW to SE Gargano, Tavoliere, Murge,
Taranto-Brindisi plain and Salento peninsula).
The Salento peninsula and the Murge block are sepa-
rated by the so-called “Soglia Messapica” [17], also
named Taranto-Brindisi depression [5], which is a high
scarp, mainly oriented E-W; the relationship between
Salento and Murge is complicated by their different rota-
tions [18] and by strike-slip movements along their
boundary (Figure 3(a), North and South Salento Fault
Zone by Gambini and Tozzi [19]).
Normal faults, with trend NW-SE, are present in both
the Salento peninsula and the Salento plateau, repre-
sented by the “Apulian Ridge or Swell” [16,20] (Figure
3(b)); these faults, active in Plio-Pleistocene, almost
transversal to the strike-slip fault in which occurred en-
ergy transfer, dissect the large antiform structure due to
the different regional uplift and the Pleistocene deposits,
witnessing a tectonic activity in Quaternary. The Apulian
Ridge is a morphological element that separates the deep
Ionian basin from the shallower Southern Adriatic basin,
extending from Salento peninsula to the island of Kefal-
3. Data Selection
All the events located in the area having a latitude be-
tween 39.5˚ and 41.5˚N and a longitude between 16.5˚
and 20.0˚E in the period 2003-2012 were extracted from
the Bulletin of the Instrumental Seismicity
( and from the Italian
Seismic Instrumental and parametric Data-basE (ISIDE)
( of INGV.
These events are distributed over the southern Apulia
and surrounding regions. In Figure 4 the 743 extracted
events, subdivided in 9 “seismic districts” (or epicentral
regions), are shown; the districts where the highest num-
ber of events occurred are “Murge” (273) and “Penisola
Salentina” (163).
However this map may be severely biased by the re-
cording of quarry blasts. It has been in fact shown [21]
that many of these events were caused by artificial ex-
plosions. This observation is supported by both the dis-
tribution of the events with respect to the day of occur-
rence and to the hour of occurrence.
In this paper we considered the seismic activity oc-
curred inside the district named “Penisola Salentina”,
where 163 earthquakes were located with a maximum
magnitude ML = 2.8.
Owing to the poor coverage of INGV stations in this
area, some low-energy events were not detected by the
INGV network. As an example, the events occurred on 6
August, 2012 (at 23:14 GMT) and on 6 September, 2012
(at 04:48) were detected only by OSUB stations.
Figure 3. (a) Structural map of the Apulian foreland [19, simplified]: 1) Front of the external Calabrian Arc; 2) main
strike-slip faults (the arrows indicate the movement versus); 3) main extensional faults; 4) other fault alignments; 5) Pescara-
Dubrovnik fault; 6) Tremiti fault; 7) Mattinata fault; 8) North-Salento fault; 9) South-Salento fault; 10) Kefallinia fault; 11)
Scutari fault. (b) Geological setting of the study area [16]: the Apulian Ridge (or Apulian Swell) represents the foreland of the
Apennines and Hellenides fold and thrust belts. With C, L e K are indicated Corfu, Lefkas and Kefallinia Island.
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0 50
Figure 4. Map of instrumentally detected earthquakes of the Southern Apulia and surrounding regions extracted from the
INGV on-line bulletins between 2003-2012. The 743 epicentres are localized in 9 seismic districts (1 and 2 represent the seis-
mic districts “Piana di Sibari” and “Sila” in the SW corner).
The southern and central part of Apulia is historically
characterised by low seismicity [8]. Figure 5 shows the
spatial distribution of historical earthquakes extracted
from the last version of the Parametric Catalogue of Ital-
ian Earthquakes (CPTI11) [15], which covers the period
1000-2006; Table 1 lists a few events located in “Peni-
sola Salentina” seismic district or very close, including
the instrumental (ML = 5.3) earthquake occurred in 1983
near Gallipoli (see Figure 2).
In this catalogue the strong event occurred in 1743 is
located (on macroseismic base obviously) offshore the
southern-eastern Salento coast (at about 50 km), on the
Salento plateau, in the ZS931 (named Otranto Channel)
of the ZS9 seismogenic zonation [22], near well located
epicentres of other earthquakes (Figure 5).
Besides, analyzing the CFTI4Med catalogue [23] other
25 earthquakes located in the examined area occurred in
XX century with M 3.0; it is important to stress that the
strong earthquake occurred in 1743 is relocated in
Salento peninsula, about 20 km north of Gallipoli (grey
star in Figure 5).
The seismic activity reported in the catalogue PFG [24]
is also shown in Figure 5: this catalogue (which covers
the period 1000 - 1980) contains 170 earthquakes oc-
curred in the extraction area, of which 47 earthquakes
with I VI MCS; several events are not present in the
other 2 catalogues, for example the earthquakes occurred
on 10 October, 1858 near Brindisi (I = VI MCS, M = 4.1)
and 26 April, 1970 in the Ionian Sea (I = VII MCS, M =
On the whole, the existing pre-instrumental seismicity
indicates the occurrence of many events of low magni-
tude (M 4.0) in Southern Apulia.
The spatial distribution of instrumentally located
events, extracted from the CSI catalogue [25] in the pe-
riod 1981-2002, has been also analysed; the seismicity is
present everywhere in all the districts, although, consid-
ering only the events with M 3.0, cluster of events can
be noted, for example in the area near Lecce. Since data
relative to these years and to “Penisola Salentina” seis-
mic district are rather poor and with a remarkable uncer-
tainty on the phase readings, we re-analysed the instru-
mental seismicity only in the period 2003-2012.
4. Discrimination between Tectonic
Earthquakes and Quarry Blasts
The concentration of events near Taranto (Figure 4) is
mainly due to an anthropic activity (explosions). We con-
sidered the approach of Wiemer and Baer [26] to analyse
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0 50
Figure 5. Map of historical detected (red circles and blue triangles for events with M 3.0) earthquakes of the Southern
Apulia and surrounding regions extracted from the CPTI11 [15] (yellow squares), PFG catalogue with Imin VI MCS [24]
(blue circles) and CFTI4Med catalogues [23] (red triangles) respectively. Red and grey stars indicate the epicentre of the 1743
earthquake located from CPTI11 and CFTI4Med respectively. The delimitation and the number of the seismogenic zones
according to the ZS9 [22] (dashed lines) is reported. The boundaries of 9 seismic districts are shown.
Table 1. List of historical earthquakes extracted from the Parametric Catalogue of Italian Earthquakes [15] occurred in the
“Penisola Salentina” seismic district or very close to it. See Figures 2 and 9 for locations.
Date Or. Time Lat. N Long. E Mw Imax Location
1087/09/10 - 41.128 16.864 4.93 VI-VII Bari
1713/01/03 - 40.589 17.113 4.51 VI-VII Massafra
1826/10/26 18 40.451 17.678 5.36 VI-VII Manduria
1932/03/30 09:56 40.633 16.900 4.80 VI Castellaneta
1983/05/07 22:09 40.062 17.890 4.96 - Gallipoli
1983/11/08 20:11 39.907 17.825 4.56 - Pen. Salent.
2001/09/23 21:16 39.767 18.001 4.96 - Pen. Salent.
the frequency distribution of the events. It is known that
dividing the number of events occurred in working hours
(generally between 08:00 a.m.-04:00 p.m. for mining
activity) or days (from Monday to Friday) by the number
of events recorded in not-working hours (00:00 a.m.-
08:00 a.m. and 04:00 p.m.-00:00 a.m.) or days (Saturday,
Sunday or holiday) the indicator Rq can be computed
dn dn
RqNNL L (1)
Nd is the number of events recorded in diurnal hours,
Nn is the number of events recorded in night hours and
the term Ld/Ln is a normalization factor equal to the ratio
between the number of diurnal and night hours (for hy-
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pothesis 8 and 16 respectively). It has been shown that
Rq is equal to unit in areas where are recorded only tec-
tonic events, whose occurrence is clearly casual at any
time or day, while anomalous values (>> 1) are obtained
if also quarry explosions are present (effectuated usually
between 08:00 a.m.-04:00 p.m.).
In our case we inferred Nd equal to 151, Nn equal to 12,
and therefore Rq 25; this result indicates that the major
part of recorded events is represented by quarry blasts.
This outcome is supported by the histograms represent-
ing the distribution of the number of the events versus
daytime and weekday (Figure 6), where the anomalous
distribution, having a minimum on Saturday and Sunday
and in intervals of not-working hours, is inferred.
It is, however, highly probable that not all 151 diurnal
events are due to quarry explosions, but the attention has
been paid only to events surely of tectonic origin, very
often easily distinguishable by the simple inspection of
waveforms and by spectral analysis. It’s important to
remember that exist a guideline for the discrimination of
earthquakes from mine explosions on the web
(http://earthquak e.usgs.g ov/earthqu akes/eqarchives/mine
blast/evidence.php); a typical feature of explosion seis-
mograms is the envelope with a “fish-like” shape (Fig-
ure 7(a)), P-onset emergent with only compressive po-
larity, lack of a clear S wave signature (the secondary
phase is due to converted phases), magnitude less than
2.0, and waveforms very different from those of tectonic
events (Figure 7(b)).
Among the 151 diurnal events, an earthquake surely of
tectonic origin, as suggested from different location and
magnitude respect to classical “anthropic” events and as
confirmed from waveform analysis (with some tensional
polarities), is the event occurred on 13 May, 2011 in
working time (06:21 GMT-08:21 local time) and day
(Friday); it was one of the most energetic events, felt by
the population.
Adding other 9 events occurred in not working-days
Figure 6. Distribution of the 163 “Penisola Salentina” events in the period 2003-2012 (a) versus days of the week and (b) three
local hour intervals.
0 5 10 15 20
time (s) 0 510 15 20
time (s)
250 (b)
(a) TAR1 BHZ
JUL 20 (201), 2007
JUL 20 (201), 2007
Figure 7. Comparison between the waveform of (a) an anthropic event (quarry blast) and (b) a tectonic earthquake recorded
at TAR1 station on 20 July, 2007 at 12:16:22 and at 04:29:29 (ID 6 in Table 2) respectively (GMT hour).
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(but in working hours), the resultant dataset includes 22
5. Event Relocation
First of all, we estimated the average Vp/Vs ratio, re-
quired by the program HYPOELLIPSE [27]. In the de-
termination of this ratio we required that the following
three criteria were jointly satisfied by each earthquake: at
least 5 stations, 8 phases and 2 OSUB phases. The last
criteria is fundamental to better constrain the solution
using the nearest stations. Based on these rules, about
700 phases of 14 earthquakes have been included and 8
earthquakes have been excluded (ID number 1, 2, 8, 10,
11, 12, 13 and 14 in Table 2), despite an accurate
re-picking of their arrival times for both the INGV and
the OSUB stations. In any case we relocated all 22 se-
lected earthquakes, with maximum local magnitude equal
to 2.8, using only data of stations having epicentral dis-
tance up to 200 km, giving the maximum weight to sta-
tions placed within 100 km.
The Vp/Vs ratio was estimated by using a modified
Wadati method [28]. In this method the average Vp/Vs
ratio is equal to DTs/DTp ratio, where DTs and DTp are
the time difference between phases Si and Sj and Pi and Pj
in two stations i and j; the slope of the least square fit of
DTs versus DTp for all available pairs of stations (Fig-
ure 8) gives the value of the slope Vp/Vs (1.78 with a
linear correlation coefficient of 98 %).
For the choice of a local velocity model, trials were
made adopting different models: the best results, in terms
of minimum RMS and error ellipsoid, were obtained
(considering whether 14 or 22 events) with the model
OSUB (reported in [8]) which was definitely assumed in
all the following processing.
Compared to the locations provided by the INGV (214
arrival times) without the OSUB stations, relocations
have an average RMS and azimuthal gap smaller (from
Figure 8. Wadati diagram with average Vp/Vs ratio ob-
tained (1.78) using linear least squares method. Linear cor-
relation coefficient is equal to 98%.
0.43 s to 0.29 s and, respectively, from 245˚ to 218˚); in
our study we exploited 324 phases.
The accuracy in the location of events is about 2 km
for the epicentral coordinates and about 5 km for the fo-
cal depth; it becomes better for the most recent events,
owing to the implementation of 3 seismic stations (CGL1,
MASS, FASA) in the period October, 2010 December,
2010. As an example, for the event occurred on 11 May,
2008 the minimum source to receiver distance is almost
60 km (see Dmin in Table 2), while the minimum distance
becomes about 15 km for the events of 2011-2012. As
expected, almost all discarded events have rather high
The depths of hypocentres, about 15 km on the aver-
age, indicate that this activity is located within the crys-
talline basement and in the deepest part of the upper
The distribution of the relocated epicentres is rather
sparse, but roughly follows an E-W striking trend that
corresponds to Soglia Messapica (Figure 9).
6. Focal Mechanisms
Since the earthquakes recorded in the “Penisola Salen-
tina” seismic district have a low magnitude (ML 2.8), it
is difficult to collect enough data for the determination of
focal mechanisms; therefore fault plane solutions were
computed only for three events (ID 9, 19, 21) having at
least eight clear observations. We used FPFIT [29] to
infer focal mechanisms. The velocity model used for the
computation of the azimuths and take-off angles is the
same as that used for the location of events.
In all cases both the low number of seismic stations
(the average number of polarities per event is 11) and
their spatial distribution does not allow to obtain well
constrained fault plane solutions. For all the events, the
best provided solution (Figure 10 and Table 3) shows
for the pressure axis P a trend of about 300˚ and a plunge
of about 30˚, whereas the tension T axis has a trend of
about 45˚ (NE-SW direction) and a plunge of 20˚. All
events reveal strike-slip faulting mechanisms along E-W
striking planes and in particular the best constrained
mechanism of the 5 May, 2012 earthquake is well repre-
sentative of this kind of solution.
Due to the limited number of polarities and the eastern
network gap (Adriatic Sea area) the reliability of the fault
plane solution is not fully assessed. However the score of
correct polarities is rather high (equal to 100% for the
third event) and the uncertainty is very low for strike and
dip of the two nodal planes (<15˚), whilst the uncertainty
is rather high for the rake (in two cases > 20˚) even if it is
worth noting that the fault plane solutions carried out
remain approximately the same.
For other events occurred in the “Penisola Salentina”
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Table 2. List of instrumentally earthquakes located from INGV in the “Penisola Salentina” seismic district and relocated in
this study. ID = identification number. Time is the earthquake origin time (UTC). In the column “Days” H represents holiday,
while days are numerated from 1 (Monday) to 7 (Sunday). Depth is the earthquake depth (fixed when marked as *). Nd1 and
Nd2 represent the number of used phases of the OSUB and INGV stations. Ns is the number of used stations. Dmin is the
minimum distance (in km). Md and Ml are the duration magnitude and the local magnitude (taken from INGV Seismic Bul-
letin). The 8 events highlighted in grey have been excluded in the estimate of the Vp/Vs ratio.
ID Date Time Days Lat. N Lon. EDepthNd1Nd2NsDminGapMdMlRMS AZ SEH1 SEH2SEZ
1 2005/08/29 15:05:54.2 1 40.833 17.1665.0* 0 6 34.7 2581.71.40.14 66 1.7 0.9 2.1
2 2006/01/06 10:21:25.9 H 40.916 17.01310.90 5 312.32041.91.10.13 43 2.8 1.8 6.6
3 2006/01/21 09:14:10.9 6 41.059 17.04410.17 6 714.92011.91.60.25 33 1.1 0.6 2.6
4 2006/04/23 07:55:07.7 7 40.679 17.3977.8 6 6 613.72031.91.30.38 87 1.7 1.0 5.2
5 2007/04/10 15:05:31.8 2 40.542 17.26010.55 6 62.4 1442.21.60.41 1 2.2 1.7 2.9
6 2007/07/20 04:29:21.2 5 40.858 17.55215.25 8 712.92902.21.90.30 21 2.5 1.4 1.2
7 2007/12/23 11:53:35.8 7 40.400 18.00825.46 141153.41642.82.50.40 33 3.6 0.8 1.7
8 2008/04/30 14:22:58.5 3 40.577 17.17711.80 6 325.32982.01.60.07 48 0.9 0.5 1.9
9 2008/05/11 23:03:13.2 1 40.843 17.8308.0 5 191758.1303/2.60.39 30 2.5 2.0 7.4
10 2008/10/16 15:19:21.5 4 40.528 16.9905.0* 0 6 316.72871.61.10.16 31 3.3 1.0 14.5
11 2009/08/23 10:15:14.6 7 40.801 17.8414.3* 0 6 323.62372.22.00.51 30 18.2 2.0 99.0
12 2009/11/15 05:07:50.4 7 40.619 16.9372.5 0 5 310.12311.50.70.21 37 9.2 1.4 65.9
13 2010/04/30 19:32:46.1 5 40.604 17.7152.1 0 8 458.73402.11.60.35 11 4.6 2.8 99.0
14 2010/05/14 03:53:16.6 5 40.625 16.9983.7 0 6 315.32441.41.10.36 37 6.4 1.4 34.6
15 2010/09/11 02:47:25.1 6 40.789 17.2758.0 7 6 7 12.11262.01.60.23 40 0.8 0.6 2.9
16 2010/10/31 07:16:19.8 7 41.120 17.0823.5 8 141317.12252.22.10.19 32 0.8 0.5 8.8
17 2011/05/07 13:19:53.0 6 40.481 17.02322.64 6 719.72281.81.10.22 4 1.7 1.0 3.1
18 2011/05/07 13:40:21.8 6 40.521 17.07312.74 8 8 13.91651.71.00.40 16 1.9 0.9 5.7
19 2011/05/13 06:21:29.8 5 40.772 17.52918.715191813.81782.52.30.38 20 1.2 0.8 0.8
20 2011/09/25 01:54:57.5 7 40.946 17.0889.5 128 1017.61672.21.40.34 38 1.0 0.5 3.4
21 2012/05/05 12:44:03.9 6 40.533 17.5427.3 14382914.799-2.80.43 15 1.1 0.5 3.3
22 2012/12/22 19:31:28.2 6 41.004 17.36523.6128 1020.9212 -2.10.27 26 1.0 0.6 1.8
17˚30'E 17˚00'E 18˚30'E 18˚00'E
Figure 9. Events re-located in the “Penisola Salentina” seismic district with representation of the error ellipse estimated for
each location (with ID number). In blue the INGV location is shown. Geographical position of main localities mentioned in
the paper is also shown.
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20080511 2011051320120505
Composite 12 ev. - 79 pol. Composite 3 ev. - 34 pol.
Figure 10. Focal mechanisms calculated with the code FPFIT [29] for three events (11 May 2008, 13 May 2011, 5 May 2012)
and for the composite solution (two different tests): white and grey triangles mark the T and P axes, white circles and small
crosses represent dilatational and compressive first arrivals.
Table 3. “Penisola Salentina” fault plane solutions carried out for the 3 major events and 2 composite solutions (A and B):
strike (S), dip (D) and rake (R) of a nodal plane, score polarities (ratio between correct and total polarities), trend and plunge
of P and T axes.
ID Date S D R Score polarities P-axis tr. P-axis pl. T-axis tr. T-axis pl.
9 2008/05/11 100 35 160 10/11 = 0.91 297 46 58 26
19 2011/05/13 95 55 180 11/13 = 0.85 314 24 56 24
21 2012/05/05 80 65 150 10/10 = 1.00 299 38 208 1
A 13 events 30 30 150 56/79 = 0.71 257 26 26 52
B 3 events 70 45 170 28/34 = 0.82 281 36 30 24
seismic district (very probably originated from the same
fault system) it has not been possible to infer stable esti-
mates of the focal mechanism owing to the small number
of available polarities. To overcome this limitation a
composite fault plane solution has been obtained by
combining the P-onset polarities of these events. Some
tests with different number of polarities (including or
excluding groups of polarities belonging to events with
the highest errors of relocation) have been carried out: in
Figure 10 and Table 3 the two most opposite tests are
shown, the first using all 79 polarities of 13 events (score
0.71) and the second joining only the polarities of the 3
above mentioned events (with 6 polarities in disagree-
ment out of a total of 34). In both cases the composite
solution fits well the solutions obtained for the single
major events.
7. Source Parameters
In this section we discuss the results of an analysis aimed
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at inferring source parameters (corner frequency, source
dimension, seismic moment and stress drop) and attenua-
tion parameters from the inversion of P wave spectra. As
we will detail in a next section, this has been possible
only for 3 events (ID 9, 19, 21).
7.1. Data Analysis
The seismic spectra were computed by considering a
time window TL = 2.56 s which starts 0.1 s before the P
wave arrival time. To reduce distortions due to the finite
length of the signals [30], a cosine taper window with a
15% fraction of tapering was applied to the P wave time
window before computing its amplitude spectrum. Traces
were deconvolved for the instrumental response.
In a preliminary analysis we noted that seismic noise
dominates over the signal at low frequencies (f < 2 Hz)
and at high frequencies (f > 40 Hz).
For this reason, we applied a band-pass filter 2 < f <
40 Hz to data and removed the mean value before com-
puting the spectra. The same data processing was applied
for the calculation of seismic spectra of noise. The analy-
sis was performed on a time window having a length of
2.56 s that is adjacent to the time window used for the
calculation of P wave spectra. Finally, an average mov-
ing window with a full width of five neighbouring points
[31] was used to smooth the spectra. To obtain suffi-
ciently stable estimates of model parameters, we selected
only those signals having an average signal to noise ratio
greater than 3.
7.2. Method
The velocity spectrum Ui,j(f) of the i-th event observed at
j-th station can be expressed as [e.g. 32]:
 
i,ji,j i,jj
where S is the source spectrum, B is the attenuation
spectrum and R is the site spectrum. In the far-field ap-
proximation, the source spectrum can be written as:
i,j 1/n
1+ f
In the previous equation, fc,i is the corner frequency,
0,i,j is the low-level spectral amplitude, γ is the high-
frequency spectral fall-off and n is a constant. The low-
level spectral amplitude is related to the seismic moment
by the equation:
M= R (4)
where r is the source to receiver distance, ρ is the density
of rocks, Rθ,Φ is the radiation pattern and c is the velocity
of the considered wave.
The source spectrum is generally described by the
Brune source model [33], which corresponds to the so
called “omega square” model (i.e. n = 1 and γ = 2). As, in
some cases, the recordings of earthquakes show a corner
sharper than the original Brune model (for a review see
[34]), a different approximation to the source model that
corresponds to γ = 2 and n = 2 is used. This choice cor-
responds to the Boatwright source model [35], that we
used in our analysis.
The attenuation spectrum is described by the equation:
Bf=exp Q
where ti,j is the travel time of the considered wave and Q
is the quality factor of the waves. Q may depend on fre-
quency; however, in many cases [e.g. 36] a constant Q
model results in a best compromise between data fitting
and simplicity of model. For this reason, in this study, we
considered a constant Q model.
As concerns the site response Rj(f), this is usually ex-
pressed as the product of the near-site attenuation func-
tion Kj(f) and the local site amplification Aj(f). The
near-site attenuation is usually described above a limiting
frequency known as fmax [37] in terms of the kj attenua-
tion factor, which was first introduced by Anderson and
Hough [38]:
Kf=exp πfk (6)
The local site amplification Aj(f) is not described by
any particular mathematical relationship, and it depends
on the elastic and geometrical properties of the rocks
near the recording site (e.g. [39]). It is generally com-
puted by averaging residuals between theoretical and
observed spectra, when many recordings of several
earthquakes at a given station are available (e.g. [40,41]).
In our case, we do not computed Rj(f) owing to the small
number of available recordings.
7.3. Inversion Technique
We used the 2-step inversion strategy developed by de
Lorenzo et al. [36]. At the first step the whole physically
admissible model parameter space is explored using a
coarse grid. The range of model parameter values in the
coarse grid is selected on the basis of the a priori analysis
of the whole data set. At each point of this grid, a misfit
function between the observed and the theoretical spec-
trum is computed. We compared the results obtained
using several misfit function, as proposed by Edwards et
al. [40] and inferred that the best results are obtained
using the following L1 misfit function of decimal loga-
rithms of amplitude:
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 
N (7)
This approach allows obtaining an initial estimate of
model parameters mbest_ini without being subjected to the
choice of an initial corner frequency.
At the second step, a refined grid is built around
mbest_ini and the misfit function is computed at each point
of this grid, allowing us to estimate the best fit model
parameters mbest for each station.
Figure 11 shows the fit of model to data for the event
occurred on 11 May, 2008.
After the inversion the event corner frequency is ob-
tained as the average of the station corner frequency,
whereas average QP and the seismic moment are obtained
by the logarithmic averages of station values.
The estimated source parameters are summarized in
Table 4.
8. Discussion and Concluding Remarks
The main goal of this study was to deduce the charac-
teristics of the low-energy seismic activity that affects
the “Penisola Salentina” seismic district, a generally con-
sidered substantially aseismic area. In the historical cata-
logues is reported only one event of magnitude greater
than 6.0, occurred in 1743, and several other earthquakes
of lower magnitude.
As regards the instrumental seismicity until 2002 the
spatial coverage of the seismic stations was too poor in
this area to obtain reliable relocations. Notwithstanding,
the study of the set of seismic parameters, albeit in an
area with a low seismic activity, might provide useful
information on its seismological characteristics. There-
fore we analysed the seismic activity from 2003 to 2012:
the larger number of stations available in the last years,
through the improvement of the OSUB network and the
implementation of new INGV stations, allowed us to
obtain more constrained relocation of low magnitude
events compared to official locations (provided in Bulle-
tin of the Instrumental Seismicity of INGV).
The “Penisola Salentina” seismic district is subjected
to very frequent quarry blasts that make more difficult
1 10 100
110 100
110 100
1 10 100
1 10 100
110100 110 100
110 100
110 100
f(Hz) f(Hz)
A(m s)
A(m s)
A(m s)
A(m s)
A(m s)
A(m s)
A(m s)
A(m s)
A(m s)
Figure 11. Fit of the P wave displacement spectra at different stations after the inversion for the event occurred on 11 May,
Table 4. Source and attenuation parameters for the studied events determined using Boatwright source model [35]. M0, fc, L,
Δσ and <QP> are the seismic moment, the corner frequency, the source dimension, the stress drop and the average attenua-
tion of the P-wave respectively.
ID Date M0 (Nm) fc (Hz) L (m) Δσ (Mpa) <QP>
9 2008/05/11 5.8E + 14 ± 1.5E + 14 8.2 ± 3.4 137 ± 57 18 2188
19 2011/05/13 3.1E + 12 ± 1.3E + 10 10.4 ± 4.4 107 ± 45 0.2 826
21 2012/05/05 4.2E + 13 ± 1.3E + 12 11.3 ± 3.5 99 ± 31 3 3982
the systematic collection of tectonic events; quarry blasts
(probably 141 out of 163 events) and tectonic earth-
quakes (at least 22) have been discriminated. These 22
events have been relocated to obtain more reliable esti-
mations on their hypocenters.
After several tests, a local velocity model, suitable for
this area, has been adopted, with the Vp/Vs ratio equal to
1.78 estimated using a modified Wadati diagram. This
value is slightly higher than the 1.73 Poissonian value
and may indicate that the crust is partially fluid-perme-
ated (e.g. [42]). Despite the high azimuthal gap and the
quality of instruments (in some cases mono-component)
the relocations are more precise. In fact a RMS reduction
of 33% is obtained and an accuracy of about 2 km for the
epicentral coordinates and about 5 km for the focal depth
is inferred. The higher degree of accuracy in the com-
puted locations, with respect to those available in the
INGV database, is due to the use of data from both the
INGV network and the OSUB network. The majority of
these events (including the three most energetic) are lo-
cated in a weakness zone, the Taranto-Brindisi depres-
sion, near the North Salento Fault Zone (see Figure
Information on the stress regime controlling this low-
energy seismic activity is rather poor, because only for
three events it has been possible to determine the focal
mechanisms. The re-picking of the recordings allows us
to detect 79 clear polarities on the whole, of which 34 for
three major events.
Our best constrained solutions of the 5 May, 2012
earthquake reveal strike-slip faulting mechanisms along
E-W striking planes. All five focal mechanisms inferred
in this study have a common characteristic with regard to
the trend of the T axes (of about 45˚), which delineates
the existence of an NE-SW extension direction. This
pattern is similar to that found by some authors in Mur-
gian area [8,43], despite the tectonic differences between
the two adjacent areas. The presence of tensional stress
in the Taranto-Brindisi depression is consistent with the
hypothesis that the margin of the Adriatic plate has un-
dergone a buckling process [44,45] following the exten-
sional rearrangement of the Apenninic belt masses. The
obtained focal mechanisms suggest that a tensional re-
gime could be still active. The NE-SW active extension
in this area was previously inferred also by Di Bucci et al.
[46] on the basis of mesostructural analysis; they pro-
posed other different geodynamic models to justify it, for
example a consequence of the convergence between Af-
rica and Europe.
We also computed source (corner frequency, source
dimension, seismic moment and stress drop) and attenua-
tion (quality factor) parameters of the three major events.
We inferred that the quality factor is generally at least a
magnitude order higher than the average value (Q 300)
determined in other parts of Italian peninsula (Central
Apennines, Campi Flegrei, Southeastern Sicily) [36,47,
48]. Therefore, even if the energy radiated by these
earthquakes is generally smaller than that of tectonic or
volcanic earthquakes, it is transferred with a greater effi-
ciency, as was previously hypothesized on the basis of
macroseismic fields of Albanian and Greek events. This
result probably reflects the differences between the geo-
dynamic context of the areas where the studied earth-
quakes occurred. In fact, whereas earthquakes occurring
in the Apennine mark the limits among plates and there-
fore occur in strongly deformed areas, the events oc-
curred in the southern Apulia are probably located inside
a plate. The variability of stress drop estimates (roughly
between 10 and 100 bar) may indicate that stress drop is
not a selective indicator of the geodynamic context, as
early proposed by Kanamori and Anderson [49].
9. Acknowledgements
We warmly thank Franco Mele and Alberto Basili of the
“Istituto Nazionale di Geofisica e Vulcanologia” who
promptly satisfied our data requests.
Most of figures were obtained by employing the GMT
freeware package by Wessel and Smith [50].
This work was supported by MIUR (Italian Ministry
of Education, University and Research) and by FCRP
(Fondazione Cassa di Risparmio di Puglia, Bari).
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