Next Article in Journal
A Proposal for a Zero-Trust-Based Multi-Level Security Model and Its Security Controls
Next Article in Special Issue
Combining Higher-Order Statistics and Array Techniques to Pick Low-Energy P-Seismic Arrivals
Previous Article in Journal
Influence of Heat Treatment on Properties and Microstructure of EN AW-6082 Aluminium Alloy Drawpieces After Single-Point Incremental Sheet Forming
Previous Article in Special Issue
Influence of Site Effects on Scaling Relation Between Rotational and Translational Signals Produced by Anthropogenic Seismicity
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Applied Sciences
Volume 15
Issue 2
10.3390/app15020784
applsci-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore

by
Pierpaolo Pierri
1,
Marilena Filippucci
1,2,*,
Vincenzo Del Gaudio
1,
Andrea Tallarico
1,2,
Nicola Venisti
1 and
Vincenzo Festa
1
1
Department of Earth and Geo-Environmental Science, University of Bari “Aldo Moro” (UniBa), 70126 Bari, Italy
2
National Institute of Geophysics and Volcanology (INGV), 00143 Rome, Italy
*
Author to whom correspondence should be addressed.
Appl. Sci. 2025, 15(2), 784; https://doi.org/10.3390/app15020784
Submission received: 31 October 2024 / Revised: 8 January 2025 / Accepted: 10 January 2025 / Published: 14 January 2025
(This article belongs to the Special Issue Advanced Research in Seismic Monitoring and Activity Analysis)
Browse Figures
Versions Notes

Abstract

:
On 23 March 2018, an event of magnitude ML 3.9 occurred about 10 km from the town of Ostuni, in the Adriatic offshore. It was the most energetic earthquake in South–Central Apulia ever recorded instrumentally. On 13 February 2019, in the same area, a second ML 3.3 event was recorded. The analysis of the 2018 event shows that the ambiguity of the solution of the fault plane reported by INGV (Istituto Nazionale di Geofisica e Vulcanologia) on the Italian National Earthquake Centre website can be solved considering existing seismic profiles, exploration well logs and the Quaternary activity of faults in the epicentral area. A seismogenic source was identified in the rupture of a small portion of a 40 km length structure with strike NW-SE, dipping at a high angle toward the south. In this work, we have relocated the recent earthquakes by using the seismic stations managed by the University of Bari (UniBa), one of which is quite close to the event’s epicenter (about 20 km), together with data coming from the RSN (Rete Sismica Nazionale). Furthermore, we have determined the focal mechanism of some events, with implications on stress field of the area. Our results show right-lateral transtensional kinematics of the seismogenic faults along approximately E-W striking planes, with a tension, T, with a trend of about 60° (NE-SW direction) and a plunge of 20°. Finally, we have tried to correlate the location of the four best constrained earthquakes with their seismogenic structures.

1. Introduction

The Apulia region, in Southern Italy, is part of the Adria plate, a microplate whose tectonic evolution is dominated by the collision with two major plates, Eurasia and Africa. Two subduction zones in opposite directions are recognized: on the western side, the subduction is considered still active beneath the Northern Apennine and the Calabrian arc [1]; and on the eastern side, beneath the Dinarides, the subduction is considered extinct, while it is considered still active in the Hellenic arc [2]. These complex tectonics involve the collision toward the northwest with the Eurasia plate, where Adria is the upper plate, involving a counterclockwise rotation in the Western Alps [3], whose Euler pole of rotation is still debated (refer to the discussion in Le Breton et al. [4]). In Figure 1, we show a schematic framework of the Adria plate and surroundings, according to the seismotectonic model proposed by Meletti et al. [5].
All the boundaries of the Adria plate are characterized by an intense seismic activity that can also be very deep (as Wadati–Benioff zone in the Hellenic and Calabrian arcs) [1,6]. The inner part of Adria, where Apulia is located, is characterized by modest recent seismic activity, if compared to the adjacent areas of the Apennine or of Albania and Greece; however, it has been the site of strong earthquakes in the past. The Apulia region has different characteristics of seismicity moving from the north to the south. The historical seismicity of Northern Apulia was analyzed in detail by using data of the CPTI Working Group [7]. They report 22 events with magnitude Mw > 5.2 [8], demonstrating that the seismogenic structures of Northern Apulia can generate damaging earthquakes. Central Apulia is characterized by low and sporadic seismicity due to a tensional stress regime, possibly related to both Apennine and Northern Apulia seismogenic activity [9]. However, it suffers from a lack of seismological knowledge as a consequence of relatively poor spatial and temporal coverage of seismic monitoring.
Seismic monitoring in Italy is managed by the National Institute of Geophysics and Volcanology (Istituto Nazionale di Geofisica e Vulcanologia—INGV) through the National Seismic Network (“Rete Sismica Nazionale”—RSN), whose code, within the International Federation of Digital Seismograph Networks—FDSN—is IV. At present, this network has only three seismic stations (NOCI, MESG and SCTE) operational in Southern Apulia, after the closure of two other stations (LCI and BRT) in 2006. In recent years, the seismic monitoring of this region has been supported by five seismic stations (TAR1, MASS, FASA, CGL1 and PE1) managed by the Seismologic Observatory of the University of Bari (OSUB). The recordings of this network, in conjunction with those of the RSN, allowed for the detection of several low energy earthquakes, such as the ML 2.8 event of 5 May 2012, which occurred near Ostuni and was felt by many inhabitants.
To improve the seismic monitoring of the Apulian territory, in 2013, thanks to the European project INTERREG, and in collaboration with INGV, the University of Bari (UniBa) installed a local seismic network covering the entire territory of Apulia, the OTRIONS network (FDSN code OT). The OT network also incorporated some stations already operating in Southern Apulia, previously managed by OSUB (Figure 2). The details on the operation of the OT network from 2013 to 2019 are described in Filippucci et al. [10]. Since 2021, other stations have been added, and in 2024, some sensors have been replaced (for further details, see the page “https://www.fdsn.org/networks/detail/OT/ (accessed on 30 October 2024)”). From May 2019, the OT registrations are available also on EIDA (European Integrated Data Archive) INGV node and can be used to detect earthquakes on Italian territory. As a result, the OT network has improved the detection of earthquakes throughout the territory of Apulia and Southern Italy, as demonstrated by the earthquakes listed in the ONT (National Earthquake Observatory) section of the INGV website.
The most energetic event ever recorded by seismic networks in Central–Southern Puglia occurred on 23 March 2018, with Mw = 3.7 and ML = 3.9; this earthquake occurred in the Southeast Murge Adriatic offshore, at the transition between the Apulian Foreland and the Dinarides–Albanides foredeep domains, about 10 km from the town of Ostuni, one of the most touristy and populated municipalities in the province of Brindisi. This event was analyzed in a previous work by Festa et al. [11] to retrieve the geometry of the seismogenic structure responsible for this earthquake, but the authors used the information as downloaded from ONT, before the OT network was integrated into the RSN.
Due to the lack of seismicity and of seismic monitoring, no information is available on the tectonic stress regime for the Apulia foreland area in the Italian Present-Day Stress Indicators database [12] (IPSI, https://ipsi.rm.ingv.it/ (accessed on 30 October 2024)). Furthermore, no potential seismogenic faults capable of producing significant permanent tectonic deformation at the surface (capable faults) are reported for this area in the Database of Individual Seismogenic Sources [13] and in the database of active capable faults of the Italian territory [14].
The aim of this paper is to integrate all the seismic data available on the ONT with the OT registrations and to obtain a more robust catalog of earthquakes that occurred in Central–Southern Apulia during the last 25 years. We also computed the focal mechanisms of some available events and stress field to improve the knowledge of the seismogenic structures in Central–Southern Apulia. Finally, we have correlated the location of the four best constrained earthquakes with their seismogenic structures.

2. Structural Setting

In the framework of the Adria plate geodynamics, the Southern Adriatic Sea area represents the Oligocene–Quaternary foreland basin of the Dinarides–Albanides–Hellenides orogen’s portion [15,16,17,18] (Figure 3). During the orogenic growth of the Dinarides–Albanides–Hellenides, the Mesozoic–Eocene Adriatic Basin was gradually involved to the west in the foreland basin, whose tectonic subsidence came to affect the southeastern part of the adjacent Apulia Platform as well [18]. To the west, the Apulia Platform progressively subsided, eastward, in the Neogene–Quaternary foredeep domain of the Southern Apennines [19,20]. Therefore, a remnant of the Apulia Platform dominates the Apulian Foreland, i.e., the Plio–Pleistocene foreland shared by Apennines and Dinarides–Albanides–Hellenides [19,21] (Figure 3).
The uplift since the Middle Pleistocene of the Apulian Foreland occurred in relation to a NW-SE striking regional, gentle buckle fold of the Adria plate, which occurred due to the difficult eastward roll-back of the continental lithosphere during Apennines subduction [20].
In its upper part, the Apulian Foreland is chiefly represented by a sedimentary cover lying above a Variscan crystalline basement [19] (Figure 3). From the bottom to the top, the sedimentary cover consists of Permo–Triassic continental deposits belonging to the Verrucano Fm (up to ca. 1000 m thick), Upper Triassic limestones/dolostones and anhydrites of the Burano Fm (up to ca. 2500 m in thickness), and Lower Jurassic limestones of the Calcare Massiccio Fm (up to ca. 1000 m thick); moreover, the Middle Jurassic–Upper Cretaceous inner platform carbonates, belonging to the Apulia Platform, are widely exposed in the Apulian Foreland with a thickness of ~4 km [19,22,23].
The Apulia Platform–basin transition and adjacent Adriatic Basin (Figure 3) are testified by marginal and pelagic carbonates (both cropping out and drilled), respectively [24,25]. Similar deposits, moreover, occupied during Upper Cretaceous narrow intra-platform basins governed by extensional faults [25,26,27].
Applsci 15 00784 g003
Figure 3. Schematic structural map of the region around the Southern Adriatic Sea (modified after [28]); M–G = Mattinata–Gondola fault; HELLEN. = Hellenides; the solid black line encloses the Murge area and the Northern Salento.
Figure 3. Schematic structural map of the region around the Southern Adriatic Sea (modified after [28]); M–G = Mattinata–Gondola fault; HELLEN. = Hellenides; the solid black line encloses the Murge area and the Northern Salento.
Applsci 15 00784 g003
The central sector of the emerged Apulian Foreland is represented by the Murge (Figure 4), a morpho-structural high where the exposed carbonates of the Apulia Platform have been grouped in the “Calcare di Bari” Fm (Lower Cretaceous) and the overlying “Calcare di Altamura” Fm (Upper Cretaceous) [22] (Figure 4). Thin Plio–Pleistocene sedimentary bodies made of calcarenites, which in turn belong to the “Calcarenite di Gravina” Fm, unconformably rest on the Cretaceous carbonates, and they crop out in some inner places and especially on the flanks of the Murge high [22], the latter controlled by normal faults [29,30,31] (Figure 4). Normal faults striking from NW-SE to W-E, toward the Adriatic Sea coastline, and dipping from the NE to N, respectively, characterize the Quaternary tectonics of the Murge area [30,31], and, together with associate faults dipping in the opposite direction, gave rise to narrow grabens [30]. According to Festa [32], these tectonic structures composed a system of faults (deformation zones, DZs) with normal and right transtensional kinematics that were active during the deposition of the limestones of the “Calcare di Altamura” Fm (Figure 4).
Oligocene–Quaternary deposits unconformably overlie the Mesozoic–Eocene platform and basin carbonates in the Adriatic Sea offshore Murge [25], and they exhibit a thickness increasing toward the inner Albanides foreland basin [15,17,25,26,33]. Dominant extensional faults affected the platform–basin transition in the southeastern Murge [34] and its Adriatic Sea offshore, where, moreover, they were active during Neogene as well [26]. Here, the Monte Giove, a narrow E-W submarine relief, and its adjacent Rosaria Mare intra-platform basin [25,26] (Figure 4) were formed as a result of faults activity up to the Quaternary [28]. In this respect, the southern fault bordering Monte Giove relief, at the transition with Rosaria Mare basin, exhibits evidence of present-day activity, representing the possible seismogenic source of the 23 March 2018 event [11].

3. Analysis of the Historical and Instrumental Seismicity

The area of Central and Southern Apulia is historically characterized by low seismicity [35] and is classified as a low-seismic-hazard area. In Figure 5, the historically documented events that occurred within 35 km from Ostuni and reported by different catalogues are plotted on the map as circles representing the focal volumes, according to Bath and Duda formula [36].
In this area, the largest earthquake and the only one with a significant damage potential is that which occurred on 26 October 1826, with Mw = 5.22 and macroseismic intensity VI-VII MCS (Mercalli–Cancani–Sieberg), near Grottaglie (see red circle, data from Parametric Catalogue of Italian Earthquakes, CPTI15 v. 4.0 [37,38]).
This event is not reported by the Catalogue of Strong Italian Earthquakes (CFTI5MED) [39,40], which has selected only a subset of the events historically documented; however, it includes four smaller earthquakes with magnitudes between 3.2 and 3.7) (grey circles in Figure 5), all based on “a single location” and therefore considered of uncertain reliability.
References to a set of additional events are found in the PFG catalogue [41], compiled within the Geodynamic Finalized Project. This set consists of 16 events (yellow circles in Figure 5), often with unreliable locations. Only four of them have intensities that could have caused damage (I ≥ VI MCS): one of these events is that of 1826, reported by the CPTI15; two others, of magnitude 4.1, that occurred in 1833 were attributed identical locations; and the fourth, which occurred on 26 February 1947, was placed in the offshore Adriatic Sea, but its location, based on few instrumental observations, was very uncertain, and the CPTI15 catalogue reports it with a completely different location (in the Tyrrhenian Sea).
With regard to instrumental seismicity, the map in Figure 5 shows, with blue circles, the earthquakes that occurred around the town of Ostuni between 1981 and 1999 (before the network was digitized; source, Italian Seismicity Catalog—CSI catalogue [42]). Only 17 earthquakes are present in the analyzed area, with Mmax = 3.2.
According to the historical record of earthquakes felt in Ostuni, reported by the Database Macrosismico Italiano (DBMI15) [43], only one event caused slight damage (VII MCS), due to the strong Salento earthquake that occurred on 20 February 1743. The other 11 earthquakes were only perceived in Ostuni, with a maximum of V MCS on the occasion of the disastrous Irpinia earthquake of 23 November 1980.

4. Data Selection and Hypocenter Re-Location

We extracted from the ONT web-catalog “https://terremoti.ingv.it/ (accessed on 30 October 2024)” a dataset consisting of earthquakes that occurred within a radius of 35 km from the town of Ostuni (lat. 40.7332 N–long. 17.5786 E). The extraction covers a period from January 2000 to September 2024, which coincides with the era of the seismic network digitization in Italy. There are a total of 32 events in the ONT web-catalog, and they are listed in Table 1 and mapped in Figure 6. The magnitude ranges from 2.0 to 3.9, with an average value M ¯ = 2.4.
Of these 32 events, seismograms were available for a review of picking only for the RSN recordings from 2008 onward, while for those up to 2007, we used the time picks provided by INGV. When possible, the picking procedure of P and S waves was carried out manually, by visual inspection. These data were integrated with the recordings acquired by the OSUB and OT networks and stored in their laboratories. So, the dataset was collected as follows:
  • From January 2000 to December 2007: time picks of P and S waves were downloaded from the INGV web-service “https://terremoti.ingv.it/?timezone (accessed 15 April 2024)” and integrated by OSUB recordings;
  • From January 2008 to March 2013: recordings were downloaded from the INGV web-service “https://terremoti.ingv.it/?timezone (accessed 15 April 2024)” and integrated by OSUB recordings;
  • From April 2013 to April 2019: recordings of the IV network were downloaded from the INGV web-service “https://terremoti.ingv.it/?timezone (accessed 15 April 2024)” and integrated by OT network recordings;
  • From May 2019 to today: recordings of the IV and OT networks were downloaded from the INGV web-service “https://terremoti.ingv.it/?timezone (accessed 15 April 2024)”.
In the Supplementary Materials, we release the txt file containing all the arrival times of the events analyzed in this paper.
The 32 events were relocated with the HYPOELLIPSE code [44], using, in addition to the previously available data, the new recordings and the revised phases derived from the re-picking. Seven different velocity models were tested: those commonly adopted by the INGV, CSTI, AK135 and PREM; and the models specifically proposed for this region by Calcagnile and Panza [45], Costa et al. [46] and Venisti et al. [47]. In addition, different values of the V p / V s ratio were tested.
Applsci 15 00784 g006
Figure 6. Circles (with ID number) and squares indicate the epicenters determined by INGV and resulting from the re-locations, respectively, of the 32 events examined in this work. In red, the events occurred offshore in the Adriatic Sea; in light blue, the events occurred on land; and in blue, the events occurred near Taranto, some of which are likely quarry blast (according to [48]). Black squares indicate municipalities.
Figure 6. Circles (with ID number) and squares indicate the epicenters determined by INGV and resulting from the re-locations, respectively, of the 32 events examined in this work. In red, the events occurred offshore in the Adriatic Sea; in light blue, the events occurred on land; and in blue, the events occurred near Taranto, some of which are likely quarry blast (according to [48]). Black squares indicate municipalities.
Applsci 15 00784 g006
The results of all of these relocations indicate that the best model for this area in terms of travel-time residuals and hypocenter location errors is that of Calcagnile and Panza [45], with V p / V s = 1.78 . The parameters of relocation of the 32 events are reported in Table 2 and plotted on the map in Figure 6. The velocity model is shown in Table 3.
Some of the events in Table 2 had already been localized by Pierri et al. [48] (ID: 10, 13, 16, 17 and 19), who analyzed the seismicity in the “Penisola Salentina” seismic district, an area much larger than that analyzed in this paper. Regarding the seismicity around the city of Taranto, some earthquakes (for example the one with ID 9) are most likely quarry blast (according to Pierri et al. [48]).
We observe a reduction in the hypocentral errors (generally less than 5 km) and in the minimum epicentral distance (Dmin), which is reflected in the lower value of the RMS (on average, from 0.37 s to 0.26 s) with respect to the ONT catalog; instead, we do not observe a significant improvement in the azimuthal gap since the azimuthal coverage of the network has not changed. The variations in the epicentral location are almost always less than 5 km: only for events 2 and 3, the variations are greater than 15 km, but for these events, no repickings were carried out.
An analysis of the relocation parameters shows a clear improvement from the first 19 events up to 2012 to the other 13 events that have occurred since 2013: the number of phases recorded by the UniBa and INGV stations (on average, equal to 6 and 18, respectively) has increased from 4 to 10 and from 12 to 27; the number of stations used in total (on average equal to 15) has increased from 9 to 25; the azimuthal gap (on average equal to 212°) has decreased from 245° to 163°; and the minimum distance (on average equal to 27 km) has decreased from 29 to 24 km.
Table 3. Velocity model [45] used in seismic event relocation by the Hypoellipse code [44]: Vp and Vs are P-wave and S-wave velocities (Vp/Vs = 1.78); D is the depth of the bottom of each layer.
Table 3. Velocity model [45] used in seismic event relocation by the Hypoellipse code [44]: Vp and Vs are P-wave and S-wave velocities (Vp/Vs = 1.78); D is the depth of the bottom of each layer.
LayerVp (km/s)Vs (km/s)D (km)
14.002.252.0
26.103.4319.0
36.803.8333.0
48.104.5590.0
58.204.61∞

5. Focal Mechanisms

Information on the orientation of the seismogenic structures can be derived from the determination of focal mechanisms. These are also necessary to calculate the stress field to which the area is subjected, since it is well inside the Adria microplate, which is highly deformed at the edges, not too far from the area under study (as discussed in Section 1 and shown in Figure 1).
We computed focal mechanisms by using P-wave polarities and the FPFIT code [49]. Since the magnitude of the earthquakes selected for this study is generally low, it is not easy to correctly distinguish polarities from the signal noise in a sufficient number to determine the focal mechanisms. P-wave polarities were determined manually and picked on the seismograms of the OSUB and OT network for the entire period examined and on those of the INGV network only for the events from 2008 onward.
The velocity model used for the computation of take-off angles is the same as that used for location [45]. Fault plane solutions were considered well constrained only if derived using a minimum of 10 clearly readable polarities homogeneously distributed on the focal sphere. This quality criterion has led us to accept the focal mechanism solution for only 7 earthquakes (ID 10, 16, 17, 23, 24, 25 and 29) out of the initial 32. Three of these earthquakes (ID 10, 16 and 17) were already analyzed by Pierri et al. [48].
For each of these events, we have chosen, from the multiple solutions proposed by FPFIT, the one that minimized the distance of the nodal planes from the mismatching polarity data points. To better constrain the FPFIT inversion, we assigned a weight to the polarity from 0 to 2 (following the picking weighting used by the SAC v. 101.6a software). Assigning weights to the polarity data used in the FPFIT inversion resulted in a reduction in the misfit, F. The best fit solution of each event was determined by minimizing the residuals between the observed and theoretical amplitudes, exploring a search grid of values of strike, φ; dip, δ; and rake, λ, spaced at 5°.
The results are shown in Table 4, where we labelled with the subscripts 1 and 2 the two nodal planes of the double-couple solution in terms of φ, δ and λ; we also reported the P and T axes orientation as the plunge (inclination measured downward relative to the horizontal plane) and trend (azimuth measured in the direction of the plunge) angles. The P and T axes represent the directions of the maximum, σ1, and the minimum, σ3, principal stress axes, respectively. The fault type (FT in Table 4) can be identified, as proposed by Frepoli et al. [50], by plotting the combination of T and P plunges: based on the position of the point representing the fault plane solution on this graph, the fault type can be defined according to the diagram legend, as shown in Figure 7.
In Table 4, Npol is the number of polarities matching with the focal solution compared to the total number of polarities, Ntot, available for each event. The quality of the solution is expressed by the quality factors, Qf and Qp. Qf gives information about the solution misfit, F, of the polarity data and assumes values that depend on F; Qp reflects the solution uniqueness in terms of the 90% confidence region for the three angular parameter uncertainties, ∆φ, ∆δ and ∆λ. Qf and Qp range from class A to class C for decreasing quality, according to Table 5.
We can observe that four of the selected focal mechanisms (ID = 10, 17, 24 and 29) represent normal/strike–slip faults, whereas only one (ID = 16) is a pure strike–slip mechanism and one (ID = 25) is a pure normal fault.
The focal mechanism of the strongest event (ID = 23) has an uncertain character, as it is of type U; the mechanism found is quite similar to that obtained by INGV using the Time-Domain Moment Tensor (TDMT) technique [51] on eight stations (“https://terremoti.ingv.it/event/18504011 (accessed on 15 April 2024)”), also classified as type U.
For most events (see Figure 8), the best solution has a pressure axis (P) with a trend of about 300° and a plunge of about 40°, whereas the tension (T) axis has a trend of about 60° (NE-SW direction) and a plunge of 20°. These events reveal normal/strike–slip faulting mechanisms along approximately E-W striking planes, and, in particular, the best constrained mechanism of the 5 May 2012 earthquake (ID = 17) is the great representative of this kind of solution.
A composite fault plane solution was also obtained by combining the 115 P-onset polarities of these seven events; in Figure 8, the beach-ball is shown with black and white quadrants; the joint solution of the normal/strike–slip type shows, on average, a trend and plunge of T and P axes similar to those obtained for the individual focal mechanisms.
Analysis of the T-axis orientations of normal and normal/strike–slip solutions suggests a widespread NE-SW extensional regime.
Table 5. Criteria for the assignment of the quality factors, Qf and Qp, to the fault plane solution (as described in [49]), based on the value of the misfit function, F (first panel), and the uncertainties affecting the parameters ΔSTR, ΔDIP and ΔRAK (second panel), respectively.
Table 5. Criteria for the assignment of the quality factors, Qf and Qp, to the fault plane solution (as described in [49]), based on the value of the misfit function, F (first panel), and the uncertainties affecting the parameters ΔSTR, ΔDIP and ΔRAK (second panel), respectively.
FQfΔSTR, ΔDIP, ΔRAKQp
F < 0.025A<20°A
0.025 < F < 0.1B20° to 40°B
F > 0.1C>40°C
Applsci 15 00784 g007
Figure 7. Diagram of T-axis plunge vs. P-axis plunge for the 7 fault plane solutions determined in this study. In this diagram the field of values near the vertices represent strike–slip (SS), reverse (RE) and normal (NO) solutions. RS and NS are oblique-type mechanisms, whereas the solutions in the U field are defined as unknown (modified by [50]). ID = identification number (see Table 1).
Figure 7. Diagram of T-axis plunge vs. P-axis plunge for the 7 fault plane solutions determined in this study. In this diagram the field of values near the vertices represent strike–slip (SS), reverse (RE) and normal (NO) solutions. RS and NS are oblique-type mechanisms, whereas the solutions in the U field are defined as unknown (modified by [50]). ID = identification number (see Table 1).
Applsci 15 00784 g007
Applsci 15 00784 g008
Figure 8. Focal mechanisms of single events (grey/white beach balls) computed in this study and relative epicentral relocation (red square); for each beach-ball date, magnitude of event, quality factors and polarities (open circles for distensive and little crosses for compressive polarities) and stress axes (grey triangle for P-axis and white triangle for T-axis) are shown. Composite focal mechanism solution is shown as black/white beach ball (black triangle for P-axis and white triangle for T-axis). Black squares indicate municipalities.
Figure 8. Focal mechanisms of single events (grey/white beach balls) computed in this study and relative epicentral relocation (red square); for each beach-ball date, magnitude of event, quality factors and polarities (open circles for distensive and little crosses for compressive polarities) and stress axes (grey triangle for P-axis and white triangle for T-axis) are shown. Composite focal mechanism solution is shown as black/white beach ball (black triangle for P-axis and white triangle for T-axis). Black squares indicate municipalities.
Applsci 15 00784 g008

6. Stress Regime

As we discussed in the Introduction, information on stress field orientation and seismogenic sources for Southern Apulia is lacking in the Italian catalogs, and an effort is needed to fill in this gap. A crucial parameter that provides information about regional tectonics and deformation mechanisms is the stress axis orientation, which can be retrieved using different techniques based on focal mechanisms.
To obtain an estimate of the orientation of the stress tensor, we performed the inversion of stress field orientation by applying the FMSI (Focal Mechanism Stress Inversion) code developed by Gephart and Forsyth [52,53]. This inversion method can retrieve four of the six independent components of the stress tensor, commonly represented by the directions of the three principal stress axes (σ1, σ2 and σ3) and a dimensionless parameter R = (σ2 − σ1)/(σ3 − σ1), which constrains the shape of the stress ellipsoid and ranges between 0 and 1. The angular difference between the shear stress on the fault plane, computed by the stress tensor inversion, and the observed slip direction on the same fault plane, obtained by focal mechanisms, measures the discrepancy (or misfit) between the data and the model.
The dataset consists of the trend and plunge angles of the T and P axes, as obtained by FPFIT inversion. To better constrain the stress values obtained by the inversion, we adopted a weighting scheme given by the weight, W, which takes into account the quality of the focal mechanism solution, described by Qf and Qp, and the event magnitude (taken as reported in Table 1). The Qf and Qp of each focal mechanism solution (Table 4) are converted from letters to numbers, according to Table 6. Earthquake magnitude is included in the value of W on the hypothesis that the regional stress should be better represented by main earthquakes since small earthquakes may represent stress accommodation near the seismogenic source of the main event. We assigned W values that decrease with the quality factors of the focal mechanism inversion and increase with the earthquake magnitude. We then assigned a weight, W, to each fault plane solution in Table 4 as a function of the total quality factor, Qt = Qf + Qp + M, according to Table 7.
The acceptability and homogeneity of the stress inversion solution was evaluated following the procedure of Lu et al. [54], based on the simultaneous satisfaction of two criteria. The first requires that the 95% confidence intervals of σ1 and σ3 do not overlap for the solution to be acceptable. The second accounts for the degree of heterogeneity of the investigated medium, requiring that the misfit angle be under a certain threshold (misfit < 6°) to consider a solution to be homogeneous.
The result is shown in Figure 9. The 95% confidence interval of the solution is very narrow, and the misfit = 2.4° indicates homogeneity of the medium. The stress ratio of R = 0.5 indicates that σ2 has a value exactly intermediate, and the misfit = 2.4° indicates a high degree of homogeneity of the solution. Following the notation of the stress regime assignment for earthquake focal mechanism data [55] in World Stress Map, we have the following:
  • σ3, corresponding to the minimum horizontal stress, Shmin, is sub-horizontal and oriented as the trend of T axes, 36° N;
  • σ1, corresponding to the maximum horizontal stress, Shmax, is approximately sub-horizontal and oriented normal to Shmin;
  • σ2, corresponding to the vertical stress or pure lithostatic pressure, Sv, is quite vertical.
In Figure 9, the double-couple focal mechanism corresponding to the mean stress tensor solution is also shown; between the two nodal planes, having a strike approximately N-S and E-W, the hypothetical fault plane is most likely the one having strike = 77° (dip = 53°; and rake = −175), according to Tropeano et al. [31], Gambini and Tozzi [56], and ZS9 [57].
This result is remarkable and reflects the general dominance of a right-lateral strike–slip regime with approximately N-S/E-W nodal planes.

7. Discussion and Conclusions

In the present paper, the seismicity of southeastern Murge was reconsidered and analyzed. Thanks to the registration of the OSUB and OTRIONS seismic networks, new recordings from stations close to the epicenters were retrieved, improving the quality of the relocations in terms of minimum epicentral distance and location uncertainty. The seismic activity is sporadic, as evidenced by the very low number of significant events recorded (only 32 earthquakes with M ≥ 2.0 in 25 years).
The collected earthquakes, distributed over a horizontal distance of ~50 km, with a range of focal depths from 4.5 km to 27 km, indicate that the whole Earth crust is involved in this sporadic seismic activity.
The seven focal mechanisms, although obtained using a limited number of P-wave polarities, appear quite well constrained and homogeneous, as can be observed in Figure 8. All the focal solutions have a common nodal plane in the approximately E-W direction, as shown by the φ1 values, which range between 80° N and 127° N (see Table 4). This is the same direction along which earthquake epicenters tend to align (Figure 6), neglecting the earthquakes in the neighboring of Taranto City, so, between the two nodal planes of the mechanism solution, the plane 1 should correspond to the actual fault plane. It should be noted that this E-W fault striking fits the seismogenic zonation of Italy [57] very well.
The stress regime inferred in this study indicates that seismicity, even if it is of low frequency and magnitude, occurs according to right-lateral transtensional kinematics of the seismogenic faults. The homogeneity of the focal plane solutions provided a high-quality stress-inversion solution. The orientation of Shmin, according to the direction of the trend of the σ3 axis (36° N), agrees with the results of the Shmin in adjacent areas (as shown in Figure 10, modified from Mariucci and Montone [12]), indicating that this region is subject to a regional stress regime that controls the tectonics of the Adria plate.
In an attempt to correlate earthquakes and their seismogenic structures, we selected the seismic events with the best constrained location (20120505, 20180323, 20190213 and 20190520), recorded by a more recent and denser network of seismographs, which provided locations with a total quality factor A or B. In agreement with Festa et al. [11], the location of the seismic event 20180323 is consistent with its association to the western branch of the fault bounding the Monte Giove structural high and the adjacent Rosaria Mare basin (Figure 11a). Moreover, the eastern branch of this fault seems to have been the seismogenic source for the seismic event 20190213 (Figure 11a). A good consistency is also observed between the eastern branch of the fault belonging to the Central Deformation Zone (Figure 4) and the location of the seismic event 20190520. Finally, the epicenter of the seismic event 20120505 is close to the fault striking from WNW (near the town of Matera) to ESE (south of the town of Brindisi), bounding the southern Murge (Figure 11a).
In the southeastern Murge area, the ca. NE-SW regional elongation has occurred since Middle Pleistocene [58]. Accordingly, such a feature of the regional strain field is coherent with the obtained stress field (Figure 9), which could determine, in the southeastern Murge, the reactivation of ca. E-W striking ancient faults (at least of the Late Cretaceous) with a right horizontal component of simple shear (Figure 11a).
Such a stress field is, moreover, tectonically coherent with the outer NW-SE striking lithospheric buckle fold of the Adria plate, which occurred during Apennines subduction and determined the uplift of the Apulian Foreland since the Middle Pleistocene (Figure 11b) [20].
Since the area is characterized by very sporadic events, the possibility of underestimating the resulting hazard and thus increasing the probability of facing unexpected earthquakes is significant. This makes uncertain the prediction of the magnitude of major local earthquakes that can be expected, their peak ground acceleration and their recurrence interval. In this regard, it is useful to introduce the concept of Maximum Credible Earthquake (MCE), which is based on the seismic history and seismotectonics of the area and is a basic ingredient in the Neo-Deterministic Seismic Hazard Assessment (NDSHA) to calculate seismic hazard [59].
With this approach, the Maximum Credible Earthquake, representing the largest physically possible scenario event at a given site, can be represented by a value, Mdesign, equal to the maximum magnitude, Mmax, observed or estimated in the study area, plus a multiple of its overall standard deviation, γEMσM [60]. To adopt a conservative approach, it is currently wise to set γEMσM as 0.7 (Panza–Rugarli law), according to [61], so that Mdesign = Mmax + γEMσM = Mmax + 0.7 [60]. For the study area, this would result in Mdesign = 3.9 + 0.7 = 4.6.
This magnitude value suggests looking at the geometry of the seismogenic structure at a wider scale than what has been considered so far, since the most common relationships between the magnitude value vs. length of the fault [62,63] show that to M ~4 corresponds to a length of the fault of about 1 km.
For this purpose, it may be wise to increase the density of the seismic network and thereby improve seismic monitoring with a view toward reliable seismic risk assessment.

Supplementary Materials

The following supporting information can be downloaded at: www.mdpi.com/article/10.3390/app15020784/s1.

Author Contributions

Conceptualization, P.P., M.F. and V.F.; methodology, P.P.; validation, V.D.G. and A.T.; formal analysis, P.P., M.F. and N.V.; investigation, P.P. and V.F.; resources, P.P., V.D.G. and A.T.; data curation, P.P., M.F. and N.V.; writing—original draft preparation, P.P., M.F. and V.F.; writing—review and editing P.P., M.F., V.D.G., N.V. and V.F.; visualization, P.P., M.F., N.V. and V.F.; funding acquisition, V.F. All authors have read and agreed to the published version of the manuscript.

Funding

This study was carried out within the (1) RETURN Extended Partnership and received funding from the European Union—NextGenerationEU (National Recovery and Resilience Plan—NRRP, Mission 4, Component 2, Investment 1.3-D.D. 1243 2/8/2022, PE0000005); (2) Progetto “GeoSciences: un’infrastruttura di ricerca per la Rete Italiana dei Servizi Geologici—GeoSciences IR” (codice identificativo domanda: IR0000037); CUP: I53C22000800006. Piano Nazionale di Ripresa e Resilienza, PNRR, Missione 4, Componente 2, Investimento 3.1, “Fondo per la realizzazione di un sistema integrato di infrastrutture di ricerca e innovazione” finanziato dall’Unione Europea—Next Generation EU.

Data Availability Statement

Data of UniBa (from the seismic networks OSUB and OTRIONS) described in Section 4, for the period preceding 2019, are available upon request.

Acknowledgments

Some figures were obtained by employing the GMT freeware package by Wessel and Smith [64] and subsequent versions by Google Earth Pro, Google, Inc., Mountain View, CA, USA (accessed on 15 April 2024). We kindly thank the four anonymous reviewers who improved the paper with their useful comments.

Conflicts of Interest

The authors declare no conflicts of interest.

References

  1. Faccenna, C.; Becker, T.W. Topographic expressions of mantle dynamics in the Mediterranean. Earth Sci. Rev. 2020, 209, 103327. [Google Scholar] [CrossRef]
  2. Handy, M.R.; Ustaszewski, K.; Kissling, E. Reconstructing the Alps-Carpathians-Dinarides as a key to understanding switches in subduction polarity, slab gaps and surface motion. Int. J. Earth Sci. 2015, 104, 1–26. [Google Scholar] [CrossRef]
  3. Serpelloni, E.; Anzidei, M.; Baldi, P.; Casula, G.; Galvani, A. Crustal velocity and strain-rate fields in Italy and surrounding regions: New results from the analysis of permanent and non-permanent GPS networks. Geophys. J. Int. 2005, 161, 861–880. [Google Scholar] [CrossRef]
  4. Le Breton, E.; Handy, M.R.; Molli, G.; Ustaszewski, K. Post-20 Ma motion of the Adriatic plate: New constraints from surrounding Orogens and implications for crust-mantle decoupling. Tectonics 2017, 36, 3135–3154. [Google Scholar] [CrossRef]
  5. Meletti, C.; Patacca, E.; Scandone, P. Construction of a seismotectonic model: The case of Italy. Pure Appl. Geophys. 2000, 157, 11–35. [Google Scholar] [CrossRef]
  6. Ninivaggi, T.; Selvaggi, G.; Mazza, S.; Filippucci, M.; Tursi, F.; Czuba, W. Nature and origin of an undetected seismic phase in waveforms from Southern Tyrrhenian (Italy) intermediate-depth and deep earthquakes: First evidence for the phase-A in the subducted uppermost lithospheric mantle? Tectonophysics 2023, 860, 229919. [Google Scholar] [CrossRef]
  7. CPTI Working Group (ING; GNDT; SGA; SSN). Catalogo Parametrico dei Terremoti Italiani; Editrice Compositori: Bologna, Italy, 2004; p. 88. Available online: https://emidius.mi.ingv.it/CPTI/home.html (accessed on 15 April 2024).
  8. Del Gaudio, V.; Pierri, P.; Frepoli, A.; Calcagnile, G.; Venisti, N.; Cimini, G.B. A critical revision of the seismicity of Northern Apulia (Adriatic microplate—Southern Italy) and implications for the identification of seismogenic structures. Tectonophysics 2007, 436, 9–35. [Google Scholar] [CrossRef]
  9. Del Gaudio, V.; Pierri, P.; Calcagnile, G.; Venisti, N. Characteristics of the low energy seismicity of central Apulia (southern Italy) and hazard implications. J. Seismol. 2005, 9, 39–59. [Google Scholar] [CrossRef]
  10. Filippucci, M.; Miccolis, S.; Castagnozzi, A.; Cecere, G.; de Lorenzo, S.; Donvito, G.; Falco, L.; Michele, M.; Nicotri, S.; Romeo, A.; et al. Seismicity of the Gargano promontory (Southern Italy) after 7 years of local seismic network operation: Data release of waveforms from 2013 to 2018. Data Brief 2021, 35, 106783. [Google Scholar] [CrossRef]
  11. Festa, V.; De Giosa, F.; Del Gaudio, V.; Moretti, M.; Pierri, P. In search of the seismogenic source of the March 23rd 2018 earthquake (Mw 3.7) near Brindisi (Puglia, Southern Italy). Geol. Croat. 2019, 72, 137–144. [Google Scholar] [CrossRef]
  12. Mariucci, M.T.; Montone, P. IPSI 1.6, Database of Italian Present-Day Stress Indicators; Istituto Nazionale di Geofisica e Vulcanologia (INGV): Rome, Italy, 2024. [CrossRef]
  13. DISS Working Group. Database of Individual Seismogenic Sources (DISS), Version 3.3.0: A Compilation of Potential Sources for Earthquakes Larger than M 5.5 in Italy and Surrounding Areas; Istituto Nazionale di Geofisica e Vulcanologia (INGV): Rome, Italy, 2021. [CrossRef]
  14. ITHACA Working Group. ITHACA (ITaly HAzard from Capable Faulting), a Database of Active Capable Faults of the Italian Territory; Version December 2019; ISPRA Geological Survey of Italy: Rome, Italy, 2019; Available online: https://sgi.isprambiente.it/ithacaweb/Default.aspx# (accessed on 15 April 2024).
  15. de Alteriis, G.; Aiello, G. Stratigraphy and tectonics offshore of Puglia (Italy, southern Adriatic Sea). Mar. Geol. 1993, 113, 233–253. [Google Scholar] [CrossRef]
  16. Bertotti, G.; Picotti, V.; Chilovi, C.; Fantoni, R.; Merlini, S.; Mosconi, A. Neogene to quaternary sedimentary basins in the south adriatic (central mediterranean): Foredeeps and lithospheric buckling. Tectonics 2001, 20, 771–787. [Google Scholar] [CrossRef]
  17. Fantoni, R.; Franciosi, R. Tectono-sedimentary setting of Po Plain and Adriatic foreland. Rend. Lincei-Sci. Fis. 2010, 21, 197–209. [Google Scholar] [CrossRef]
  18. Cicala, M.; Festa, V.; Sabato, L.; Tropeano, M.; Doglioni, C. Interference between Apennines and Hellenides foreland basins around the Apulian swell (Italy and Greece). Mar. Pet. Geol. 2021, 133, 105300. [Google Scholar] [CrossRef]
  19. Ricchetti, G.; Ciaranfi, N.; Luperto Sinni, E.; Mongelli, F.; Pieri, P. Geodinamica ed evoluzione sedimentaria e tettonica dell’Avampaese Apulo. Mem. Soc. Geol. Ital. 1988, 41, 57–82. [Google Scholar]
  20. Doglioni, C.; Mongelli, F.; Pieri, P. The Puglia uplift (SE Italy): An anomaly in the foreland of the Apenninic subduction due to buckling of a thick continental lithosphere. Tectonics 1994, 13, 1309–1321. [Google Scholar] [CrossRef]
  21. Selli, R. Il Paleogene nel quadro della geologia dell’Italia meridionale. Mem. Soc. Geol. Ital. 1962, 3, 737–790. [Google Scholar]
  22. Ciaranfi, N.; Pieri, P.; Ricchetti, G. Note alla Carta Geologica delle Murge e del Salento (Puglia centromeridionale). Mem. Soc. Geol. Ital. 1988, 41, 449–460. [Google Scholar]
  23. Spalluto, L.; Pieri, P.; Ricchetti, G. Le facies carbonatiche di piattaforma interna del Promontorio del Gargano: Implicazioni paleoambientali e correlazioni con la coeva successione delle Murge. Boll. Soc. Geol. Ital. 2005, 124, 675–690. [Google Scholar]
  24. Borgomano, J.R.F. The Upper Cretaceous carbonates of the Gargano-Murge region, southern Italy: A model of platform-to-basin transition. AAPG Bull. 2000, 84, 1561–1588. [Google Scholar] [CrossRef]
  25. Nicolai, C.; Gambini, R. Structural architecture of the Adria Platform-and-Basin System. Boll. Soc. Geol. Ital. 2007, 7, 21–37. [Google Scholar]
  26. de’ Dominicis, A.; Mazzoldi, G. Interpretazione geologico-strutturale del margine orientale della Piattaforma Apula. Mem. Soc. Geol. Ital. 1987, 38, 163–176. [Google Scholar]
  27. Mastrogiacomo, G.; Moretti, M.; Owen, G.; Spalluto, L. Tectonic triggering of slump sheets in the Upper Cretaceous carbonate succession of the Porto Selvaggio area (Salento peninsula, southern Italy): Synsedimentary tectonics in the Apulian Carbonate Platform. Sediment. Geol. 2012, 269–270, 15–27. [Google Scholar] [CrossRef]
  28. Cicala, M.; De Giosa, F.; Festa, V.; Lisco, S.; Moretti, M. The northern fault of the onshore-offshore Monte Giove relief in the southern Adriatic Sea, Italy: Implications for tectonic reactivation in the Apulian Foreland. Geol. Q. 2023, 67, 11. [Google Scholar] [CrossRef]
  29. Martinis, B. Sulla tettonica delle Murge Nord-Occidentali. Acc. Naz. Lincei Rend. Sc. Fis. Mat. Nat. 1961, 31, 299–305. [Google Scholar]
  30. Iannone, A.; Pieri, P. Caratteri neotettonici delle Murge. Geol. Appl. Idrogeol. 1982, 17, 147–159. [Google Scholar]
  31. Tropeano, M.; Pieri, P.; Moretti, M.; Festa, V.; Calcagnile, G.; Del Gaudio, V.; Pierri, P. Quaternary tectonics and seismotectonic features of the Murge area (Apulian foreland, SE Italy). Il Quat. 1997, 10, 543–548. [Google Scholar]
  32. Festa, V. Cretaceous structural features of the Murge area (Apulian Foreland, Southern Italy). Eclogae Geol. Helv. 2003, 96, 11–22. [Google Scholar] [CrossRef]
  33. de Alteriis, G. Different foreland basins in Italy: Examples from the central and southern Adriatic Sea. Tectonophysics 1995, 252, 349–373. [Google Scholar] [CrossRef]
  34. Pieri, P.; Laviano, A. Tettonica e sedimentazione nei depositi senoniani delle Murge sud-orientali (Ostuni). Boll. Soc. Geol. Ital. 1989, 108, 351–356. [Google Scholar]
  35. Del Gaudio, V.; Festa, V.; Ripa, R.R.; Iurilli, V.; Pierri, P.; Calcagnile, G.; Moretti, M.; Pieri, P.; Tropeano, M. Evidence of Apulian crustal structures related to low energy seismicity (Murge—Southern Italy). Ann. Geophys. 2001, 44, 1049–1066. [Google Scholar] [CrossRef]
  36. Bath, M.; Duda, S.J. Earthquake volume, fault plane area, seismic energy, strain, deformation and related quantities. Ann. Geofis. 1964, 17, 353–368. [Google Scholar]
  37. Rovida, A.; Locati, M.; Camassi, R.; Lolli, B.; Gasperini, P. The Italian earthquake catalogue CPTI15. Bull. Earthq. Eng. 2020, 18, 2953–2984. [Google Scholar] [CrossRef]
  38. Rovida, A.; Locati, M.; Camassi, R.; Lolli, B.; Gasperini, P.; Antonucci, A. Catalogo Parametrico dei Terremoti Italiani (CPTI15), Versione 4.0 [Data Set]; Istituto Nazionale di Geofisica e Vulcanologia (INGV): Rome, Italy, 2022. [CrossRef]
  39. Guidoboni, E.; Ferrari, G.; Mariotti, D.; Comastri, A.; Tarabusi, G.; Sgattoni, G.; Valensise, G. CFTI5Med, Catalogo dei Forti Terremoti in Italia (461 a.C.-1997) e Nell’area Mediterranea (760 a.C.-1500); Istituto Nazionale di Geofisica e Vulcanologia (INGV): Rome, Italy, 2018. [CrossRef]
  40. Guidoboni, E.; Ferrari, G.; Tarabusi, G.; Sgattoni, G.; Comastri, A.; Mariotti, D.; Ciuccarelli, C.; Bianchi, M.G.; Valensise, G. CFTI5Med, the new release of the catalogue of strong earthquakes in Italy and in the Mediterranean area. Sci. Data 2019, 6, 80. [Google Scholar] [CrossRef]
  41. Postpischl, D. Catalogo dei Terremoti Italiani Dall’anno 1000 al 1980; C.N.R.—Progetto Finalizzato Geodinamica—Quaderni della Ricerca Scientifica: Bologna, Italy, 1985; Volume 114 2B, 239p. [Google Scholar]
  42. Castello, B.; Di Stefano, R. Catalogo della Sismicità Italiana (CSI): Fasi (Version 2) [Data Set]; Istituto Nazionale di Geofisica e Vulcanologia (INGV): Rome, Italy, 2022. [CrossRef]
  43. Locati, M.; Camassi, R.; Rovida, A.; Ercolani, E.; Bernardini, F.; Castelli, V.; Caracciolo, C.H.; Tertulliani, A.; Rossi, A.; Azzaro, R.; et al. Database Macrosismico Italiano (DBMI15), Versione 4.0 [Data Set]; Istituto Nazionale di Geofisica e Vulcanologia (INGV): Rome, Italy, 2022. [CrossRef]
  44. Lahr, J.C. HYPOELLIPSE/Version 2.0: A Computer Program for Determining Local Earthquakes Hypocentral Parameters, Magnitude, and First-Motion Pattern; Open-File Rep. n. 89-116; U.S. Geological Survey: Reston, VA, USA, 1989; 95p. [CrossRef]
  45. Calcagnile, G.; Panza, G.F. The main characteristics of the lithosphere-asthenosphere system in Italy and surroundings regions. Pure Appl. Geophys. 1980, 119, 865–879. [Google Scholar] [CrossRef]
  46. Costa, G.; Panza, G.F.; Suhadolc, P.; Vaccari, F. Zoning of the Italian territory in terms of expected peak ground acceleration derived from complete synthetic seismograms. J. Appl. Geophys. 1993, 30, 149–160. [Google Scholar] [CrossRef]
  47. Venisti, N.; Calcagnile, G.; Pontevivo, A.; Panza, G.F. Tomographic Study of the Adriatic Plate. Pure Appl. Geophys. 2005, 162, 311–329. [Google Scholar] [CrossRef]
  48. Pierri, P.; de Lorenzo, S.; Calcagnile, G. Analysis of the low-energy seismic activity in the Southern Apulia (Italy). Open J. Earthq. Res. 2013, 2, 91–105. [Google Scholar] [CrossRef]
  49. Reasenberg, P.; Oppenheimer, D. FPFIT, FPPLOT and FPPAGE: FORTRAN Computer Programs for Calculating and Displaying Earthquake Fault Plane Solutions; Open File Rep. n. 85-739; U.S. Geological Survey: Reston, VA, USA, 1985; 109p. [CrossRef]
  50. Frepoli, A.; Cimini, G.B.; De Gori, P.; De Luca, G.; Marchetti, A.; Monna, S.; Montuori, C.; Pagliuca, N.M. Seismic sequences and swarms in the Latium-Abruzzo-Molise Apennines (central Italy): New observations and analysis from a dense monitoring of the recent activity. Tectonophysics 2017, 712–713, 312–329. [Google Scholar] [CrossRef]
  51. Scognamiglio, L.; Tinti, E.; Michelini, A. Real-time determination of seismic moment tensor for Italian region. Bull. Seism. Soc. Am. 2009, 99, 2223–2242. [Google Scholar] [CrossRef]
  52. Gephart, J.W. FMSI: A Fortran program for inverting fault/slickenside and earthquake focal mechanism data to obtain the regional stress tensor. Comput. Geosci. 1990, 16, 953–989. [Google Scholar] [CrossRef]
  53. Gephart, J.W.; Forsyth, D.W. An improved method for determining the regional stress tensor using earthquake focal mechanism data: Application to the San Fernando earthquake sequence. J. Geophys. Res. 1984, 89, 9305–9320. [Google Scholar] [CrossRef]
  54. Lu, Z.; Wyss, M.; Pulpan, H. Details of stress directions in the Alaska subduction zone from fault plane solutions. J. Geophys. Res. 1997, 102, 5385–5402. [Google Scholar] [CrossRef]
  55. Heidbach, O.; Rajabi, M.; Reiter, K.; Ziegler, M. World Stress Map 2016 [Dataset]; GFZ Data Services: Potsdam, Germany, 2016. [Google Scholar] [CrossRef]
  56. Gambini, R.; Tozzi, M. Tertiary geodynamic evolution of Southern Adria Microplate. Terra Nova 1996, 8, 593–602. [Google Scholar] [CrossRef]
  57. Meletti, C.; Valensise, G. Zonazione Sismogenetica ZS9—App. 2 al Rapporto Conclusivo. Gruppo di Lavoro per la Redazione della Mappa di Pericolosità Sismica Prevista Dall’ordinanza PCM 3274 del 20 Marzo 2003; Istituto Nazionale di Geofisica e Vulcanologia: Rome, Italy, 2004; 38p.
  58. Di Bucci, D.; Caputo, R.; Mastronuzzi, G.; Fracassi, U.; Selleri, G.; Sansò, P. Quantitative analysis of extensional joints in the southern Adriatic foreland (Italy), and the active tectonics of the Apulia region. J. Geodyn. 2011, 51, 141–155. [Google Scholar] [CrossRef]
  59. Panza, G.F.; Bela, J. NDSHA: A new paradigm for reliable seismic hazard assessment. Eng. Geol. 2020, 275, 105403. [Google Scholar] [CrossRef]
  60. Rugarli, P.; Vaccari, F.; Panza, G.F. Seismogenic nodes as a viable alternative to seismogenic zones and observed seismicity for the definition of seismic hazard at regional scale. Vietnam. J. Earth Sci. 2019, 41, 289–304. [Google Scholar] [CrossRef]
  61. Wen, Z.P.; Wang, G.X. Earthquakes and Sustainable Infrastructure-Neodeterministic (NDSHA) Approach Guarantees Prevention Rather Than Cure. Earthq. Sci. 2024, 37, 493–496. [Google Scholar] [CrossRef]
  62. Leonard, M. Earthquake Fault Scaling: Self-Consistent Relating of Rupture Length, Width, Average Displacement, and Moment Release. Bull. Seism. Soc. Am. 2010, 100, 1971–1988. [Google Scholar] [CrossRef]
  63. Trippetta, F.; Petricca, P.; Billi, A.; Collettini, C.; Cuffaro, M.; Lombardi, A.M.; Scrocca, D.; Ventura, G.; Morgante, A.; Doglioni, C. From mapped faults to fault-length earthquake magnitude (FLEM): A test on Italy with methodological implications. Solid Earth 2019, 10, 1555–1579. [Google Scholar] [CrossRef]
  64. Wessel, P.; Smith, W.H.F. New, Improved Version of Generic Mapping Tools Released. Eos Trans. Am. Geophys. Union 1998, 79, 579. [Google Scholar] [CrossRef]
Applsci 15 00784 g001
Figure 1. Structural sketch of Italy and surrounding areas (modified from Meletti et al. [5]). The black arrow indicates the slip vectors of Africa vs. Europe and of Adria vs. Europe obtained from geodetic data. Adria RP is the Adria rotation pole. The red square delimits the area analyzed in this paper.
Figure 1. Structural sketch of Italy and surrounding areas (modified from Meletti et al. [5]). The black arrow indicates the slip vectors of Africa vs. Europe and of Adria vs. Europe obtained from geodetic data. Adria RP is the Adria rotation pole. The red square delimits the area analyzed in this paper.
Applsci 15 00784 g001
Applsci 15 00784 g002
Figure 2. Seismic stations used in this study: in red, those belonging to seismic network managed by UniBa (OSUB/OTRIONS); in blue, those belonging to other seismic networks (IV, IX, GE, MN, CR, AC and TV).
Figure 2. Seismic stations used in this study: in red, those belonging to seismic network managed by UniBa (OSUB/OTRIONS); in blue, those belonging to other seismic networks (IV, IX, GE, MN, CR, AC and TV).
Applsci 15 00784 g002
Applsci 15 00784 g004
Figure 4. Structural sketch map of the Murge area and the Northern Salento (modified after [32]); the fault striking from WNW (near the town of Matera) to ESE (south of the town of Brindisi) borders the Murge (to the north) from the Salento (to the south); the Monte Giove submarine relief and Rosaria Mare basin (modified after [11,28]) are also indicated; red filled squares represent epicentral relocation of seismic events for which the focal mechanism was determined in the present paper.
Figure 4. Structural sketch map of the Murge area and the Northern Salento (modified after [32]); the fault striking from WNW (near the town of Matera) to ESE (south of the town of Brindisi) borders the Murge (to the north) from the Salento (to the south); the Monte Giove submarine relief and Rosaria Mare basin (modified after [11,28]) are also indicated; red filled squares represent epicentral relocation of seismic events for which the focal mechanism was determined in the present paper.
Applsci 15 00784 g004
Applsci 15 00784 g005
Figure 5. Seismic events located within 35 km from Ostuni (red dashed circle), identified by (i) the CPTI15 version 4.0 catalogue (red circles, [37,38]), (ii) the CFTI5MED (grey circles, [39,40]), (iii) the PFG catalogue (yellow circles, [41]) and (iv) the CSI catalogue (blue circles, [42]). Circles represent focal volumes according to Bath and Duda formula [36]. The geographical position of main localities mentioned in the paper is also shown as black squares.
Figure 5. Seismic events located within 35 km from Ostuni (red dashed circle), identified by (i) the CPTI15 version 4.0 catalogue (red circles, [37,38]), (ii) the CFTI5MED (grey circles, [39,40]), (iii) the PFG catalogue (yellow circles, [41]) and (iv) the CSI catalogue (blue circles, [42]). Circles represent focal volumes according to Bath and Duda formula [36]. The geographical position of main localities mentioned in the paper is also shown as black squares.
Applsci 15 00784 g005
Applsci 15 00784 g009
Figure 9. Results of the FMSI inversion for the considered event group. From the left: stereonet plot with 95% confidence limits for the principal stress axes σ1 (in fuchsia), σ2 (in green) and σ3 (in light blue); plunge/trend angles of σ1, σ2, σ3, R value and misfit angle of solution; double-couple focal mechanism corresponding to the mean stress tensor solution (black triangle for P-axis and white triangle for T-axis).
Figure 9. Results of the FMSI inversion for the considered event group. From the left: stereonet plot with 95% confidence limits for the principal stress axes σ1 (in fuchsia), σ2 (in green) and σ3 (in light blue); plunge/trend angles of σ1, σ2, σ3, R value and misfit angle of solution; double-couple focal mechanism corresponding to the mean stress tensor solution (black triangle for P-axis and white triangle for T-axis).
Applsci 15 00784 g009
Applsci 15 00784 g010
Figure 10. Map extracted from the IPSI database (modified from [12]). Colored segments refer to minimum horizontal stress (Shmin) orientations; see legend for color explanation. The result of this study is also superimposed.
Figure 10. Map extracted from the IPSI database (modified from [12]). Colored segments refer to minimum horizontal stress (Shmin) orientations; see legend for color explanation. The result of this study is also superimposed.
Applsci 15 00784 g010
Applsci 15 00784 g011
Figure 11. (a) Structural sketch map showing the attempt to correlate the major faults with the seismic events 20120505, 20180323, 20190213 and 20190520 in the southeastern Murge area (legend for the faults as in Figure 4). (b) Schematic block diagram showing the Apennines subduction in correspondence of the Apulian Foreland, adjusted for the Murge area (modified after [20]); the red arrows indicate the regional elongation (as in Figure 11a) on the outer lithosphere buckle fold.
Figure 11. (a) Structural sketch map showing the attempt to correlate the major faults with the seismic events 20120505, 20180323, 20190213 and 20190520 in the southeastern Murge area (legend for the faults as in Figure 4). (b) Schematic block diagram showing the Apennines subduction in correspondence of the Apulian Foreland, adjusted for the Murge area (modified after [20]); the red arrows indicate the regional elongation (as in Figure 11a) on the outer lithosphere buckle fold.
Applsci 15 00784 g011
Table 1. List of earthquakes located by INGV between 2000 and September 2024 within 35 km from Ostuni (40.7332 N–17.5786 E), with M ≥ 2.0. ID = identification number. Date is expressed in Year-Month- Day. Time is the earthquake origin time (UTC). Depth (km) is the earthquake depth (fixed if marked with *). RMS (s) is the root mean square of the travel time residuals; M is the “preferred” magnitude taken from INGV Seismic Bulletin (ML, Md or Mw).
Table 1. List of earthquakes located by INGV between 2000 and September 2024 within 35 km from Ostuni (40.7332 N–17.5786 E), with M ≥ 2.0. ID = identification number. Date is expressed in Year-Month- Day. Time is the earthquake origin time (UTC). Depth (km) is the earthquake depth (fixed if marked with *). RMS (s) is the root mean square of the travel time residuals; M is the “preferred” magnitude taken from INGV Seismic Bulletin (ML, Md or Mw).
IDDateTimeLat (N)Long (E)DepthRMSMLocation
12000-08-2610:27:15.0240.994017.77505 *0.302.7Costa Adriatica Brindisina (Brindisi)
22001-05-1523:00:46.9040.808017.340010.80.202.33 km SW Fasano (BR)
32001-07-1511:17:29.8840.665017.54005 *0.603.03 km NE Ceglie Messapica (BR)
42002-06-0602:18:22.5940.778017.315010 *0.202.43 km NW Locorotondo (BA)
52006-05-1609:04:23.5140.544017.30506.10.102.13 km SW Montemesola (TA)
62006-08-1812:01:54.4840.525017.368010 *0.452.03 km NW Monteiasi (TA)
72006-08-2312:44:37.7440.548017.364010 *0.332.03 km SE Montemesola (TA)
82007-06-0413:06:28.4040.569017.354010 *0.442.02 km E Montemesola (TA)
92007-07-2012:16:17.2040.542017.367010 *0.272.04 km SE Montemesola (TA)
102008-05-1123:03:14.3940.821017.69801 *0.842.6Costa Adriatica Brindisina (Brindisi)
112009-05-1209:46:10.4940.556017.27406.90.082.35 km W Montemesola (TA)
122009-06-1609:27:08.0940.568017.24807.10.392.04 km E Statte (TA)
132009-08-2310:15:13.5340.805017.82704 *0.652.0Costa Adriatica Brindisina (Brindisi)
142009-09-0713:25:53.3140.791017.666010 *0.472.1Costa Adriatica Brindisina (Brindisi)
152010-07-0709:01:56.7440.562017.26106.30.402.35 km E Statte (TA)
162011-05-1306:21:29.6140.748017.51607.90.412.36 km W Ostuni (BR)
172012-05-0512:44:02.9440.539317.54155.10.442.84 km W Francavilla Fontana (BR)
182012-06-1309:11:41.2940.551317.24735 *0.322.04 km E Statte (TA)
192012-12-2219:31:28.3140.999217.357210 *0.322.2Costa Adriatica Barese (Bari)
202013-08-1106:37:09.2740.737817.41505.00.342.21 km W Cisternino (BR)
212015-06-2514:37:47.5540.514217.58652.80.222.52 km S Francavilla Fontana (BR)
222015-10-2818:53:31.6940.780817.41954.90.342.85 km N Cisternino (BR)
232018-03-2323:31:56.8140.800317.693829.70.273.9Costa Adriatica Brindisina (Brindisi)
242019-02-1321:56:44.0640.825317.814329.80.483.2Costa Adriatica Brindisina (Brindisi)
252019-05-2000:57:24.2940.793817.66084 *0.422.5Costa Adriatica Brindisina (Brindisi)
262019-10-0922:36:49.7540.938217.38636.80.252.2Costa Adriatica Barese (Bari)
272020-01-1605:00:47.6640.768217.200225.80.372.34 km SW Alberobello (BA)
282021-02-1119:36:08.1840.689317.218329.90.292.69 km N Crispiano (TA)
292021-04-2106:10:18.3740.755717.954725.50.512.9Costa Adriatica Brindisina (Brindisi)
302021-05-3122:41:20.3240.895817.312722.70.372.56 km S Monopoli (BA)
312021-05-3123:11:36.7440.892317.302821.10.322.37 km S Monopoli (BA)
322022-04-0913:45:22.6740.907817.739827.20.322.6Costa Adriatica Brindisina (Brindisi)
Table 2. List of relocated earthquakes: Nd1 and Nd2 represent the number of used phases (P and S) of the UniBa and INGV stations; Ns is the number of used stations; Dmin (km) is the minimum epicentral distance; Gap (°) is the azimuthal gap; SEH1, SEH2 and SEZ are the horizontal and vertical 68% confidence limits of the error ellipsoid; refer to Table 1 caption for other parameters.
Table 2. List of relocated earthquakes: Nd1 and Nd2 represent the number of used phases (P and S) of the UniBa and INGV stations; Ns is the number of used stations; Dmin (km) is the minimum epicentral distance; Gap (°) is the azimuthal gap; SEH1, SEH2 and SEZ are the horizontal and vertical 68% confidence limits of the error ellipsoid; refer to Table 1 caption for other parameters.
IDDateTimeLat (N)Long (E)DepthNd1Nd2NsDminGap RMSSEH1SEH2SEZ
12000-08-2610:27:15.0341.084317.818814.8610961.12330.201.23.31.8
22001-05-1523:00:46.6040.954817.360318.048617.72040.315.714.45.3
32001-07-1511:17:30.0040.787217.598917.66262115.41920.343.37.02.4
42002-06-0602:18:22.0340.827417.308621.46879.61560.302.84.34.9
52006-05-1609:04:24.1340.532717.306010.806335.03250.051.63.49.1
62006-08-1812:01:56.6340.532417.20586.336530.92460.161.53.429.4
72006-08-2312:44:39.0840.549117.244616.626430.73050.143.16.710.8
82007-06-0413:06:29.9640.535817.22765.046531.32260.151.51.935.5
92007-07-2012:16:17.8240.535517.332718.908436.12950.050.83.11.0
102008-05-1123:03:13.1740.842517.82978.05191758.13030.392.02.57.4
112009-05-1209:46:11.0640.580117.283016.606329.73180.010.20.30.4
122009-06-1609:27:08.7440.599517.247314.2016826.02860.181.53.24.2
132009-08-2310:15:13.6140.821017.87114.506326.02410.234.535.899.0
142009-09-0713:25:53.6940.819717.67665.906329.52440.243.514. 569.3
152010-07-0709:01:57.5740.610817.251816.009525.32870.192.55.13.3
162011-05-1306:21:29.8240.772217.528618.715191813.81780.380.81.20.8
172012-05-0512:44:03.9240.532517.54187.314382914.7990.430.51.13.3
182012-06-1309:11:41.7040.568817.30456.707431.83130.2517.925.899.0
192012-12-2219:31:28.2441.003817.365023.61281020.92120.270.61.01.8
202013-08-1106:37:09.9540.768017.415610.010121213.61680.150.51.03.1
212015-06-2514:37:48.4440.503717.59675.9016923.51210.140.51.414.0
222015-10-2818:53:32.5340.785317.44015.00523431.71690.601.74.64.1
232018-03-2323:31:57.4240.813417.703524.223606126.1990.511.21.81.4
242019-02-1321:56:45.2240.848017.743213.66141530.12120.281.93.18.9
252019-05-2000:57:25.3940.794817.66196.74272427.92000.380.91.84.4
262019-10-0922:36:50.8140.952817.38507.518292432.61850.571.62.53.4
272020-01-1605:00:48.2840.759217.169323.3721169.51400.140.60.91.7
282021-02-1119:36:08.8340.676217.193727.21023216.41240.180.61.01.3
292021-04-2106:10:17.7140.862117.981917.38141432.32150.261.24.62.2
302021-05-3122:41:20.9740.901117.307616.416414424.01190.373.65.93.7
312021-05-3123:11:37.3940.887417.292516.516252722.11650.282.95.23.6
322022-04-0913:45:23.6240.876117.706225.811182031.31960.321.83.34.7
Table 4. List of fault plane solutions of the events. For each event, the table reports the identification number (ID); the angles of strike (φ), dip (δ) and rake (λ) of the 2 nodal planes; the trend and plunge angles of the P and T axes; the number of polarities, Npol, that match compared to the total; the quality factors (Qf and Qp); and the fault type (FT: NS, normal/strike–slip; SS, strike–slip; U, unknown; and NO, normal). The fault plane solution obtained for the composite mechanism is also reported.
Table 4. List of fault plane solutions of the events. For each event, the table reports the identification number (ID); the angles of strike (φ), dip (δ) and rake (λ) of the 2 nodal planes; the trend and plunge angles of the P and T axes; the number of polarities, Npol, that match compared to the total; the quality factors (Qf and Qp); and the fault type (FT: NS, normal/strike–slip; SS, strike–slip; U, unknown; and NO, normal). The fault plane solution obtained for the composite mechanism is also reported.
IDφ1δ1λ1φ2δ2λ2Trend PPlunge PTrend TPlunge TNpol/NtotQfQpFT
1010035−16035379−5729746582610/11CCNS
169555−180590−3531424562411/13CBSS
178065−15033663−2829938208110/10AANS
2311784125215351017931594144/48BBU
248080−14034251−13309352051910/10ABNS
2528131−10712060−8055732031410/10AANO
2912731−1612080−6032147862911/13CANS
joint7030−16032380−6226247302984/115CBNS
Table 6. Weighting criteria assigned to the fault plane solution in the FMSI inversion.
Table 6. Weighting criteria assigned to the fault plane solution in the FMSI inversion.
Qf = A, Qp = AQf = B, Qp = BQf = C, Qp = C
321
Table 7. Relative weight as a function of the total quality factor (Qt).
Table 7. Relative weight as a function of the total quality factor (Qt).
QtWeight W
Qt < 4.53.0
Qt ≥ 4.54.0
Qt ≥ 5.55.0
Qt ≥ 6.56.0
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Pierri, P.; Filippucci, M.; Del Gaudio, V.; Tallarico, A.; Venisti, N.; Festa, V. New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore. Appl. Sci. 2025, 15, 784. https://doi.org/10.3390/app15020784

AMA Style

Pierri P, Filippucci M, Del Gaudio V, Tallarico A, Venisti N, Festa V. New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore. Applied Sciences. 2025; 15(2):784. https://doi.org/10.3390/app15020784

Chicago/Turabian Style

Pierri, Pierpaolo, Marilena Filippucci, Vincenzo Del Gaudio, Andrea Tallarico, Nicola Venisti, and Vincenzo Festa. 2025. "New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore" Applied Sciences 15, no. 2: 784. https://doi.org/10.3390/app15020784

APA Style

Pierri, P., Filippucci, M., Del Gaudio, V., Tallarico, A., Venisti, N., & Festa, V. (2025). New Insights on the Seismic Activity of Ostuni (Apulia Region, Southern Italy) Offshore. Applied Sciences, 15(2), 784. https://doi.org/10.3390/app15020784

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop