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Analysis of small magnitude seismic sequences along the Northern Apennines (Italy)

D. Piccinini a,⁎, N. Piana Agostinetti a, P. Roselli a, M. Ibs-von Seht b, T. Braun a

a Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italyb Federal Institute for Geosciences and Natural Resources, Hannover, Germany

a b s t r a c ta r t i c l e i n f o

Article history:Received 31 December 2007Received in revised form 26 March 2009Accepted 6 April 2009Available online 16 April 2009

Keywords:Northern ApenninesSeismicityFocal mechanismsStress field

We analyze the seismicity of a small sector of the Northern Apennines merging data from the Italian seismicbulletin with original data collected by temporary seismic networks. Our attention is focused on the regionenclosed between the Apenninic watershed and the Adriatic Sea. This portion of belt is interested by theoccurrence of diffuse crustal seismicity and small-to-moderate earthquakes. In this paper we study the fivesmall sequences with mainshock having Mwb4.7 that in the past 15 years hit the area. Our interest isaddressed to better understand the relationship between these events and the regional seismotectonicsetting in terms of seismicity distribution and stress field. Two regions with different behavior in the seismicrelease can be distinguished: (i) along the watershed where seismicity is clustered at shallow depths(b15 km) and where strong earthquakes occurred in the past, (ii) an eastern portion where the seismicity isdistributed across all of the crustal volume, locally reaching depths down to 30 km. The focal mechanism ofthe seismic sequences shows mainly normal fault kinematics coherent with the regional stress field. Detailedstress field analysis suggests a rotation of the principal stress axis moving from the axial part of the chaintoward the Adriatic Sea to the east.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

In the last decades, Central and Northern Italy was the setting for aconsiderable number of moderate earthquakes with magnitudes up toM=6 (Gubbio, 1984; Colfiorito, 1997/1998; Gualdo Tadino, 1998;Ciaccio et al., 2005 among others). Nearly all major events occurred onSW-dipping (40°–50°) normal faults, an observation that gives one ofthe main seismological evidence for the predominant extensionalstress regime that characterize the Northern Apennines. Analysis ofseismic reflection profiles revealed that the crustal structure of theNorthern Apennines is characterized by a set of NE-dipping, low anglenormal faults (LANF). As recently demonstrated, the easternmost ofthese LANFs, the Alto Tiberina Fault (hereinafter ATF), shows a strongmicroseismic activity (Boncio et al., 2000; Collettini and Barchi, 2002;Piccinini et al., 2003; Chiaraluce et al., 2007).

The topography of the ATF is well imaged by passive seismicobservations up to Sansepolcro. On the adjacent northern part,towards the Casentino area, microseismicity is virtually absent, butevaluation of reflection seismic profiles near Sansepolcro (Barchi et al.,1998) and Pieve Santo Stefano (Ciaccio et al., 2007) give evidence for acontinuation of the ATF towards north for at least 15 km. The furthercontinuation of the ATF towards NWhas been proposed also by Boncio

et al. (2000) who postulated the aggregation of the ATF with faults ofthe Northern Apennines along the so called Etrurian Fault System(EFS) passing along the Casentino area.

In the present study we review and analyze the seismicitydistribution occurring in a sector of the Northern Apennines (Fig. 1)including new data recorded by temporary local seismic networks.Then we compare and correlate the occurrence of low-magnitudeearthquakes, moderate small seismic sequences (3.5bMb4.7), andhistorical earthquakes (MN5.5).

2. Seismotectonic setting of the area

The Northern Apennines are composed of a NE-verging thrust-foldbelt, formed as the result of the collision between the Europeancontinental margin (Sardinia–Corsica block) and the Adriatic litho-sphere (e.g. Alvarez, 1972; Reutter et al., 1980). At the present day, thisportion of the belt is subject to active crustal extension, accompaniedby moderate normal faulting earthquakes that occur on NW-trendingSW-dipping normal faults with lengths of usually less than 10 km(Deschamps et al., 1984; Haessler et al., 1988; Westaway et al., 1989;Boncio and Lavecchia, 2000; Chiaraluce et al., 2004). Focal mechan-isms of seismic events and borehole break-outs (Montone et al., 2004)reveal a regional stress field with a nearly vertical α1 axis and a NE-trending sub-horizontal α3 axis, and an extensional strain rate in theorder of 2.5 mm/yr (Hunstad et al., 2003). Recently, the interpreta-tion of seismic reflection profiles provided by the CROP03-NVR

Tectonophysics 476 (2009) 136–144

⁎ Corresponding author.E-mail address: piccinini@ingv.it (D. Piccinini).

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seismic active experiment (CROsta Profonda Project Near VerticalReflection; Pialli et al., 1998) showed that a significant amount ofextension within the brittle upper crust is accommodated by a set ofeast-dipping low angle normal faults associated with high-angleantithetic structures. According to Boncio et al. (2000), the eastern-most part of the Apenninic LANF system developed during the lastca. 3.5 Ma (Late Pliocene Quaternary) and formed graben and half-graben systems.

The LANF geometry is clearly imaged in seismic reflection profiles(Barchi et al., 1998) and is consistent with movements along an east-dipping extensional fault. This main active fault can be imaged con-tinuously for at least 60 km fromSansepolcro to Perugia (Fig.1). Recentstudies of the microseismic activity of the ATF, performed by Piccininiet al. (2003) and Chiaraluce et al. (2007), showed that in the areabetween Sansepolcro and Città di Castello the seismicity seems to bealmost absent.

In the past, the study area was hit by moderate earthquakes (seeFig. 1). Following the catalogue of strong earthquakes in Italy(Guidoboni et al., 2007) we could group the historical events for thenorth-western sector (S. Sofia) as represented by three historicalstrong earthquakes with 5.8bMeb6.1, in 1279, 1661 and 1768. Duringthe 20th century a M=5.9 struck the same area (1918/11/10). Thecentral part of the study area was hit by numerous events withmaximum equivalent magnitude M=6.3 (1781). The last moderateevent reported in the catalogue is represented by the 1917 M=5.9

eventwhich struck the area close Città di Castello. In the south-easternpart the historical seismicity is represented by the two events locatedin the Gualdo Tadino area in 1747 (M=5.8) and in 1751 (M=6.3)which reaches the maximum value of intensity X.

3. Dataset and analysis procedures

In order to better understand the location and the evolution ofthe local seismicity we merge data from two permanent seismicnetworks and those coming from a number of temporary seismicdeployments carried out inside the study area. In this study, weanalyze in detail the local seismicity recorded by six different data-sets (Fig. 1): (1) the Italian Seismic Network (ISN), (2) the MarcheRegional Seismic Network (RSM), (3, 4, 5) three temporary arrayinstallations of 2003 and 2005, and (6) a small seismic networkdeployed north of Sansepolcro (2005/2006). Data from the ISNbelong to two different sources: (i) the Catalogue of the Seismicity ofItaly (CSI 1.1, Chiarabba et al., 2005), which reports the Italianseismicity between 1981 and 2002, and (ii) the seismic bulletin ofthe Istituto Nazionale di Geofisica e Vulcanologia (INGV), whichcontains phases and localizations of earthquakes that occurred alongthe Italian peninsula between 2003 and present. Datasets from theabove mentioned permanent and temporary deployments have beenmerged to obtain a homogeneous dataset of the seismicity in thestudy area for the period 1981–2005.

Fig. 1. Introductive map of the study area. Seismic stations used in the study are represented by triangles and labeled by colors. Temporary experiments are indicated by theoperating period: gray/red framed—ISN, yellow—2002/2003, blue and red—2005, green—2005/2006. Black solid lines represent the cross-sections described in the text. The graybold line represents the CROP03 (Pialli et al., 1998) interesting the area, while the outcrop of the ATF and its hypothetical continuation towards NW (EFS) are plotted in solid/dotted red, respectively. Names of the geographic regions and cities are reported in bold gray. Orange stars represent the locations of the historical earthquakes discussed in thetext.

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We analyzed the complete dataset using the following scheme: tobetter constrain earthquake locations, we first computed a minimum1-D velocity model (Kissling et al., 1994), by simultaneously invertinghypocentral and velocity parameters. Afterwards, we locate theseismicity in the study area inverting P and S-waves arrival times(Lahr, 1989). Finally, to refine the localization of the individual seismicsequence or clusters we apply a relative relocation (double-difference,DD) algorithm (Waldhauser and Ellsworth, 2000). For each sequence,we computed focal mechanisms solutions using the P-wave firstmotion polarity (Reasenberg and Oppenheimer, 1985). An inversion ofthe resulting fault plane solutions was performed (Michael, 1984) tocompute direction and plunge of the principal stress axes.

In order to calculate an appropriate starting velocity model for thelocation procedure, data from the CSI 1.1 catalogue were used. The CSI1.1 contains arrival times of all available stations of the Italian SeismicNetwork and of all regional networks operating in Italy. We extracted3644 events with epicenter inside the area of interest, containing P- andS-wave arrivals at 70 stations. A subset of the complete dataset wasformed by selecting 543 events with horizontal locations errors (ERH)less than 2 km and vertical location errors (ERZ) less than 3 km. Thissubset comprising 6440 P- and S-phases was used to compute theminimum 1-D velocity model of the area. The resulting 1-D velocitymodel (Table 1) produced a final RMS residual of 0.16 s whichcorresponds to an RMS reduction of about 60%. The upper crustalvolume iswell resolveddown to adepth8 km,withmore than2000 rayspassing through. Although the area investigated in the present study islarge and geologically heterogeneous, the found P-wave velocities are inaccordance with those reported by Piccinini et al. (2003) and they arealso consistent with similar values derived from drilling logs andlaboratory experiments (Bally et al., 1986). The VP/VS ratio of 1.80 wascomputed using a Wadati diagram and is similar to values found inadjacent regions (Haessler et al.,1988; Piccinini et al., 2003).We locatedthe original dataset composed of 3644 seismic events using theminimum 1-D model and obtained a final average RMS of all eventsequal to 0.22 s. Despite the low average RMS, we decided to furtherreduce the dataset in order to analyze only those events with locationerrors lower than ERH=3 km and ERZ=5 km and with azimuthal gapslower than270°. Thefinal dataset (Fig. 7)was composedof 1731events ofmagnitudes between 2.2bMLb4.4. Their depths aremainly ranging from0 to 15 km in thewestern portion and tomore than 30 km in the easternsector. Thefinalmean formal errors of the locationprocedure are small. Inparticular,more than80%of the events are locatedwith ahorizontal errorless than 1.5 km, while the vertical errors are around 2 km.

Analyzing the catalogue we recognize five seismic sequencesoccurred in the study area during the 1997–2005 time window(Figs. 2–5) with main event having Mwb4.7. These minor sequenceswere occasionally recorded also by temporary networks running inthe study area. To get a more detailed image of the individual seismicsequences, we relocated the data of each subset applying the DDalgorithm (Waldhauser and Ellsworth, 2000), using the best-fitvelocity model previously obtained (Table 1). For each subset, weperformed a number of iterations to optimize the damping and con-trol parameters of the program, and to obtain good conditioning andconvergence of the solution.

Fault plane solutions were calculated for all events of each seismicsequence subset with more than seven polarity readings. For eachsubset, we show the fault plane solution of three major events only(except for the Chiusi della Verna cluster). The stress inversion resultsassociated to each subset are shown in the inset of Figs. 3–5.

4. Analysis of the seismicity

In the following paragraphs we will describe in detail the seismicsequences denominated according to the nearest town: Pieve SantoStefano, Carpegna, Chiusi della Verna, Santa Sofia, and Sestino(Figs. 2–6). The two clusters near Pieve Santo Stefano have beenpreviously analyzed in Braun et al. (2002) and Ciaccio et al. (2007)where the authors named them as Sansepolcro sequences.

4.1. Pieve Santo Stefano

On 26th of November 2001 an earthquake of Mw=4.7 occurred inthe area near Pieve S. Stefano northward of Sansepolcro (Fig. 1),followed by a two month lasting aftershock sequence. The same areawas hit by a moderate seismic event of Mw=4.4 on 2nd of October1997 at 19:38 and a second event about 2 hours later with similarmagnitude (ML=4.1). Even if the station coverage of the 1997sequence was not optimal, we associated the two sequences,relocating the catalogue data listed in the CSI 1.1. Although the formalerrors obtained for the hypocenters of the 2001 dataset (76 events)are in the range of a few tens of meters, the error reduction for the1997 (62 events) sequence is minor, resulting in a less sharp image ofseismicity. Fig. 2 shows the epicentral map (Fig. 2a) and a NE-trendingvertical cross-section (Fig. 2b; see cross-sections traces in Fig. 1) of the1997 and 2001 seismic sequences. The grey NE-dipping line in Fig. 2b

Table 1Table describing the best 1-D velocity model used to locate the seismicity presented inthe paper.

VP (km/s) Depth range (km)

5200 0.0–105700 1.0–4.05900 4.0–6.06100 6.0–8.06200 8.0–10.06400 10.0–34.07800 34.0–

Fig. 2. Seismicity distribution of the Pieve Santo Stefano clusters shown as map (a) andcross-section (b) in SW–NE direction (A–A′): 1997 and 2001 hypocenters arerepresented with red and green dots, respectively. In red, focal mechanisms for themainshock and the second major event of 1997 seismic sequence (RCMT solution,Mw=4.4, polarity solution—ML=4.1). Also the mainshock of 2001 (green RCMT for theMw=4.7) is plotted. The projection of the position of the ATF, as evidenced by CROP03(Pialli et al., 1998), is plotted in light gray.

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represents the ATF, as imaged by the CROP03, located about 5 km SE ofthe clustered hypocenters. The focal solutions reported (green andred) are those reported in the RCMT (Regional Centroid MomentTensor) database (Pondrelli et al., 2002, 2004). We also computed the

focal mechanism of the 1997 second event by using first motionpolarities (see Fig. 2 for details). The seismicity distribution of the1997 sequence developed on a high-angle SW-dipping structure thatcould be interpreted as antithetic fault to the ATF. However, thealignment of the 2001's aftershock sequence seems to be steeper thanthe dip angle of the ATF. Both observations are confirmed by the focalmechanisms of the mainshocks indicating a normal fault solution.Discrepancies between the focal solutions and the aftershockdistribution will be discussed in the final paragraph.

4.2. Carpegna

We analyzed about 150 seismic events in the Carpegna area with amagnitudeMLb3 (Fig. 3a) in the period from 15th of September 2005 to

Fig. 3.Map (a) and cross-section (b) showing the seismicity distribution of the Carpegnacluster. The focal solutions of the three strongest events of the sequence (MLN2.6) arerepresented in blue. Notice the direction of the principal stress axis (inset): in the topstereographic projection the directions derived from the entire dataset are plotted, in thebottom one those retrieved from the focal solution of the deeper events (see detail in thetext). In gray the confidence region for each direction are plotted.

Fig. 4.Map (a) and cross-section (b) for the Chiusi della Verna cluster. Cumulative focalsolution and direction of principal stress axis in maps are shown.

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15th of October 2005. Despite the final RMS is small (0.17 s), and theformal location errors are even small (ERHb1 km; ERZb2 km), theseismicity distribution does not show a clear fault geometry. Therefore,the local seismicity of the Carpegna area appears to be substantiallydifferent from the other clusters described in the present paper and, ingeneral, from the seismicity commonly located along the chain.Although the seismogenic volume interested by this cluster is small,

we observe that diffuse crustal seismicity seems to be a behavior of theeastern side of the study area. However, microseismicity release in theCarpegna area extends almost vertically at greater depths than the onthe Tyrrhenian side. We also observe that the majority of the Carpegnacluster is released in the deeper part of the crust (ZN14 km) (PianaAgostinetti et al., 1999; Levin et al., 2002). The magnitude rangesbetween 1bMLb3.

Focal mechanisms of the three major events (MLN2) are shown inFig. 3a, plotted together with local stress axes retrieved from theinversion of 52 focal mechanisms of the complete seismic sequence.Focal mechanisms of the three major events and stress axes for thecomplete seismic sequence showcoherent directions: themajor stressaxis α1 is almost horizontal and oriented NW, while α3 lies in thehorizontal plane (strike slip solution). Because of the high residuals inthe stress inversion solution we also computed the stress axis of asubset of deeper events (ZN14 km). We notice that the analysis of thedeeper events shows a different stress configuration. In fact, for depthsgreater than 14 km α1 and α2 switch their position, while α1 remainunchanged. The resulting stress field is compressive although themajor stress axis is parallel to the strike of the thrusts of the area.These results are in good agreement with results obtained by otherauthors based on geological data (Lavecchia et al., 2003).

4.3. Chiusi della Verna

A small seismic sequence composed of 39 small events (MLb2.3)was localized at the beginning of October 2005 near the small townChiusi della Verna (Fig. 4). The cluster is well confined in space andtime, and about 80% of the events occurred within the subsequent18 h. Due to the lack of a mainshock and the small magnitude of theevents, a composite focal mechanism was calculated using the firstarrival polarities of the sequence. However we also inverted the stressfield obtained by using 19 focal solutions. The resulting principalstress axes are partly in agreement with the regional stress field,showing an almost vertical α1, and an α3 axis orthogonal to theregional one. Due to the very small magnitude of these events wecannot speculate too much about this peculiarity; we just suggest thatit could be related to the NW–SE trending faults affecting the area or topossible local rearrangement of the regional stress field.

4.4. Santa Sofia

On 26th of January 2003 twomoderate earthquakes ofMw=4.5 andMw=4.7 hit the Santa Sofia area. These events occurred within a timespan of less than 20 min. As evident from the seismicity distribution(Fig. 5) bothmainshocks occurred in the same small crustal volume at adepth of about 10 km. The resulting location errors are small(RMS=0.10) and the seismicity distribution imaged by the seismiccross-sections shows a SW-dipping seismicity cloud. This is in goodagreementwith the twonormal fault solutions (Fig. 5), indicatingone ofthe fault plane dipping between about 40 and 60° towards SW. Thecomposite fault plane solution of the sequence is quite similar to the twomajor events with an almost pure normal fault mechanism. The stressfield obtained inverting the available fault plane solutions (19 events)indicates direction and plunge of the stress axes equivalent to those ofthe regional stress field. Although the Santa Sofia area is located in theexternal sector of the chain, its stress fieldwell correlate the extensionalregime of the chain itself.

4.5. Sestino

During the 1st of December 2002 a temporary network recorded anearthquake swarmofmore than250 events.Most of the eventswere toosmall to be recorded by more than two stations of the network andhence we locate the events using only S–P time and first arrival particlemotion. Althoughwe could not estimate the error location,we observed

Fig. 5. Map and cross-section of the Santa Sofia earthquake cluster of 2003. The focalsolution of the two larger events (Mw=4.5 and Mw=4.7) are shown. The insetsrepresent the direction of the principal stress axis derived from the focal mechanisms.

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that the earthquakes occurredwithin a short time span of only 26 h andoriginated from a spatially restricted area about 5 km south of stationRSP (station location in Fig.1, seismicity location in Fig. 7a and c). In fact,a closer look on the records indicates that most of the recordedseismicity shows high waveform similarities and virtually identical S–Pdelay times (Fig. 6). In particular we cross-correlate the waveformsseparating at least 3 group of almost co-located events. The S–P delaytimes differ of less than 0.1 s between the three groups, indicating a

source volumeof about 0.5 km(Fig. 6). Each group shows indications fora co-location of the seismic sources and similar source mechanisms ofthe events. Moreover, the generally small energy released by the singleearthquakes and the lack of a typicalmainshock–aftershock pattern leadus to speculate that the earthquakes could have been generated along acreeping fault patch (i.e. repeating earthquakes of similar size located inthe same area which repeatedly rupture the same fault section) or bedue to diffusivity process.

Fig. 6.Waveform similarity of events as recorded at station RSP. The filtered (4–25 Hz) waveforms recorded on the vertical component are aligned using a cross-correlation algorithmover 3.0 s starting from the P-wave arrival time (solid vertical line). S-wave time arrival is represented with dashed line. The three groups of events are composed by waveforms witha cross-correlation index greater than 0.9. Each group show slightly different S–P delay time corresponding approximately to a source separation of less than 1 km.

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Fig. 7. (a) 1731 epicenters of the data set between 1997 and 2006 are represented by small dots: A) Pieve S. Stefano 1997 and 2001 (red and green), B) Carpegna (blue), C) Chiusidella Verna (yellow), D) Santa Sofia (light blue), the background seismicity around Città di Castello (black), and CSI catalogue seismicity (light grey). The Sestino seismicity isrepresented by a black asterisk. Cross-sections showing (b) the seismicity distribution in the Upper Tiber Valley along the NW–SE direction (E–E′) and (c) SW–NE direction (F–F′)are also shown. The fault plane solutions presented in the paper are plotted by colored beach balls. In gray, we label the location of the historical seismicity (H1, H2 and H3) asdiscussed in the text.

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5. Discussion and conclusions

The present study analyzes the seismicity of the NorthernApennine, using datasets combined from temporary local seismicexperiments and the available seismic catalogue. Five distinct clustersof small or moderate size earthquakes have been observed andlocalized during the last 10 years (from SE to NW): Pieve S. Stefano(A1, A2), Carpegna (B), Chiusi della Verna (C) and Santa Sofia (D).Fig. 7b and c shows the depth and spatial distributions of the clustersplotted on two NE-trending cross-sections. The clusters A1–A2, C, Dlie exactly on the Apenninic orientated section line (Fig. 7b), whilecluster A1–A2 and B are situated along the anti-Apenninic sectionline (Fig. 7c). The Sestino cluster lies in the midway between clustersA1–A2 and B, along the section line in Fig. 7c. In Table 2 we reportthe source parameters for the nine focal solutions presented in thispaper.

Comparative observation of the peculiarities of each cluster andtheir spatial distribution, together with the position of the historicalearthquakes, suggests the following seismotectonic characterizationof the study area: in the eastern domain, the seismicity distributioncharacterized by the Carpegna seismicity (Figs. 3 and 7b) reachesdepths of ~30 km and our dataset suggests a switching of the stressfield from transpressive to almost compressive at depths greater than14 km. Although the small magnitude events poorly constrain thestress inversion results, stress rotationwith depth is a behavior knownfrom adjacent areas. Piccinini et al. (2006) interpreted the compres-sion highlighted by the occurrence of a Mw=5.3 event and relatedsequence in the Northern Apennines (2003 Mw=5.3 Monghidoroearthquake) as a secondary process caused by differential motioninside the flexed Adriatic lithosphere. Following this interpretationcompressional fault mechanisms could be observed within the lowercrust were at surface and in the shallow crust are present extensionalbasins (e.g. Mugello basin).

However, in the western sector of the study area, the hypocentresof A1–A2 are almost completely confined in the upper crust(Zb15 km). In particular, the seismicity distribution of these clusterssuggests that the ATF continues towards the northern part of the studyarea, acting as the lower boundary for the seismic energy release. Theseismicity of the Pieve Santo Stefano 1997 and 2001 sequences seemsto be confined in the hangingwall block of the fault in agreement withprevious studies (Boncio et al., 2000; Collettini and Barchi, 2002;Piccinini et al., 2003; Chiaraluce et al., 2007). These authors reportthat the diffuse seismicity in the southern adjacent area is confined atdepth by the ATF. The pattern found for the hypocenters of the 2001sequence indicates that the ATF continues also in the NW part respectto the CROP03 line. Comparing our locations to those previouslyproposed (Braun et al., 2002; Ciaccio et al., 2007), we motivate theclustering of the seismicity due to the use of different data, differentvelocity model (see text for details) and location method. From thelocated earthquakes distribution we prove the presence of an

antithetic faults (Figs. 2 and 7c) limited in depth by the presence ofthe ATF. The discrepancy between the seismicity distribution and thefocal mechanisms could be caused either to a mislocation of theseismic sequence, or to a badly constrained focal solution, obtained byusing different methods (see Section 4.1 for details). Analyzing thefault geometry obtained by relocating the seismicity, we state that thetwo 1997 Pieve Santo Stefano mainshocks probably occurred at theintersection between the ATF and the antithetic high-angle SW-dipping fault, while the aftershock sequence developed along thehigh-angle normal fault.

For the 2001 Pieve Santo Stefano seismic sequence we observed asimilar discrepancy between the focal mechanism of the mainshockand the hypocenters distribution of the aftershock sequence. In thiscase the focal solution and the CROP03 line indicates a similar dipangle of the ATF, while the aftershock locations align along a steeperplane. A plausible explanation for this observation could be, that theaftershock sequence involved one of the steeper and shallower splaysof the ATF.

On the midway between the Pieve Santo Stefano and Carpegnaclusters, we observed the microseismicity associated to the Sestinocluster, described in Section 4.5 (black asterisk in Fig. 7). The presenceof a low-magnitude microseismic activity in the Sestino area suggeststhat the seismic energy release in this area could be associated to asmall source volume able to generate earthquakes of similar sizewhich repeatedly rupture the same fault section.

In our analysis we observe that seismic energy release across theApenninic chain displays very different characteristics. Some peculia-rities become evident when looking at the spatial distribution alongthe chain. As shown in Fig. 7b the seismicity clusters of profiles A1–A2,C, D (compare Fig. 1) are confined in the upper crust with depths of9 km, 13 km and 18 km, respectively, showing increasing depths fromSE to NW. Looking at the historical seismicity (Fig. 7a and b), thestrong earthquakes seem to be spatially grouped. Independently oftheir time of occurrence, we distinguish the following historicalclusters that lie near the section E–E′: H1 (Gualdo Tadino), H2(Sansepolcro), H3 (Santa. Sofia). While the diffuse microseismicity ofCittà di Castello (Chiaraluce et al., 2007) falls exactly between H1 andH2, the moderate earthquakes plus aftershock sequences A, C, Doccurred in an area with a low seismicity release, between H2 and H3.In particular between Città di Castello and the H2 events, we localizedfewer seismic events compared to the frequency of small earthquakesin the southernmost part of the study area. Between the clusters A1–A2 and C, neither instrumental seismicity nor stronger historicalearthquakes are present (Fig. 7b).

Finally the stress field, resulting from the inversion of fault planesolutions for each individual cluster along the profile E–E′, depicts ahomogeneous and unexpected image. Results from clusters A1–A2(Pieve Santo Stefano) are in agreement with the extensional regimeinteresting the Apenninic chain. On the other hand retrieved stressfields for cluster D (Santa Sofia) showa near verticalα1, suggesting thepresence of the extensional regime beyond the Apenninic watershedtowards the Adriatic Sea, where compressive regime in the lower crustis expected (i.e. Carpegna seismicity). These results contribute todefine a more complete image of the small-to-moderate magnitudeseismic sequences in the area and help to better understand thecorrelation between seismic release and regional seismotectonic ofthe Northern Apennines.

Acknowledgements

Weare grateful toT. Plenefisch, K. Klinge, R.M. Azzara, F. Bergamaschi,M. Ohrnberger, F. Krüger, and D. Vollmer for the help during field work.Part of theworkwas supported by theDeutsche Forschungsgemeinschaft(KL 776/4-1) and by the Regione Toscana (Convenzione Valtiberina).Useful suggestions to improve the paper were given by P. Boncio, L.Chiaraluce and an anonymous referee.

Table 2Focal solutions discussed in the text.

Data M Strike Dip Rake Location

19971002 19:38 4.4 127 41 −58 Pieve S. Stefano⁎19971002 21:38 4.2 100 50 −120 Pieve S. Stefano20011126 00:52 4.7 127 26 −57 Pieve S. Stefano ⁎

– – 40 35 −130 Chiusi della Verna20030126 19:57 4.5 140 41 −101 Santa Sofia20030126 20:15 4.7 291 37 −130 Santa Sofia20050930 21:19 3.0 95 70 −170 Carpegna20051007 21:45 2.6 3 85 −20 Carpegna20051010 09:45 2.6 105 80 −160 Carpegna

All the focal mechanisms presented in the paper were obtained inverting first P-wavearrival except those marked with an asterisk (RCMT database).

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Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.tecto.2009.04.005.

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