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SEISMoLoGICAL InvEStIGAtIon of thE 2013 – 2014 SEISMIC SEqUEnCE of thE MAtESE MASSIf �SoUthERn APEnnInES, ItALy��G. MilanoOsservatorio Vesuviano – Sezione di Napoli INGV, Napoli, Italy

Introduction. The degree of knowledge about the location of the active faults, as wellThe degree of knowledge about the location of the active faults, as well as the relation between the occurrence of seismic sequence and the regional tectonics, is at present matter of studies for the Apennines Chain. The increasing number of seismic stations running in the Apennine Chain in the last 15 years, ensure also accurate earthquake locations and focal mechanism solutions of low-magnitude earthquakes (M < 2.5). These seismological data yield some new information on active earthquake faults, their kinematics and on strain deformation. The study of the background seismicity, in particular those of low and moderateThe study of the background seismicity, in particular those of low and moderate seismic sequences, can provide significant information towards the comprehension of rupture mechanism of the faults that can generate large earthquakes. This is of particular relevance for regions in which large historical earthquakes, suspended by long quiescence time periods, occurred. In the Apennine Chain, a region with the above characteristics is the Sannio-Matese. Between the end of December 2013 and January 2014 a seismic sequence, started with a ML = 4.9 earthquake, affected the southern side of the Matese Massif. The main shock was feltThe main shock was felt in a vast area, with the largest MCS intensities located to the south of the Matese Massif. This sequence struck the internal part of the Massif where no remarkable seismic events werestruck the internal part of the Massif where no remarkable seismic events were where no remarkable seismic events were located in the previous years. The sequence (red ellipse in Fig. 1) is located in between the macroseismic epicentral areas of historical destructive events with 6.5 ≤ Mw ≤ 7.0; further, it occurred less than 20 km far from the area where 15 years before a different low-energy seismic sequence took place. Here, a seismological investigation of the sequence is carried out using standard methods already used to study the seismicity of the area. The aim is to document the sequence and to valuate, if any, the main differences with the previous seismicity in order to provide a contribution to deepen the knowledge of the area.

The seismicity of the area. The Sannio-Matese area (Fig. 1), extending between southern Abruzzi and northern Campania administrative Regions, represents one of the most seismically active segments of the Apennines chain. This area includes the towns of Isernia and Campobasso to the North and Benevento to the SE (Fig. 1). In terms of seismic-hazard evaluation, this sector of the Apennines is considered one of the most dangerous region of Italy since, during the historical period, it was hit many times by highly destructive earthquakes with I0 > IX MCS, as well as by low-energy seismic sequences and swarms. Notable historical earthquakes, separated by long periods of relative quiescence, occurred in 1349, 1688, 1702, 1732, 1805. These events nucleated along NW-SE striking faults whose kinematics is consistent with the

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large scale NE-SW extensional regime affecting the Apennine Chain. Detailed study on the instrumental seismicity occurred since 1996 in the Sannio-Matese area (e.g. Milano et al., 2008; Fracassi and Milano, 2014) have shown that the seismic activity close to the Matese Massif is characterized by the occurrence of few single events, generally with magnitude less than 3.0, typically 2.5, with hypocenters within the uppermost 15 km of the crust (black dots in Fig. 1). These events occur sparse along the Massif without a preferred alignment even if the general distribution of the seismicity follow the NW-SE Apennine Chain axis. To this background seismicity are superimposed low magnitude seismic sequences and swarms. Key examples are the 1997-98 (MD < 4.2) and 2001 (MD < 3.6) sequences occurred at the eastern border of the Matese Massif and to NE of Isernia, respectively. Examples of low-magnitude swarms (MD < 3.2), lasting few days and consisting of some tens of events, occurred in 1999 and 2005 (Fig. 1). The 1997-98 seismic sequence started on March 19, 1997 at 23:10 (UTC) The 1997-98 seismic sequence started on March 19, 1997 at 23:10 (UTC) with an MD = 4.1 earthquake. The sequence, marked by alternating periods of intense activity and relative quiescence, consisted of more than 4000 events with MD between 0.8 and 3.7, 25 of these had MD between 3.0 and 3.7. The epicentral distribution of the sequence and the hypocentral distribution (Fig. 1, blue circles) show that earthquakes occurred along a nearly vertical plane striking NNE-SSW and dipping towards the east, located in the ESE edge of the in the ESE edge of the Matese Massif. Standard statististical techniques of strain analysis of faults applied to 50 focalStandard statististical techniques of strain analysis of faults applied to 50 focal mechanisms of events with MD > 2.5 shown that: 1) poles and strike of nodal planes evidence

Fig. 1 – Top: Main structural lineaments of the Matese Massif (modified from: Neotectonic map of Italy, CNR-PFG, 1983) on which is reported the background seismicity occurred since 1996 (black circles), the 1997-98 (bleu circles) and the 2001 (green circles) seismic sequences. Red ellipse represent the area in which the 2013-2014 seismic sequence occurred. The rectangular box with the year represent the seismogenic sources of the strong historical earthquakes (DISS, 2010). Triangles identify the permanent seismic stations close to the Matese Massif. Bottom: Hypocentral distribution of the events along the SW-NE (A-A’) and NW-SE (B-B’) directions.

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a maximum striking NE-SW and a second-order maximum striking WNW-ESE; 2) rakes have two maxima with values of about –50 and –120; 3) P axes are sub-vertical to oblique and show two preferred strike: NNE-SSW and ESE-WNW; 4) T axes are sub-horizontal with a maximum strike striking NNW-SSE. These results suggest that the strain field is heterogeneous even if a clear preferred rupture direction striking NE-SW is recognized. Rakes associated to this rupture as well as the preferred strike and dip of rupture plains are consistent with a NNE-SSW striking fault dipping towards SE and with normal to oblique dextral movements (Milano et al., 1999; Milano et al., 2002).

Between January and September 2001 the area to NE of Isernia was interested by a seismic sequence (Fig. 1, green circles). The seismic activity started with a sensible increase of the background seismicity but the magnitude of the events was comparable with the magnitude of the background seismicity of the area. The sequence consisted of about 1000 events with MD between 0.8 and 3.6 and only 5 events had MD > 3.0. Both the epicentral and hypocentral distributions of the events belonging to the sequence suggest that seismic activity took place along a near vertical plane NNE-SSW striking and dipping steeply eastward (Fig. 1, green circles) located close to the WNW edge of the Matese Massif. Standard statististical techniques close to the WNW edge of the Matese Massif. Standard statististical techniquesStandard statististical techniques of strain analysis of faults applied to the nodal planes of 30 well constrained focal mechanisms of events with MD > 2.5, shown that: 1) poles and strike evidence a maximum striking NE-SW with a maximum in rakes between –50 and –100; 2) P axes are sub-vertical to obliques and strike N-S; 3) T axes are sub-horizontal with a NW-SE preferred strike. The 2001 events record ruptures along a NE-SW preferred plane dipping towards SE. The rake values are consistent with prevailing normal movements. These ruptures are consistent with a NW-SE striking extension (Milano et al., 2005).

The 2013-2014 sequence. The sequence started on December 29, 2013 (17:08 UTC) withThe sequence started on December 29, 2013 (17:08 UTC) with a ML = 4.9 earthquake and was characterized by an intense seismic activity till the first week of January, 2014. The main shock was preceded a few minutes earlier by an event with ML = 2.7 (17:03 UTC). After a period of relative quiescence characterized by very few events, a second earthquake with ML = 4.2 occurred on January 20, 2014 (7:12 UTC), and, in turn, was followed by low seismic activity till January 23. Seismicity totally ceased afterwards. SGG station, the oldest seismic station operating in the area (since 1977; OV-INGV network; Fig. 1) and about 5 km away from the epicentral area of the sequence, recorded few events since January 2013 (about 20; ML ≤ 2.0). No events were detected in the days before the main-shock. With regard to the sequence, this station recorded about 1000 events between December 29, 2013 and the end of January 2014 with magnitude greater than 0.8. More than 50% of the events occurred during the first 3 days of the sequence. The magnitude of the events of the sequence was very low. Till the first week of January, only 8 events had 3.0 < ML < 4.0 and about 100 events had 2.0 < ML < 3.0, whereas, following the ML = 4.2 earthquake of January 20, only 2 events with 3.0 < ML < 4.0 and about 15 events with 2.0 < ML <3.0 occurred. With the exception of the two main shocks, no events with 4.0 < ML < 4.9 were recorded.

For location purposes, data recorded by the Italian Telemetered Seismic Network (ITSN) operated by Istituto Nazionale di Geofisica e Vulcanologia (INGV) have been used. Consisting of more than 300 stations, the present-day ITSN configuration ensures a fairly good azimuthal coverage of the Italian territory. In particular, the present-day spatial distribution of the seismic stations in the Sannio-Matese area (triangles in Fig. 1) allows also to locate low-magnitude events (ML ≤ 2). Data of the permanent OV-INGV seismic stations as well as data of two seismic stations installed in the area the days after the December 29, 2014 event (De Gori et al., 2014) have been used. Digital waveforms of recorded data related to the sequence have been collected to perform a re-picking of the events, focusing the attention on S-phase, for the reliability of the focal depth. To compare the results of this study with the results of previous studies performed on the area, the procedure reported in Milano et al. (2008) has been adopted. Events recorded by a minimum of five stations and with at least five P- and four S-phase readings have been located

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by means of the standard HYPO71 algorithm (Lee and Lahr, 1975), using the 1D velocity model reported in Milano et al. (2005). This velocity model has been repeatedly utilized to locate the seismicity occurred in the Matese Massif and surrounding in the last 20 years (e.g. the 1997-98 seismic sequence). Re-picking the seismic events also led to obtain a P-wave polarity dataset, that has been used to compute focal mechanisms by means of the standard FPFIT grid-search algorithm (Reasenberg and Opphenheimer, 1985). The number of polarity data used (≥ 14) and the fairly good azimuthal coverage of the seismic stations at short epicentral distance (< 80 km) led to stable solutions, with average errors on the maximum likelihood solutions < 10 degree for strike, dip and rake.

The epicentral distribution of about 260 best located events of the sequence, whose magnitude is 1.8 ≤ ML ≤ 4.9, is shown in Fig. 2 (top, left panel). Maximum error location on horizontal position and depth is 1.5 for both; maximum rms value is 0.45 s. Maximum gap is < 100° (<50° for the M(<50° for the ML > 2.5 events). Seismicity is concentrated in an area of about 10x4 km. Seismicity is concentrated in an area of about 10x4 km and the epicentral distribution depicts an about 10 km long, NW-SE trending alignment (Fig. 2,he epicentral distribution depicts an about 10 km long, NW-SE trending alignment (Fig. 2, trending alignment (Fig. 2,(Fig. 2, top) within the south-eastern part of the Matese Massif. Whereas the location of the first main- within the south-eastern part of the Matese Massif. Whereas the location of the first main-Massif. Whereas the location of the first main-Whereas the location of the first main-shock (ML= 4.9) falls in the middle of the sequence alignment, the second main-shock (ML= 4.2) is shifted about 2 km to the SE relative to the first one (red dots in Fig. 2 top, left panel). About 15 km divide the epicentral area of this sequence from the epicentral area of the 1997-98he epicentral area of this sequence from the epicentral area of the 1997-98 sequence (Fig. 1).

Fig. 2 – Top, left: Epicentral distribution of the 2013-14 sequence. The size of the circles is proportional to the magnitude of the events. Red dots represent the ML = 4.9 and the ML = 4.2 main-shocks. Top, right: Fault plane solutions of the events with magnitude greater than 2.8 and their location. Bottom: Hypocentral distribution of the events of the 2013-14 sequence (red circles) and of the events of the 1997-98 sequence (blue circles) along the NW-SE (B-B’) and SW-NE (A-A’) directions. The size of the circles is proportional to the magnitude of the events.

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The hypocentral distribution shows that focii are confined between 10 and 20 km depth (red circles in Fig. 2, bottom). Two clusters of events can be observed. The deeper one is at around 16-19 km, and includes the hypocentral depth for the two main-shocks (about 18 km). A well-defined shallower concentration of events is about 14-15 km depth and shows a sub-horizontal trend. Very few events occurred between 5-10 km depth and no events are located above 5 km and below 20 km in depth. The hypocentral distribution in a cross-section orthogonal to the main regional structures outlines a steep (about 75°-80°) southwest-dipping alignment between 10-20 km (Fig. 2 bottom). The computed fault-plane solutions show normal dip-slip mechanisms (Fig. 2 top, right panel). The focal mechanism of the first main-shock has about NW-SE striking planes (n. 1 in Fig. 2 top, right panel), as well as those related to events with ML > 2.8 (n. 2, 3, and 5 to 8). Instead, solutions n. 4 and 9 show about N-S striking planes. Fault-plane solutions relative to the second main-shock and those related to two events who followed it, have almost the same NW-SE striking planes like the previous (n. 10 to 12 in Fig. 2 top, right panel). The nodal planes of almost all computed fault-plane solutions agree with the sequence alignment and the P- and T-axes position of each focal mechanism are consistent with the large-scale extensional regime that characterize the Apennine Chain.

Discussion and concluding remarks. The sequences occurred in 2013-14 and 1997-98, although the epicentral areas are very close, shown considerable differences between them. The main differences concern: their temporal duration, very few days the first and over a year the second, the number of event, about 1000 vs more than 4000, the epicentral distribution, about NW-SE vs NNE-SSW, and the depth of the events. With respect to the last, the seismicity of the 2013-14 sequence does not propagate towards the surface being confined between 10 and 20 km in depth (red circles in Fig. 2, bottom). On the contrary, the seismic events of the 1997-98 sequence propagate towards the surface starting from the depth of about 15 km (blue circles in Fig. 2, bottom). This depth correspond to the depth of the MD = 4.1 earthquake that triggered the sequence. The hypocentral distribution of the events belonging to the 1997-98 sequenceThe hypocentral distribution of the events belonging to the 1997-98 sequenceof the events belonging to the 1997-98 sequence shows that seismic activity took place along a plane NNE-SSW striking and dipping steeplytook place along a plane NNE-SSW striking and dipping steeply eastward, whereas the hypocentral distribution of the events belonging to the 2013-14 sequence whereas the hypocentral distribution of the events belonging to the 2013-14 sequencewhereas the hypocentral distribution of the events belonging to the 2013-14 sequencehypocentral distribution of the events belonging to the 2013-14 sequence distribution of the events belonging to the 2013-14 sequence suggest that seismic activity took place along a near vertical plane NW-SE striking and gently dipping south-westward. Further, strain analysis of faults applied to the focal mechanisms of the 1997-98 sequence is consistent with a local NNW-SSE extension, as also reveled for the 2001 sequence, whereas the focal mechanism of the investigated sequence agree with the NE-SW large-scale extensional regime.

The spatial distribution of the seismicity since 1996, particularly, the epicentral areas of the 1997-98, between the seismogenic sources of the 1805 and 1688 events, and the 2001, at the NW tip of the 1805 source, sequences suggest that, before to the beginning of the 2013-14 sequence, the seismic release in the Matese Massif was concentrated in its WNW and ESE edges. Tomographic image of the Matese area and surrounding (Chiarabba and Amato, 1997; Bisio et al., 2004) evidence the presence of positive velocity anomalies all NW-elongated and the observation of high Vp regions at depths below 6 km beneath the Matese Massif. In the tomographic images proposed by Bisio et al. (2004), the most interesting feature is the high heterogeneity observed between 6 and 15 km depth. In particular, at about 9-10 km depth, where the Apulian platform is expected, the large NW-SE striking high velocity anomaly is found in the central part of the model. This structure extends parallel to the chain beneath the Matese Massif and is composed by two local highs that, laterally, go up till about 6-7 km ingo up till about 6-7 km in depth. These local high Vp These local high Vp regions are located close to the 1688 and 1805 seismogenic sources, and close to the 1688 seismogenic source, and are characterized by scarce seismicity, it being concentrated at their borders as showed by instrumental seismicity occurred in the last about 20 years. As evidenced by Bisio et al. (2004), the high Vp regions can be interpreted as high strength materials able to store large stress, representing the main asperities. The presence of high Vp bodies at 6 to 9 km depth beneath the Matese Massif was interpreted by Chiarabba

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and Amato (1997) as deep carbonates stacked in large scale thrust by the former compression. These authors speculated that the lower crustal material can have been involved in the thrusting. Improta et al. (2000), based on a joint interpretation of a 2D seismic profile and deep well data, explained the observed shallow high Vp material in terms of the Apulian platform involved in the compressional tectonics. The above suggests that the top of the Apulian platform has a strong wavy trend beneath the Matese Massif. The hypocenter of the main shock suggests that theThe hypocenter of the main shock suggests that the causative fault of the 2013-14 sequence has a focal depth at about 18 km. Under the hypothesis that just beneath the Matese Massif, where the sequence nucleate, the Apulian platform is atMatese Massif, where the sequence nucleate, the Apulian platform is at a depth of about 10 km, the focal volume is largely confined within the crust of Apulia, beingfocal volume is largely confined within the crust of Apulia, being only very few events occurred sparsely within its sedimentary cover. In other words, seismicity of this sequence stopped within the Apulia platform and did not interest the overlying Molisan and Matese units. The top of the Apulian platform could have acted as a barrier for the upwardsApulian platform could have acted as a barrier for the upwardsacted as a barrier for the upwards propagation of the seismic release. Alternatively, the presence of a strong absorption small the seismic release. Alternatively, the presence of a strong absorption small Alternatively, the presence of a strong absorption small layer, with very low rigidity, between the top of the Apulian platform and its sedimentary coverits sedimentary cover might be the cause of the no upwards propagation of the seismic release. However However, in case of case of earthquake with magnitude greater than that with ML= 4.9 that occurred on December 29, 2013, it is not to exclude that a little layer with strong absorption could be transparent and the upwards propagation of the seismic release could activate little superficial fault segments.ReferencesBisio, L., Di Giovambattista R., Milano, G., Chiarabba, C., 2004. Three-dimensional earthquake locations and upperThree-dimensional earthquake locations and upper

crustal structure of the Sannio-Matese region (southern Italy). Tectonophysics 385, 121– 136.Tectonophysics 385, 121– 136.CNR-PFG, 1983. Neotectonic map of Italy. Sottoprogetto Neotettonica, Dir. Barberi, Coor. Bosi, Scale 1:500,000,Neotectonic map of Italy. Sottoprogetto Neotettonica, Dir. Barberi, Coor. Bosi, Scale 1:500,000,ri, Coor. Bosi, Scale 1:500,000,Bosi, Scale 1:500,000,

Sheet 4.Chiarabba, C., Amato, A., 1997. Upper-crustal structure of the Benevento area (southern Italy): fault heterogeneities

and potentialfor large earthquakes. Geophys. J. Int. 130, 229– 239.De Gori, P., Moretti, M., Margheriti, L., Cecere, G., Aladino Covoni, A., Criscuoli, F., Mazza, S., Lauciani, V.,

D’ambrosio, C., Castagnozzi, A., Mammolo, A., Migliari F., Minichiello, F., Cardinale, V., Falco, L., Zarrilli, L., De Luca, G., Abruzzese, L., Delladio, A., Scognamiglio, L., Nichelini, A., (2014). Le attività di pronto intervento sismico dell’INGV a seguito del terremoto del 29 Dicembre 2013 (area del Matese). Rapporti Tecnici INGV 2014, No 281, ISSN 2039-7941.

DISS Working Group, 2010. Database of Individual Seismogenic Sources (DISS), Version 3.1.1: A compilation of potential sources for earthquakes larger than M 5.5 in Italy and surrounding areas. http://diss.rm.ingv.it/diss/, © INGV 2010 - Istituto Nazionale di Geofisica e Vulcanologia - All rights reserved. doi: 10.6092/INGV.IT-DISS3.1.1.

Fracassi, U., Milano, G., 2014. A soft linkage between major seismogenic fault systems in the central-southern Apennines (Italy): Evidence from low-magnitude seismicity. Tectonophysics (2014) in press; http://dx.doi. Tectonophysics (2014) in press; http://dx.doi.Tectonophysics (2014) in press; http://dx.doi.org/10.1016/j.tecto.2014.08.002

Improta, L., Iannaccone, G., Capuano, P., Zollo, A., Scandone, P., 2000. Inferences on the upper crustal structureInferences on the upper crustal structure of Southern Apennines (Italy) from seismic refraction investigations and subsurface data. Tectonophysics 317,Tectonophysics 317, 273– 297.

Lee, W.H.K., Lahr, J.C., 1975. HYPO71 (revised): a computer program for determining hypocenter, magnitude, and first motion pattern of local earthquakes. US Geological Survey Open File Report 75-311. US Geological Survey, Washington, DC.

Milano, G., Di Giovambattista, R., Alessio, G., 1999. Earthquakes swarms in the Southern Apennines chain (Italy):Earthquakes swarms in the Southern Apennines chain (Italy): the 1997 seismic sequence in the Sannio-Matese mountains. Tectonophysics 306, 57-78.

Milano, G., Ventura, G., Di Giovambattista, R., 2002. Seismic evidence of longitudinal extension in the SouthernSeismic evidence of longitudinal extension in the Southern Apennines chain (Italy): the 1997-1998 Sannio-Matese seismic sequence. Geophysical Research Letters 29, NOGeophysical Research Letters 29, NO 20, 65.1-65.4.

Milano, G., Di Giovambattista, R., Ventura, G., 2005. The 2001 seismic activity near Isernia (Italy): ImplicationsThe 2001 seismic activity near Isernia (Italy): Implications for the seismotectonics of the Central-Southern Apennines. Tectonophysics 401, 167-178. doi: 10.1016/Tectonophysics 401, 167-178. doi: 10.1016/j.tecto.2005.03.010.

Milano, G., Di Giovambattista, R., Ventura, G., 2008. Seismic activity in the transition zone between Southern andSeismic activity in the transition zone between Southern and Central Apennines (Italy): Evidences of longitudinal extension inside the Ortona-Roccamonfina tectonic line. Tectonophysics 457, 102-110. doi: 10.1016/j.tecto.2008.05.034.

Reasenberg, P., and D. Oppenheimer, FPFIT, FPPLOT and FPPAGE: Fortran computer programs for calculating and displaying earthquake fault plane solutions, U.S. Geol. Surv., Open File Rep., 85– 739, 1985.

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