48,06,2005misc 995 ANNALS OF GEOPHYSICS, VOL. 48, N. 6, December 2005 Key words seismic sequences – stress field evolu- tion – Southern Apennines – Calabrian Arc – Polli- no Chain 1. Introduction Earthquakes play a fundamental role in de- picting the ongoing geological processes because they make possible the location of the active structures and the characterization of the stress and strain fields. Therefore the occurrence of seismic events becomes the occasion for clarify- ing tectonic relationships in areas of increased in- terest. This is especially true when they hit areas of moderate seismicity, where even a magnitude 5.0 event can furnish significant information on the geodynamical regime and in particular where the boundaries between tectonic major domains are not well defined. For these reasons we focus on the seismic sequence that followed an earthquake located on the western slope of the Pollino Chain (Southern Italy). This seismic event occurred on 9th September 1998 and caused some dam- age in several towns and villages located in the mesoseismal area (Galli et al., 1998), attaining a maximum intensity of VII MCS. The whole sequence was confined to the up- per 15 km of the crust and involved the sedi- mentary cover of the region where two impor- tant contacts exist. At the surface there is the boundary between the Apenninic Chain and the Calabrian Arc; more in depth there is the limit between the Adriatic and the African plate. These contacts are not yet well understood ei- ther at subduction on crustal levels. 2. Geodynamic setting From a geodynamic viewpoint, Southern Italy is divided into two regions. The southern- The 1998-1999 Pollino (Southern Apennines, Italy) seismic crisis: tomography of a sequence Ignazio Guerra (1), Paolo Harabaglia (2), Anna Gervasi (1) (3) and Anna B. Rosa (2) (1) Dipartimento di Fisica, Università degli Studi della Calabria, Arcavacata di Rende (CS), Italy (2) Dipartimento di Strutture, Geotecnica e Geologia Applicata, Università della Basilicata, Potenza, Italy (3) Istituto Nazionale di Geofisica e Vulcanologia - CESIS, Grottaminarda (AV), Italy Abstract In 1998-1999 a seismic sequence occurred in the Southern Apennines, after the moderate size (mb = 5.0) 9th Sep- tember 1998 Pollino earthquake. It lasted about 14 months and was clearly localized to the sole north-west area of the main shock epicenter. Its peculiarity consisted in sudden changes of activity from a series of normal faults with Apenninic (NW-SE) trend and transfer, presumably strike slip, faults with Antiapenninic (NE-SW) and E-W trend. The complexity of the behavior and the different orientations of the activated systems suggest that the area acts as a hinge between the NW-SE trending Southern Apennines and the locally N-S trending Calabrian Arc. Mailing address: Prof. Ignazio Guerra, Dipartimento di Fisica, Università degli Studi della Calabria, Via P. Bucci, Cubo 31C, 87036 Arcavacata di Rende (CS), Italy; e-mail: guerra@unical.it 996 Ignazio Guerra, Paolo Harabaglia, Anna Gervasi and Anna B. Rosa most one, the so-called Calabrian Arc, is the area where the Ionian lithosphere still subducts beneath the Tyrrhenian Sea; the subduction is characterized by an eastward rollback (e.g., Malinverno and Ryan, 1986; Doglioni et al., 1996). North of the Calabrian Arc there are the so-called Southern Apennines that constitute the accretionary prism of the Adriatic plate sub- duction (e.g., Doglioni et al., 1996, and refer- ences therein); on the basis of geological evi- dence the hinge of this subduction has been mi- grating northeastward up to the Pliocene; at present it is thought to be quiescent. These two geodynamically separated regions meet in the Pollino Chain (fig. 1a,b). This latter, broadly shaped as a triangle due to its short E-W exten- sion, is bordered to the south by the Sibari Plane, to the north-east by the Metaponto plane and to the west by the Mercure Basin. The main structural lineaments shown in fig. 1a,b are taken from Valensise and Pantosti (2001). It is noteworthy that the tectonic style of the Southern Apennines is broadly dominated by a NE-SW oriented tensile stress field (Amato and Montone, 1997) that generates a series of nor- mal fault systems with longitudinal trend. They are usually recognized to dip northeastward and are displaced by shear fault systems with Anti- apenninic trend. The southernmost normal sys- tem that exclusively yields this feature is the Agri Valley, bordered to the south by the Poli- castro Gulf-S. Arcangelo Line. The extensional systems are characterized by large earthquakes. According to the CPTI catalogue (Gruppo di Lavoro CPTI, 1999), the main known seismic event in the Agri Valley is the 1857 earthquake that caused over 10 000 ca- Fig. 1a,b. a) Relative position of the investigated area with respect to the location of the thrust belt. b) Main structural lineaments according to Valensise and Pantosti (2001) and seismic stations in Southern Italy. Blue lines: Apenninic features; red lines: other tectonic features; black squares: permanent seismic stations; red squares: temporary seismic stations. a b The 1998-1999 Pollino (Southern Apennines, Italy) seismic crisis: tomography of a sequence sualties (fig. 2). This is the famous «Great Neapolitan Earthquake», that is considered the starting point of the modern seismology, be- cause of the in-depth inspection of its effects and of the subsequent first attempt of a quanti- tative interpretation of macroseismic observa- tions carried out by Mallet (1862) on behalf of the Royal Society of London. The Calabrian Arc in its northern portion has an almost North-South trend. It is also apparent- ly characterized by extensional activity oriented along its axis: this is testified by the graben struc- ture of the Crati Valley (fig. 1a,b) and the fault plane solutions of the two seismic events of Feb- ruary 20, 1980, the strongest ones in the last few decades (Cello et al., 1982). In the same area E- W trending lineaments exist too, that probably displace the extensional systems in a similar manner to what happens in the Southern Apen- nines, even though there is scant geological evi- dence (Finetti and Del Ben, 1986; Moretti and Guerra, 1997; Moretti, 2000; Valensise and Pan- tosti, 2001). The Cetraro-Rossano line represents the northern boundary of the area where Calabri- an Arc tectonic features are exclusively ob- served. The Calabrian Arc showed in historical times earthquakes of such an energetic level that it has to be considered among the regions with highest seismic risk in the world. The Pollino Chain and the strictly adjacent ar- eas represent a minimum of the seismic activity in terms of moment release along the ridge of the Italian Peninsula. They are characterized by the occurrence of a relatively large number of events, all however of moderate size. Maximum intensi- ties, observed in 1831, 1836, 1894 never attained severity higher than degree VIII on the MCS macroseismic scale. From the tectonic point of view, the boundaries of the area can be identified with the Policastro Gulf-S. Arcangelo Line to the north and with the Cetraro-Rossano Line to the south: in between these limits both trends, Antia- penninic and east-west, coexist generating a com- plex surface tectonic pattern. One open question 997 Fig. 2. Historical seismicity (red circles, radius increasing with macroseisimic intensity), main tectonic linea- ments (solid red lines), reported active faults on the base of geological evidence (dashed red rectangles – inter- section with the surface, solid red). Black circles show instrumental locations of the 1982 and 1988 seismic events and their relative dislocation with respect to the macroseismic epicentres. The star refers to the 1998 main event (sources: Gruppo di Lavoro CPTI, 1999; Valensise and Pantosti, 2001). 998 Ignazio Guerra, Paolo Harabaglia, Anna Gervasi and Anna B. Rosa is whether they are still active at the present time throughout the whole area. The 1998 seismic sequence followed an earthquake that occurred in the Mercure Basin, immediately to the northwest of the Pollino Chain, in an area broadly comprised between the Policastro Gulf-S. Arcangelo Line and the Verbicaro-S. Maria del Cedro Line. It is noteworthy that aside from the 1831, 1836 and 1894 events, in more recent years the area suffered damaging earthquakes only in 1982, 1988 and 1998 (Esposito et al., 1988; Gas- parini and Tertulliani, 1988; Galli et al., 1998). It is quite obvious on the basis of the huge shift be- tween the instrumental and macrosesimic epicen- tres observed in these occasions (fig. 2), that his- torical locations might be misleading (Moretti et al., 1994): they can give a hint on the level of activity in the area but not on the tectonic setting. 3. Data acquisition and treatment Around the Pollino Chain several seismic sta- tions belonging to different organizations oper- ate. Among these there are the National Central- ized Seismic Network of the Istituto Nazionale di Geofisica e Vulcanologia, Rome (INGV) and the Calabria University Seismic Network. Data use- ful for its seismographic monitoring are also ac- quired by Osservatorio Vesuviano (Naples), presently a section of INGV. The stations active in 1998 are drawn in fig. 1a,b. Soon after the main shock of 9th September, 1998 the above described network was integrat- ed by 14 temporary seismic stations that were installed all around the mesoseismal area. In particular, 6 REFTEK acquisition systems man- aged by INGV remained active up to mid Octo- ber. The other temporary stations, belonging to the University of Calabria, remained active up to 20th December 1998, but two are still in ac- tivity (LB2 and MM1). These stations were six Lennartz Mars-88/FD, one locally engineered telemetered digital seismograph, and one ana- logic Sprengnether MEQ-800 (fig. 3). Because of the elongated shape of the Ital- ian Peninsula, it is often quite difficult to obtain an optimal azimuthal coverage. Since the tem- porary network operated only for a short span Fig. 3. Event locations throughout the seismic sequence and seismographic stations. Blue squares: INGV sta- tions; black squares: University of Calabria stations. Fig. 5. Local velocity model. The 1998-1999 Pollino (Southern Apennines, Italy) seismic crisis: tomography of a sequence of time, it was necessary to verify whether the locations obtained on the basis of the perma- nent network only could be considered satisfac- tory enough for an in-depth investigation of the whole sequence. To perform the check, the envisaged proce- dure was to select a subset of events with az- imuthal gap and number of available arrival times respectively below and above given threshold values. The successive step was the comparison of their locations obtained using separately the data from the permanent and the temporary network. As a result, we verified that by using the pre- ferred velocity model, the locations of events with gaps up to 200° and a minimum set of 10 available readings, almost never differed by more than 6 km, with the overwhelming majority falling within a 3 km difference (fig. 4). This pro- cedure finally led to the selection of 881 events from September 1998 to October 1999 (fig. 3). The process of hypocentral location re- quired the treatment of phase arrivals recorded both at the temporary network installed in the aftermath of the main event, and at the regional networks – INGV and University of Calabria – that operated in the area (stations within 150 km from the main shock). This imposed a com- plex procedure to design a velocity model that could satisfy both data sets. The regional model is a 3D model – SLAB5 – developed at the Geophysical Laboratory of Calabria University (Guerra, 1998) that con- sists of a 30 × 30 km horizontal grid extending down to the depth of 550 km. It proved to yield a good match for the whole Calabrian Arc and surrounding regions (Bruno et al., 1999). This was then integrated with an ad hoc model of the epicentral area that essentially resulted in a 1D description (fig. 5), to be inserted in the 30 × 30 km grid of the regional model. The epicentral area model was initially de- rived by extrapolation of seismic refraction da- ta available in the surrounding regions (Colom- bi et al., 1973; Morelli et al., 1975; Milano et al., 1989) and further refined by an iterative procedure of rms minimization. This process resulted in a family of possible models out of which we selected our preferred solution. The criteria we applied were: – Depth distribution of the events consis- tent with the structural asset of the area, located between the nearby Tyrrhenian Sea and the ax- is of the Apennine mountain chain. – Pronounced clustering of the aftershocks. To better focus possible fault planes activat- ed during the sequence, the method proposed by Bossu (2000) was applied. It is based on the assumption that being 2σ the 95% confidence 999 Fig. 4. Histogram of the event frequency with re- spect to the epicentre displacements as obtained using separately temporary and permanent network data. 1000 Ignazio Guerra, Paolo Harabaglia, Anna Gervasi and Anna B. Rosa interval of an hypocentral location, it is possi- ble to substitute the location of a given event with that arising by the average locations of all the events that fall within the confidence inter- val, without any net gain or loss of information. As an example, hypocentral locations of the events that occurred in spring 1999 are report- ed in fig. 6a,b before and after the application Fig. 6a,b. Comparison of the surface distribution of the epicentres a) before and b) after the application of the Bossu (2001) method. Here and in the following figures, colours correspond to different depth ranges. a b The 1998-1999 Pollino (Southern Apennines, Italy) seismic crisis: tomography of a sequence of the Bossu method: in both cases hypocenters tend to align along an Antiapenninic trending direction but this trend is better evidenced in the second case. 4. The 1998 event and the sequence The main event took place on 9th Septem- ber 1998 at 11:28:00.9 UTC. Its localization is 1001 Fig. 7. Events observed on 9th September 1998. The star refers (in the following figures too) to the main event (at 11.28 UTC), larger squares to the foreshocks (with relative UTC timing), smaller squares refer to aftershocks locations. Fig. 8. Event locations in the time interval 10/09/1998-29/09/1998. 1002 Ignazio Guerra, Paolo Harabaglia, Anna Gervasi and Anna B. Rosa 39.9811°N ± 1.3 km, 16.0499°E± 1.9 km, at the depth of 14.3 ± 4.3 km. Magnitude estimates vary widely – e.g., BJI (MS = 5.6), ISC (MS = 5.2, mb = 5.3), NEIC (MS = 5.2, mb = 5.2), MOS (MS = = 5.3, mb = 5.9), ROM (md = 5.0). The mean of the values arising from the records at the sta- tions of the Calabria University Seismic Net- work is mL = 5.0. It is noteworthy that the Har- vard CMT catalogue reports a moment release of 3.26∗1024 dyne cm, corresponding to a Mw = = 5.6: this high value from teleseismic record- ings in association with the lower value from regional ones, suggests a relatively low spectral corner frequency. It is also important to observe that the focal mechanism derived by the CMT inversion yields a normal faulting event with Apenninic trend and a rather significant minor to major double couple ratio. The centroid time is set at about 5 s after the nucleation time. This is a quite long de- lay for a moderate sized earthquake and might indicate a rather complex rupture process. Fig. 9. Event locations in the time interval 30/09/1998-30/11/1998. Fig. 10. Fault plane solutions for 41 selected aftershocks in the time interval 12/09/1998-07/10/1998. FPS for events in the same day are in an unique column, at height increasing with time. 1003 The 1998-1999 Pollino (Southern Apennines, Italy) seismic crisis: tomography of a sequence The time evolution of the energy release pri- or to the main shock was also rather remark- able. In the two months preceding the main event, almost no activity was observed in a wide area surrounding the epicenter of the main shock event: the catalogue of the Seismic Net- work of the Calabria University reports in the area represented in fig. 3 only 4 shocks, all with mL ≤ 2.8. On 9th September 1998 a burst of fore- shocks occurred (fig. 7). The first and largest one was an md = 3.7 event at 6:20 UTC. Other foreshocks occurred at 7.42, 9.32, 10.53 UTC. There was another feature that differentiated the event spatial distribution throughout the first day of activity with respect to the rest of the sequence: both the foreshocks and the first few aftershocks were distributed in a rather wider area that embodied the main shock. From 10th September onward, the aftershocks oc- curred within a narrower area located to the NNW of the main event location (fig. 8). The activity that can be ascribed to the se- quence lasted up to October 1999. However af- ter June 1999 the decrement of energy release was significant. As a consequence, the Bossu method can be applied only up to the end of May 1999. On this basis, taking into account the time evolution of the hypocentral location, the se- quence can be divided into five major phases. The first lasted about 20 days (fig. 8). Dur- ing this period most of the activity was concen- trated along a narrow NNW-SSE elongated area at depths between 6 and 9 km. This first phase was therefore compatible with the focal mecha- nism of the main event. The second phase (fig. 9) went through the end of November 1998. During this period the hypocenters delineate a fault plane that dips to the south and meet the surface in correspon- dence of the Sapri-Nocara Line. The hypocen- ters appear to lay along an E-W oriented plane with a low dipping angle that makes hard a straightforward interpretation of the space dis- tribution. The relatively few stable fault plane solutions that could be obtained (fig. 10) by us- ing first arrivals polarities, show a prevalence of extensional focal mechanisms with a signifi- cant number of strike slip events: only one re- flects a locally compressive stress field. The in- homogeneous orientation of the stress axes sug- gests that the area behaved as a transfer zone. The third phase (fig. 11) lasted through the end of January 1999 and coincided with the ac- tivation of a shear plane dipping southwest- Fig. 11. Event locations in the time interval 01/12/1998-31/01/1999. 1004 Ignazio Guerra, Paolo Harabaglia, Anna Gervasi and Anna B. Rosa wards. This is in sharp contrast with the geolog- ical observation of a northeastward dipping fault plane (Valensise and Pantosti, 2001). Its peculiarity is a sort of activity gap at depths of 7-8 km. The fourth phase (fig. 12) lasted till mid March 1999. It marked a return to the same regime as the second phase, with an E-W trend- ing plane that was dipping southward. The ac- tivity level undergone an evident decrease. Fig. 12. Event locations in the time interval 01/02/1999-15/03/1999. Fig. 13. Event locations in the time interval 16/03/1999-15/06/1999. 1005 The 1998-1999 Pollino (Southern Apennines, Italy) seismic crisis: tomography of a sequence The fifth and last phase (fig. 13) was char- acterized by a noticeable increment of seismic activity. The tectonic element activated in this case shows an Antiapenninic trend, already en- visaged by Moretti et al. (1994). Its main fea- ture was the southwestward deepening of hypocenters that suggests a coexistence of ac- tivity with that of the south dipping fault ob- served during the second and fourth phase. To characterize the time evolution of the en- ergy and moment release, we show in fig. 14 the cumulative moment obtained by applying the re- lationship between magnitude and seismic mo- ment proposed by Selvaggi et al. (1997). The plot accounts for earthquakes with epicenter in the area represented in the preceding figures accord- ing to the catalogue of the Geophysical Laborato- ry of Calabria University since 1997, excluding however the main shock of 9th September 1998 in order to enhance the variation before and after the begin of the sequence. Arrow marks the start- ing date of the above described last phase. Re- markable features are the perfectly flat trend in the years preceding the sequence and the sharp jump corresponding to the fifth phase. 5. Conclusions We observe that the major tectonic features along the whole Apenninic ridge are directed mostly in directions parallel and perpendicular to the axis of the chain and are referred to as Apenninic and Antiapenninic elements respec- tively. Analogously, south of the Pollino Chain, the most evident tectonic features are directed Fig. 14. Cumulative moment release (dyne·cm∗1020) in the investigated area as resulting by considering all the events with the exclusion of the main shock. md values have been converted to moment applying the relation by Selvaggi et al. (1997). Arrow marks the starting date of the last phase of the seismic sequence. Fig. 15. Scheme of the proposed limit of the hinge between the Southern Apennines and the Calabrian Arc at crustal levels. 1006 Ignazio Guerra, Paolo Harabaglia, Anna Gervasi and Anna B. Rosa NS and EW: since in its northern sector the Cal- abrian Arc is directed NS, they result respec- tively parallel and perpendicular to the strike of the arc. Therefore we can define them calabrian and anti-calabrian. The seismic sequence started on 9th Sep- tember 1998 on the western slope of the Polli- no Chain shows the alternate activation of tec- tonic elements characterized by different strike and dip, that seems to well agree with the loca- tion at the contact between different major geo- logical domains, in such way that the character- istics of both them are evidenced: Apennic, An- tiapenninic and Anticalabrian, the latter being represented by the Sapri-Nocara Line. The atypical character of the studied area is demonstrated by the southwestward dipping trend of the shear plane activated in the third phase of the seismic sequence, contrasting with that of the major Apenninic normal structure that typically deepens northeastward. The peculiar features of the area, classifi- able in the typology of transfer zones due to their heterogeneity, are also confirmed by the wide variety of focal mechanisms observed on each lineament. The major features include the E-W trend- ing fault that was activated throughout the sec- ond and the fourth phase of the sequence and the Antiapenninic fault that was activated throughout the last phase. From the moment re- lease point of view, they seem to be the major transfer structures at the hinge between the Southern Apennines and the Calabrian Arc. This is particularly true with regard to the Anti- apenninic fault, whose activation at the end of the sequence, was characterized by the most in- tense moment release. Its existence had already been suggested by Moretti et al. (1994). On the basis of a few dense microearthquake se- quences both at sea and on land, these authors identified an Antiappeninic trending lineament on the prolonging of that activated in the last phase of the 1998-1999 sequence (fig. 15). At the time, they hypothesized the existence of a major feature that would extend for over 100 km from the submerged Palinuro volcano up to the Upper Sinni Valley (S. Arcangelo Basin). The observations described in the present work seem to confirm this model, so that the lineament can be more correctly called Palin- uro-S. Arcangelo Lineament. It is intersected in proximity of Palinuro volcano by another major lineament, the Palinuro-Cetraro Line (Finetti and Del Ben, 1986; Moretti et al., 1990) and its land expression, the Cetraro-Rossano Line. 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