Re-Evaluation of the Early Ms=5.8 1984 Abruzzo-Lazio Instrumental Earthquake (Central-Southern Apennines) G

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Concluding remarks. We have noted here the importance of active tectonic studies in the field of seismic hazard, and how they necessarily need detailed and unambiguous field surveys, ranging from Quaternary geology to paleoseismology. In particular, we believe that a great caveat has to be adopted for the evaluation of fault activity only via morphological analyses, and hence without inte- grated stratigraphic, chronometric, morphological and structural approaches. According to our data, the San Pio fault really controlled the opening of a basin (Paganica-San Demetrio-Barisciano basin) even larger that its current intermountain depression (San Pio basin), but its activity died out just before the end of the Lower Pleistocene sedimentation. Successively, the tectonic activity migrated progressively south-westward, as witnessed finally by the current activity of the Paganica-San Demetrio fault system, responsible for the Mw 6.3, 2009 eatrhquake (Galli et al., 2010). References Bosi C., Bertini T.; 1970: Geologia della media valle dell’Aterno. Memorie Società Geologica Italiana, 9, 719-777. Bertini T., Bosi C.; 1993: La tettonica quaternaria della conca di Fossa (L’Aquila). Il Quaternario, 6, 293-314. D’Agostino N., Speranza F., Funiciello R.; 1997: Le Brecce Mortadella dell’Appennino Centrale: primi risultati di stratigrafia magnetica. Il Quaternario, 10, 385-388. Di Bucci D., Vannoli P., Burrato P. Fracassi U., Valensise G.; 2011: Insights from the Mw 6.3, 2009 L’Aquila earthquake (central Apennines) to unveil new seismogenic sources through their surface signature: the adjacent San Pio Fault. Terra Nova, 23, 108–115. Galli P., Giaccio B., Messina P.; 2010: The 2009 central Italy earthquake seen through 0.5 myr-long tectonic history of the L’aquila faults system, Journal of Quaternary Science Reviews, 29, 3768-3789, doi:10.1016/j.quascirev.2010.08.018. Giaccio B., Galli P., Messina P., Scardia G., Falcucci E., Galadini F., Gori S., Peronace E., Sposato A., & Zuppi G.M.; 2011: Quaternary tectonic and sedimentary evolution of the L’Aquila 2009 mesoseismic region, central Apennine: stratigraphic, paleomagnetic and 40Ar/39Ar constraints, Riassunti di Geoitalia 2011, VIII Forum Italiano di scienze della Terra, Epitome 4, 165. Messina P., Galli P., Giaccio B., Falcucci E., Galadini F., Gori S., Peronace E., Scardia G., Sposato A.; 2010: Evoluzione geologica e tettonica quaternaria dell’area epicentrale del terremoto aquilano del 2009 (bacino di Paganica–Barisciano, Appennino centrale), Riassunti del 29° Convegno Nazionale GNGTS, Prato 26-28 Ottobre 2010, 73-75. Messina P., Galli, P., Giaccio, B.; 2011: Comment on ‘Insights from the Mw 6.3, 2009 L’Aquila earthquake (Central Apennines) – unveiling new seismogenic sources through their surface signatures: the adjacent San Pio Fault’ by Di Bucci et al. (2011), Terranova, 23, 280-282.

RE-EVALUATION OF THE EARLY MS=5.8 1984 ABRUZZO-LAZIO INSTRUMENTAL EARTHQUAKE (CENTRAL-SOUTHERN APENNINES) G. Milano1, R. Di Giovambattista1,2 1 Osservatorio Vesuviano – Istituto Nazionale di Geofisica e Vulcanologia, Napoli, Italy 2 Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy The 1984 Abruzzo-Lazio earthquake (Ms=5.8; NEIS) occurred at the border between Central and Southern Apennines in a restrict area crossed by fault segments with different orientation. The sequence is characterized by two main shocks having low spatial and temporal separation: the first (Ms=5.8; NEIS) occurred on May 7 and the second (Ms=5.2) on May 11, 1984. Although the CMT focal mechanism of the two main shocks point to a NNW-SSE seismogenic structure, according to the faults direction along which the major earthquake of the Apennine Chain occurred, the epicentral distribution of the aftershocks is characterized by a complex geometry arising from the pres- ence of a NE-SW oriented cluster. Several geophysical and geological studies have been performed on this sequence in order to explain the divergence between the nodal planes of the two main shocks and the spatial distribution of the aftershocks (e.g. Pace et al., 2002; Westaway et al., 1989), but, still, some questions are unresolved. Major open questions concern: 1) whether the May 7 and the May 11mainshocks nucleated on the same fault segment or on different seismogenic structures; 2) the divergence between the CMT focal mechanisms and the spatial distribution of the aftershocks; 3) the relation, if any, between this sequence and the Aremogna-Cinque Miglia source, being this one considered capable of substantially large events even if it has not been active historically. We re-evaluate the 1984 seismic sequence integrating most of the original data collected by different Institutions both in analogue and in digital format and discuss the results in light of the wave-

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Fig. 1 - Epicentral and hypocentral distributions on N-S and W-E oriented cross-sections of the events occurred in the study area between January and December 1984. Triangles represent the seismic stations close to the epicentral area. MS1 and MS2 indicate the two main shocks. Faults are from Pace et al. (2002). form analysis and previous interpretations from other authors. We utilized all available seismic data from ING, OV, ENEAcatalogues, this last containing also data from temporary mobile seismic stations, to built a data-set as fully as possible. We also recovered same original seismograms of the seismic stations belonging to the permanent networks that allowed to integrate the new data set reviewing, when possible, P- and S-phases previously unpicked or mis-picked. The dataset allowed us to improve several poor locations due to timing, mis-interpretation and mis-association of phases and to compute fault plane solutions for some aftershocks of the sequence, never obtained before. To ensure reliable hypocentral locations, we selected the seismic events with at least 5 P and 2 S phase arrival times. Seismicity has been relocated by means of the standard HYPO71 algorithm (Lee and Lahr, 1975) utilizing the 1D velocity model reported in Milano et al. (2008), already utilized to locate seismic events in the study area. Focal mechanisms for some events of the sequence with MD > 3.5 occurred during the first ten days and with at least 10 P-wave polarities have been computed by means of the standard FPFIT grid-search algorithm (Reasenberg and Opphenheimer, 1985). The results of: i) qualitative analysis on the temporal evolution of the 1984 sequence, corrobo- rated by waveform analysis (Console and Di Giovambattista, 1987), ii) epicentral distribution of the

70 GNGTS 2011 SESSIONE 1.1 events (Fig. 1) and iii) focal mechanism suggest that the 1984 sequence developed in several stages with a different strain pattern. The first stage occurred between the May 7 and May 11, following the Ms=5.8 event, and it was characterized by the occurrence of few events with magnitude gener- ally less than 3.0. Inside the epicentral distribution, a roughly concentration of events along the ENE-WSW direction is observable and the focal mechanisms show normal dip-slip solution with NE-SW striking planes on average whose kinematics is consistent with a NW-SE extension. The second stage started with the Ms=5.2 event and was characterized by a strong productivity of aftershocks, some of them with 3.5≤MD≤4.5. The epicentral distribution shows that the seismic events cluster roughly along a W-E and NE-SW directions and the fault plane solutions show a prevalence of normal and normal/oblique solutions with striking planes in three main directions on average: W- E, NW-SE and NE-SW. On the basis of the results in each stage a different interpretation for the 1984 sequence may be proposed. The sequence originated in a narrow area, adjacent to the major structures belonging to the Apenninic Chain, crossed by fault segments with different orientation. We assume that the sequence involved small scale fault segments with different orientation and that the rupture of one of them can induce the activation of adjacent segments. The focal mechanisms of the first stage and their epicenters suggest that the rupture nucleated on a NE-SW oriented normal fault and spread along strike northeastward. The propagation of the aftershocks is interrupted in correspondence of the NNW-SSE M. Barrea and W-E Mt. Greco faults. These faults and their geometry would have represented a structural complexity sufficient to halt the propagation of the ruptures. The static stress change due to the May 7 mainshock and the halt of the cracks would have generated a high

Fig. 2 - Changes in Coulomb failure stress (dCFS, MPa) due to faulting in the 1984 sequence at a depth of 10 km. The panel shows the dCFS using data from our revaluated focal mechanism of the first mainshock. White circles show epicentres.

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Coulomb stress increase in the area enclosed between the Barrea and Mt. Greco faults. Consequent- ly, the change of static stress on a pre-existing fault segment could have triggered the beginning of the second stage with the Ms 5.2 mainshock. The location of the second mainshock, few km towards NE with respect to the location of the first mainshock, its focal mechanism and those related to its aftershocks, have consistent position and orientation with the minor E-W fault segments recognized in the area. These observations suggest that the second stage originated on a fault segment belonging to the minor E-W fault system. In turn, the 11 May mainshock could have inducted the activation of adjacent cross fault segments NE-SW and NW-SE striking, according to the spatial distribution of the events and the focal mechanisms of the second stage. To corroborate the above interpretation, we evaluated the static stress change (Fig. 2) due to the May 7 mainshock by means of GNStress2 algorithm (Robinson, 2002). We used data from our revaluated focal mechanism of the first mainshock considering a 10 km-long fault segment and 7 km wide according also to the 18 MO=0.59 x 10 N m valuated by Westaway et al. (1989). References Console, R., Di Giovambattista R., 1987. Local earthquake relative location by digital records. Phys. Earth Planet. Inter., 47 (1987) 43-49. 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., Ventura, G., 2008: Seismic 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. Pace, B., Boncio, P., Lavecchia, G., 2002. The 1984 Abruzzo earthquake: an example of seismogenic process controlled by interaction between differently oriented synkinematic faults. Tectonophysics 350, 237-254. Reasenberg, P., Oppenheimer, D., 1985. FPFIT, FPPLOT and FPPAGE: Fortran computer programs for calculating and displaying earthquake fault plane solutions. U.S. Geol. Surv. Open File Rep., 85-739. Robinson, R., 2002. GnStress_2: A Computer Programme for Inverting First Motion Observations to a Regional Stress Tensor. Institute of Geological & Nuclear Sciences Box 30368, Lower Hutt, New Zealand. Westaway, R., Gawthorpe R., Tozzi, M., 1989. Seismological and field observations of the 1984 Lazio-Abruzzo earthquakes: implications for the active tectonics of Italy. Geophys. J. Int., 98, 489-514.

SHALLOW GEOPHYSICAL IMAGING OF THE MT. MARZANO FAULT ZONE; A KALEIDOSCOPIC VIEW THROUGH ERT, GPR AND HVSR ANALYSES E. Peronace1, P. Galli1,2, A. Giocoli3, S. Piscitelli3, B. Quadrio1,2, J. Bellanova1 1 CNR-IGAG, Monterotondo, Italy 2 Dipartimento Protezione Civile Nazionale, Roma, Italy 3 CNR-IMAA, Tito Scalo, Italy Introduction. The Mount Marzano fault system (Fig. 1) is one of the few seismogenetic structures that has been individuated and studied in the most seismically hazardous area of Italy, i.e. southern Apennines. Indeed, apart from the deep structural complexity of the inherited fold-and- thrust belt chain, the general difficulty in identifying active faults in southern Apennines is mainly due to the high erodibility of the siliciclastic units that form the outcropping structure of the seismogenic belt. Under climatic conditions of the late Quaternary, rates of erosion in these rocks pre- cluded the preservation of tectonic surface landforms (e.g., fault scarps) generated by short-term, low-rate (<1mm/yr) slip rates. Thus, it is not by chance that the only paleoseismic investigations eastward of the Ortona-Roccamonfina line Auctorum have focused on faults affecting erosion- resistant carbonate rocks, as with the N-Matese faults (Galli and Galadini, 2003), the Aquae Iuliae fault (Galli and Naso, 2009), the Caggiano fault (Galli et al., 2006), and the Mount Marzano fault system (Pantosti et al., 1993). As a matter of fact, the present activity of the Mt. Marzano fault system has been definitely witnessed by the surface faulting related to the 1980, Mw 6.9 earthquake (Westaway and Jackson, 1984), which occurred along more than 30 km of length, with offsets exceeding also 1 m. However, as the fault trace runs mainly through steep forested slopes (usually 35° dipping), traditional paleoseismic trenching aimed at the characterization of the seismogenetic behaviour of this structure has been focused to two flat areas (Piano di Pecore and Pantano di San