The 2001 seismic activity near Isernia (Italy): Implications for the seismotectonics of the Central–Southern Apennines

Abstract
We analyze the seismic activity that occurred in 2001 near the city of Isernia (Southern Apennines) with the aim to add
information on the seismotectonics of the transition zone between Central and Southern Apennines. Seismic activity lasted for 9
months showing the typical behaviour of swarms: a large number of events of low magnitude without a distinct main shock and
clustered in space and time. The maximum magnitude MD of the events was 3.3, comparable with the magnitude MD of the
background seismicity of the area. Hypocentral distribution of the events (maximum depth=15 km) suggests that seismic
activity took place along a best fitting plane striking N338E and steeply dipping eastward. The results of the strain analysis
performed on selected, well-constrained focal mechanisms indicate that most of the ruptures associated to the 2001 seismic
sequence occurred along N–S to NE–SW-striking faults characterized by strike-slip to normal movements. This kinematics is
consistent with a prevailing NW–SE extension. This direction of extension, already observed in the 1997–98 seismic sequence,
which occurred at about 35 km toward SE from the 2001 epicentral area, is not consistent with that one assumed at regional
scale on the basis of the analysis of focal mechanisms of stronger earthquakes. The results of the 1997–98 and 2001 seismic
sequences, discussed in light of the available geological and geophysical data, evidence that: (i) in this portion of the Apennine
Chain low magnitude seismic sequences cluster along NE–SW striking faults that move in response to a NW–SE extension and
(ii) the NE–SW striking faults along which the 1997–98 and the 2001 seismic sequences occurred mark the Southeastern and
Northwestern tips of the NW–SE fault segment responsible for the 1805 earthquake.

References
Alessio, G., Godano, C., Gorini, A., 1990. A low magnitude seismic
sequence near Isernia (Molise, Central Italy) in January 1986.
Pageoph 134 (2), 243– 260.
Alessio, G., Gorini, A., Vilardo, G., Iannaccone, G., 1996. Low
energy sequences in areas with high seismic potential: Benevento
(Southern Apennines), April 1990. In: Schenk, V. (Ed.),
Advances in Natural and Technological Hazard Research—
Earthquake Hazard and Risk. Kluwer Academic Publishers,
Dordrecht, pp. 3– 16.
Bisio, L., Di Giovambattista, R., Milano, G., Chiarabba, C., 2004.
Three-dimensional earthquake locations and upper crustal
structure of the Sannio–Matese region (Southern Italy). Tectonophysics
385, 121– 136.
Boschi, E., Guidoboni, E., Ferrari, G., Valensise, G., Gasperini, P.,
1997. Catalogue of Strong Italian Earthquakes, 461 B.C. to
1990, ING-SGA, Bologna. 644 pp. and CD-ROM.
Brozzetti, F., Lavecchia, G., 1994. Seismicity and related extensional
stress field: the case of the Norcia seismic zone (central
Italy). Annales Tectonicae VIII, 36– 57.
Calabro` , R.A., Corrado, S., Di Bucci, D., Robustini, P., Tornaghi,
M., 2003. Thin-skinned vs. thick-skinned tectonics in the
Matese Massif, Central–Southern Apennines (Italy). Tectonophysics
377, 269– 297.
Carrozzo, M.T., Luzio, D., Margiotta, C., Quarta, T., 1992. Gravity
map of Italy. Scale 1:500,000. CNR, P.F.G., Quaderni de bLa
ricerca ScientificaQ, 114, 3.
Chiarabba, C., Amato, A., 1997. Upper crustal structure of the
Benevento area (Southern Italy): fault heterogeneities and
potential for large earthquakes. Geophys. J. Int. 130, 229– 239.
Chiarabba, C., Frepoli, A., 1997. Minimum 1D velocity models in
Central and Southern Italy: a contribution to better constrain
hypocentral determinations. Ann. Geofis. XL (4), 937– 954.
CNR-PFG (Consiglio Nazionale delle Ricerche-Progetto Finalizzato
Geodinamica), 1983. Synthetic structural-kinematic map of
Italy. In: Structural model of Italy, Scale 1:500.000, Sheet 5.
Di Bucci, D., Corrado, S., Naso, G., 2002. Active faults at the
boundary between Central and Southern Apennines (Isernia,
Italy). Tectonophysics 359, 47– 63.
Doglioni, C., 1996. Geological remarks on the relationships
between extension and convergent geodynamic settings. Tectonophysics
252, 253– 268.
Ferranti, L., Oldow, J.S., Sacchi, M., 1996. Pre-Quaternary orogenparallel
extension in the Southern Apennine belt, Italy.
Tectonophysics 260, 325–347.
Frepoli, A., Amato, A., 2000. Spatial variation in stresses in
peninsular Italy and Sicily from background seismicity. Tectonophysics
317, 109– 124.
Frohlich, C., 1992. Triangle diagrams: ternary graphs to display
similarity and diversity of earthquake focal mechanisms. Phys.
Earth Planet. Inter. 75, 193– 198.
Galadini, F., Meletti, C., Rebez, A. (Eds.), Le ricerche del GNDT
nel campo della pericolosita sismica (1996–1999). C.N.R.,
Rome, pp. 107– 136.
Gasparini, C., Iannaccone, G., Scarpa, R., 1980. On the focal
mechanisms of Italian earthquakes. Rock Mech. 9, 85–91.
Gasparini, C., Iannaccone, G., Scarpa, R., 1985. Fault plane
solutions and seismicity of the Italian peninsula. Tectonophysics
117, 59– 78.
Hippolyte, J.C., Angelier, J., Roure, F., 1994. A major geodynamic
change revealed by Quaternary stress patterns in the Southern
Apennines (Italy). Tectonophysics 230, 199–210.
Iannaccone, G., Improta, L., Capuano, P., Zollo, A., Biella, G., De
Franco, R., Deschamps, A., Cocco, M., Mirabile, L., Romeo, R.,
1998. A P-wave velocity model of upper crust of Sannio region
(Southern Apennines). Ann. Geofis. XLI (4), 567– 582.
Lee, W.H.K., Lahr, J.C., 1975. HYPO71 (revised) 1975. A
computer program for determining hypocentre, magnitude and
first motion pattern of local earthquakes. U.S. Geol. Surv., Open
File Rep. 75 – 311. 114 pp.
Marrett, R., Allmendinger, R.W., 1990. Kinematic analysis of faultslip
data. J. Struct. Geol. 12, 973– 986.
Marrett, R.A., Allmendinger, R.W., 1991. Estimates of strain due to
brittle faulting: sampling of fault populations. J. Struct. Geol.
13, 735–738.
Michetti, A.M., Blumetti, A.M., Esposito, E., Ferrelli, L.,
Guerrieri, L., Porfido, S., Serva, L., Vittori, E., 2000. Earthquakes
ground effects and seismic hazard assessment in Italy:
examples from the Matese and Irpinia areas, Southern
Apennines. Active fault research for the new millenium.
Proceedings of the Hokudan Symposium and School on Active
Faulting, pp. 279– 284.
Milano, G., Di Giovambattista, R., Alessio, G., 1999. Earthquakes
swarms in the Southern Apennines chain (Italy): the 1997
seismic sequence in the Sannio–Matese mountains. Tectonophysics
306, 57– 58.
Milano, G., Ventura, G., Di Giovambattista, R., 2002. Seismic
evidence of longitudinal extension in the Southern Appennines
chain (Italy): the 1997–1998 Sannio–Matese seismic sequence.
Geophys Res. Lett., 29, 20, 2004, 65–1 65–4.
Oldow, J.S., D’Argenio, B., Ferranti, L., Pappone, G., Marsella, E.,
Sacchi, M., 1993. Large-scale longitudinal extension in the
southern Apennines contractional belt, Italy. Geology 21,
1123–1126.
Patacca, E., Scandone, P., 1989. Post-Tortonian mountain building
in the Apennines. The role of the passive sinking of a relic
lithospheric slab. In: Boriani, A., Bonafede, M., Piccardo, G.B.,
Vai, G.B. (Eds.), The Lithosphere in Italy, Acc. Naz. Lincei vol.
80, pp. 157– 176.
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.
Scandone, P., Patacca, E., Meletti, C., Bellatalla, M., Perilli, N.,
Santini, U., 1990. Struttura geologica, evoluzione cinematica e
schema sismotettonico della Penisola italiana. Atti Conv.
Gruppo Naz. Dif. Terremoti 1, 119– 135.
Selvaggi, G., Castello, B., Azzara, R., 1997. Spatial distribution of
scalar seismic moment release in Italy (1983–1996): seismotectonic
implication for the Apennines. Ann. Geofis. XL (6),
1565– 1578.
Serpelloni, E., Anzidei, M., Baldi, P., Casula, G., Galvani, A., Pesci,
A., Riguzzi, F., 2001. Geodetic deformations in the Central–
Southern Apennines (Italy) from repeated GPS surveys. Ann.
Geofis. 44 (3), 627– 647.
Valensise, G., Pantosti, D., 2001. Seismogenic faulting, moment
release patterns and seismic hazard along the central and
southern Apennines and Calabrian Arc. In: Vai, G.B., Martini,
I.P. (Eds.), Anatomy of an Orogen: The Apennines and Adjacent
Mediterranean Basins. Kluwer Academic Publishing, Dordrecht,
pp. 495– 512.
Valensise, G., Pantosti, D. (Eds.), Database of Potential Sources for
Earthquakes Larger than M 5.5 in Italy, Ann. Geofis. Suppl. to
Vol. 44 (4), 180pp. with CD-ROM.