Stress Directions and Shear-Wave Anisotropy: Observations from Local

The spatial distribution of 414 earthquakes (1.0 ≤ ML ≤ 4.6), recorded from 1994 to 2002 in Southeastern Sicily (Italy), has been analyzed; it generally coincides with mapped Plio-Quaternary faults, including the NNW-SSE offshore fault system which is the most important tectonic structure of the area. For the best located events, we computed 70 focal mechanisms by combining P-wave polarities with S-wave polarizations. A predominance of strike-slip and normal faults was observed. Focal mechanisms were then inverted for stress tensor parameters using the algorithm of Gephart and Forsyth. The results highlighted a region governed mainly by a compressional stress regime. Moreover, anisotropy analysis of shear-waves showed a polarization of fast S-waves prevalently aligned in the NNW-SSE to NW-SE direction over the whole area.
A finer analysis of stress tensor evidenced three regions characterized by slightly differing orientation of the greatest principal stress axis, 1. The eastern sector displays a nearly horizontal 1 trending NW-SE; the central sector is affected by a low dip NNW-SSE 1; whereas in the western sector a 1 NNW-SSE oriented, with a higher dip angle, was detected. Finally, the comparison of the spatial distribution of seismicity occurring during 1994-2002, with locations of previous instrumental earthquakes and larger (M ≥ 5.0) historical events showed that the seismic patterns are persistent.

Adam, J., M. Grasso, C.M. Reuther, and L. Torelli (2000). Neotectonic stress analysis and active fault kinematics along the Ionian margin of the Hyblean Plateau (Augusta peninsula and adjacent areas, SE Sicily), Tectonophysics 326, 217-239.
Aki, K., and P.G. Richards (1980). Quantitative Seismology, Theory and Methods, W. H. Freeman, San Francisco, 932 pp.
Amato, A., R. Azzara, A. Basili, C. Chiarabba, M. Cocco, M. Di Bona, and G. Selvaggi (1995). Main shock and aftershocks of the December 13, 1990, Eastern Sicily earthquake, Ann. Geofis. 38, 255-266.
Ambrosetti, P., C. Bartolini, C. Bosi, F. Carraro, N. Ciaranfi, F. Ghisetti, G. Papani, L. Vezzani, A. Zanferrari, and N. Zitellini (1987). Neotectonic Map of Italy (scale 1:1.500.000). CNR-P.F.Geodinamica, LAC – Florence.
Azzaro, R., and M.S. Barbano (2000). Analysis of the seismicity of Southeastern Sicily: a proposed tectonic interpretation, Ann. Geofis. 43, 171-188.
Ben-Avraham, Z., and M. Grasso (1990). Collisional zone segmentation in Sicily, Ann. Tectonicae 4, 131-139.
Bernard, P., and A. Zollo (1989). Inversion of near-source S polarization for parameters of double-couple point like sources, Bull. Seismol. Soc. Am. 79, 1779-1809.
Bianchi, F., S. Carbone, M. Grasso, G. Invernizzi, F. Lentini, G. Longaretti, S. Merlini, and F. Moscardini (1987). Sicilia orientale: profilo geologico Nebrodi-Iblei, Mem. Soc. Geol. It. 38, 429-458 (in italian).
Bonforte, A., M. Anzidei, G. Puglisi, M. Mattia, O. Campisi, G. Casula, A. Galvani, A. Pesci, G. Puglisi, S. Gresta, and P. Baldi (2002). GPS surveys in the foreland-foredeep tectonic system of southeastern Sicily: first results, Ann. Geophys. 45, 673-682.
Booth, D.C., and S. Crampin (1985). Shear wave polarizations on a curved wavefront at an isotropic free surface, Geophys. J. R. Astron. Soc. 83, 31-45.
Boschi, E., E. Guidoboni, G. Ferrari, D. Mariotti, and G. Valensise (2000). Catalogue of strong italian earthquakes, Ann. Geofis. 43, 268 pp.
Brillinger, D.R., A. Udias, and B.A. Bolt (1980). A probability model for regional focal mechanism solutions, Bull. Seismol. Soc. Am. 70, 149-170.
Cassinis, R., I. Finetti, P. Giese, C. Morelli, L. Steinmetz, and O. Vecchia (1969). Deep seismic refraction research on Sicily, Boll. Geof. Teor. Appl. 11, 140-160.
Cernobori, L., A. Hirn, R. Nicolich, J. McBride, L. Petronio, M. Romanelli, and the Streamers/Profiles group (1996). Crustal image of the Ionian Basin and its Calabrian margins, Tectonophysics 264, 175-189.
Cocina, O., G. Neri, E. Privitera, and S. Spampinato (1997). Stress tensor computations in the Mount Etna area (southern Italy) and tectonic implications, J. Geodynamics 23, 2, 109-127.
Colombi, B., I. Guerra, G. Luongo, and S. Scarascia (1979). Profilo sismico a rifrazione Acireale-Termini Imerese, in Contributi preliminari alla sorveglianza e rischio vulcanico (Etna-Eolie), CNR-P.F.Geodinamica, publ. nr. 235, 155-170 (in italian).
Continisio, R., F. Ferrucci, G. Gaudiosi, D. Lo Bascio, and G. Ventura (1997). Malta escarpment and Mt. Etna: early stages of an asymmetric rifting process? Evidences from geophysical and geological data, Acta Vulcanol. 9, 45-53.
Crampin, S., R. Evans, and B. K. Atkinson (1984). Earthquake prediction: a new physical basis, Geophys. J. R. Astron. Soc. 76, 147-156.
Crampin, S., and S. Chastin (2003). A review of shear wave splitting in the crack-critical crust, Geophys. J. Int. 155, 221-240.
Currie, C.A., J.F. Cassidy, and R.D. Hyndman (2001). A regional study of shear wave splitting above the Cascadia subduction zone: Margin-parallel crustal stress, Geophys. Res. Lett. 28, 659-662.
Davis, J.C. (1986). Statistics and Data Analysis in Geology, John Wiley & Sons, New York, second edition, 646pp.
De Mets, C., R.G. Gordon, D.F Argus, and S. Stein (1990). Current plate motions, Geophys. J. Int. 101, 425-478.
De Natale, G., A. Ferraro, and J. Virieux (1991). A probability method for earthquake focal mechanisms, Geophys. Res. Lett. 18, 613-616.
Evans, R. (1984). Effects of the free surface on shear wavetrains, Geophys. J. R. Astron. Soc. 76, 165-172.
Frepoli, A., and A. Amato (2000). Spatial variation in stresses in peninsular Italy and Sicily from background seismicity, Tectonophysics 317, 109-124.
Galadini, F., C. Meletti, and A. Rebez (2000). Le ricerche del GNDT nel campo della pericolosità sismica (1996-1999), CNR-Gruppo Nazionale per la Difesa dai Terremoti - Roma, 397 pp. (in italian).
Gephart, J.W. (1990). Stress and the direction of slip on fault planes, Tectonics 9, 845-858.
Gephart, J.W., and D.W. Forsyth (1984). An improved method for determining the regional stress tensor using earthquake focal mechanism data: application to the San Fernando earthquake sequence, J. Geophys. Res. 89, 9305-9320.
Ghisetti, F., and L. Vezzani (1980). The structural features of the Iblean plateau and of the Monte Iudica area (South Eastern Sicily). A microtectonic contribution to the deformational history of the Calabrian Arc, Boll. Soc. Geol. It. 99, 57-102.
Gledhill, K.R. (1991). Evidence for shallow and pervasive seismic anisotropy in the Wellington region New Zealand, J. Geophys. Res. 96, 21503- 21516.
Grasso, M., C.D. Reuther, H. Baumann, and A. Becker (1986). Shallow crustal stress and neotectonic framework of the Malta Platform and the Southeastern Pantelleria Rift (Central Mediterranean), Geol. Romana 25, 191-212.
Grasso, M., and C.D. Reuther (1988). The western margin of Hyblean plateau: a neotectonic transform system on the SE Sicilian foreland, Ann. Tectonicae 2, 107-120.
Hirn, A., R. Nicolich, J. Gallart, M. Laigle, L. Cernobori, and ETNASEIS Scientific Group (1997). Roots of Etna volcano in faults of great earthquakes, Earth Planet. Sci. Lett. 148, 171-191.
Hollenstein, Ch., H.G. Kahle, A. Geiger, S. Jenny, S. Goes, and D. Giardini (2003). New GPS constraints on the Africa-Eurasia plate boundary zone in southern Italy, Geophys. Res. Lett. 30, 1935, doi:10.1029/2003GL017554.
Iannaccone, G., and A. Deschamps (1989). Evidence of shear wave anisotropy in the upper crust of central Italy, Bull. Seismol. Soc. Am. 76, 1905-1912.
Jurkevics, A. (1988). Polarization analysis of three-component array data, Bull. Seismol. Soc. Am. 78, 1725-1743.
Karnassopoulou, A., R.G. Pearce, and D.C. Booth (1996). A method to determine microearthquake focal mechanisms in the presence of seismic anisotropy, Tectonophysics 261, 115-126
Lahr, J.C. (1999). HYPOELLIPSE: A computer program for determining local earthquake hypocentral parameters, magnitude, and first motion pattern, U.S. Geol. Surv., Open-File Rep. 99/23, 112 pp.
Michael, A.J. (1987). Use of focal mechanisms to determine stress: A control study, J. Geophys. Res. 92, 357-368.
Montalbetti, J.F., and E.R. Kanasevich (1970). Enhancement of telesismic body phase with a polarization filter, Geophys. J. R. Astron. Soc. 21, 119-129.
Montone, P., A. Amato, A. Frepoli, M.T. Mariucci, and M. Cesarò (1999). Crustal stress regime in Italy, Ann. Geofis. 40, 741-757.
Musumeci, C., G. Di Grazia, and S. Gresta (2003). Minimum 1D velocity model in Southeastern Sicily (Italy) from local earthquake data, J. Seismol. 7, 469-478.
Neri, G., G. Barberi, B. Orecchio, and A. Mostaccio (2003). Seismic strain and seismogenic stress regime in the crust of the southern Thyrrenian region, Earth Planet. Sci. Lett. 213, 97-112.
Nicolich., R., M. Laigle, A. Hirn, L. Cernobori, and J. Gallart (2000). Crustal structure of the Ionian margin of Sicily: Etna volcano in the frame of regional evolution, Tectonophysics 329, 121-140.
Parker, R.L., and M.K. McNutt (1980). Statistics for the one norm misfit measure, J. Geophys. Res. 85, 4429-4430.
Patanè, D., and F. Ferrari (1998). A Visual-Basic computer program for interactive and automatic earthquakes waveform analysis, Comp. & Geosci. 23, 1005-1012.
Patanè, D., and E. Privitera (2001). Seismicity related to 1989 and 1991-93 Mt. Etna (Italy) eruptions: kinematic constraints by fault solution analysis, J. Volcanol. Geotherm. Res. 109, 77-98.
Ragg, S., M. Grasso, and B. Müller (1999). Patterns of tectonic stress in Sicily from borehole breakout observations and finite element modeling, Tectonics 18, 669-685.
Rivera, L., and A. Cisternas (1990). Stress tensor and fault plane solutions for a population of earthquakes, Bull. Seismol. Soc. Am. 80, 600-614.
Scarfì, L., H. Langer, and S. Gresta (2003). High-precision relative locations of two microearthquake clusters in southeastern Sicily, Italy, Bull. Seismol. Soc. Am. 93, 1479-1497.
Sharp, A.D.L., P.M. Davis, F. Gray, and G.C.P. King (1980). Preliminary results from a seismic array study of Mount Etna, United Kingdom Research on Mount Etna, 1977-1979. The Royal Society, London, 15-18.
Slejko, D., G. Neri, I. Orozova, G. Renner, and M. Wyss (1999). Stress field in Friuli (NE Italy) from fault plane solutions of activity following the 1976 main shock, Bull. Seismol. Soc. Am. 89, 1037-1052.
Waldhauser, F., and W.L. Ellsworth (2000). A double-difference earthquake location algorithm: method and application to North Hayward Fault, California, Bull. Seismol. Soc. Am. 90, 1353-1368.
Waldhauser, F. (2001). HypoDD: A computer program to compute double-difference earthquake locations, U.S. Geol. Surv., Open File Rep. 01/113, 25 pp.
Wyss, M., B. Liang, W.R. Tanigawa, and W. Xiaoping (1992). Comparison of orientations of stress and strain tensor based on fault plane solutions in Kaoiki, Hawaii, J. Geophys. Res. 97, 4769-4790.
Wyss, M., and Z. Lu (1995). Plate boundary segmentation by stress directions: southern San Andreas fault, California, Geophys. Res. Lett. 22, 547-550.
Yellin-Dror, A., M. Grasso, Z. Ben-Avraham, and G. Tibor (1997). The subsidence history of the northern Hyblean Plateau margin, Eastern Sicily, Tectonophysics 282, 277-289.
Zatsepin, S.V., and S. Crampin (1997). Modelling the compliance of crustal rock: I - response of shear-wave splitting to differential stress, Geophys. J. Int. 129, 477-494.
Zhang, Z., and S.Y. Schwartz (1994). Seismic anisotropy in the shallow crust of the Loma Prieta segment of the San Andreas fault system, J. Geophys. Res. 99, 9651-9661.
Zinke, J.C., and M. Zoback (2000). Structure-related and stress-induced shear wave velocity anisotropy: observation from microearthquakes near the Calaveras Fault in Central California, Bull. Seismol. Soc. Am. 90, 1305-1312.
Zollo, A., and P. Bernard (1991). Fault mechanism for near source data: joint inversion of P polarities and S polarizations, Geophys. J. Int. 104, 441-452.