Natural Hazards and Earth System Science www.nat-hazards-earth-syst-sci.net 1561-8633 1684-9981 9 1 2009 10.5194/nhess-9-161-2009 http://www.nat-hazards-earth-syst-sci.net/9/161/2009/ http://www.nat-hazards-earth-syst-sci.net/9/161/2009/nhess-9-161-2009.html http://www.nat-hazards-earth-syst-sci.net/9/161/2009/nhess-9-161-2009.pdf 161 174 2009-02-17 Seismogenic zonation and seismic hazard estimates in a Southern Italy area (Northern Apulia) characterised by moderate seismicity rates V. Del Gaudio delga@geo.uniba.it P. Pierri G. Calcagnile Dipartimento di Geologia e Geofisica, Università degli Studi di Bari, Bari, Italy Osservatorio Sismologico, Università degli Studi di Bari, Bari, Italy The northernmost part of Apulia, in Southern Italy, is an emerged portion of the Adriatic plate, which in past centuries was hit by at least three disastrous earthquakes and at present is occasionally affected by seismic events of moderate energy. In the latest seismic hazard assessment carried out in Italy at national scale, the adopted seismogenic zonation (named ZS9) has defined for this area a single zone including parts of different structural units (chain, foredeep, foreland). However significant seismic behaviour differences were revealed among them by our recent studies and, therefore, we re-evaluated local seismic hazard by adopting a zonation, named ZNA, modifying the ZS9 to separate areas of Northern Apulia belonging to different structural domains. To overcome the problem of the limited datasets of historical events available for small zones having a relatively low rate of earthquake recurrence, an approach was adopted that integrates historical and instrumental event data. The latter were declustered with a procedure specifically devised to process datasets of low to moderate magnitude shocks. Seismicity rates were then calculated following alternative procedural choices, according to a "logic tree" approach, to explore the influence of epistemic uncertainties on the final results and to evaluate, among these, the importance of the uncertainty in seismogenic zonation. The comparison between the results obtained using zonations ZNA and ZS9 confirms the well known "spreading effect" that the use of larger seismogenic zones has on hazard estimates. This effect can locally determine underestimates or overestimates by amounts that make necessary a careful reconsideration of seismic classification and building code application. Aki, K.: Maximum likelihood estimate of $b$ in the formula $\logN=a-bM$ and its confidence limits, B. Earthq. Res. I. Tokyo, 43, 237–239, 1965. Bath, M. and Duda, S. J.: Earthquake volume, fault plane area, seismic energy, strain, deformation and related quantities, Ann. Geofis., 17(3), 353–368, 1964. Bender, B.: Maximum likelihood estimation of b values for magnitude grouped data, B. Seismol. Soc. Am., 73, 831–851, 1983. Bender, B. and Perkins, D. M.: SEISRISK III: a computer program for seismic hazard estimation, US Geological Survey Bulletin 1772, 48~pp., 1987. Castello, B., Selvaggi, G., Chiarabba, C., and Amato, A.: CSI Catalogo della sismicità italiana 1981–2002, versione 1.1. INGV-CNT, Roma, available at: http://csi.rm.ingv.it, 2006. Cornell, C. A.: Engineering seismic risk analysis, B. Seismol. Soc. Am., 58, 1583–1606, 1968. CPTI Working Group (ING; GNDT; SGA; SSN): Catalogo parametrico dei terremoti italiani, Ed. Compositori, Bologna, http://emidius.mi.ingv.it/CPTI04/, 2004. Davis, S. D. and Frohlich, C.: Single-link cluster analysis. synthetic earthquake catalogues, and aftershocks identification, Geophys. J. Int., 104, 289–306, 1991. Del Gaudio, V., Pierri, P., Calcagnile, G., and Venisti, N.: Characteristics of the low energy seismicity of central Apulia (southern Italy) and hazard implications, J. Seismol., 9, 39–59, 2005. Del Gaudio, V., Pierri, P., Frepoli, A., Calcagnile, G., Venisti, N., and Cimini G.: A critical revision of the seismicity of Northern Apulia (Adriatic Microplate – Southern Italy) and implications for the identification of seismogenic structures, Tectonophysics, 436(1/4), 9–35, 2007. Gasperini, P.: Local magnitude revaluation for recent Italian earthquakes (1981–1996), J. Seismol., 6, 503–524, 2002. Giardini, D.: The Global Seismic Hazard Assessment Program (GSHAP) – 1992/1999, Ann. Geofis., 42(6), 957–974, 1999. Gruppo di Lavoro CSTI: Catalogo Strumentale dei terremoti italiani dal 1981 al 1996, Versione~1.0. ING-GNDT, CD-ROM, 2001. Gruppo di Lavoro "Mappa della Pericolosità Sismica": Redazione della mappa di pericolosità sismica prevista dall'Ordinanza PCM 3274 del 20 marzo 2003, Rapporto Conclusivo per il Dipartimento della Protezione Civile, INGV, Milano-Roma, aprile 2004, 65~pp.+5 appendici, 2004. Gutenberg, B. and Richter, C. F.: Earthquake intensity, energy and acceleration, B. Seismol. Soc. Am., 32, 163–191, 1942. Gutenberg, B. and Richter, C.F.: Frequency of earthquakes in California, B. Seismol. Soc. Am., 34, 185–188, 1944. Kulkarni, R. B., Youngs, R. R., and Coppersmith, K. J.: Assessment of confidence intervals for results of seismic hazard analysis, in Proceedings of the Eighth World Conference on Earthquake Engineering, San Francisco, 1, 263–270, 1984. Reasenberg, P.: Second-order moment of Central California seismicity, 1969–1982, J. Geophys. Res., 90(B7), 5479–5495, 1985. Sabetta, F. and Pugliese, A.: Estimation of response spectra and simulation of non stationary earthquake ground motion, B. Seismol. Soc. Am., 86(2), 337–352, 1996. Sandri, L. and Marzocchi, W.: A technical note on the bias in the estimation of the b-value and its uncertainty through the Least Squares technique, Ann. Geophys. – Italy, 50(3), 329–339, 2007. Slejko, D., Peruzza, L., and Rebez, A.: Seismic hazard maps of Italy, Ann. Geofis., 41(2), 183–214, 1998. Tinti, S. and Mulargia, F.: Completeness Analysis of a Seismic Catalog, Ann. Geophys., 3, 407–414, 1985. Tinti, S. and Mulargia, F.: Confidence intervals of b values for grouped magnitudes, B. Seismol. Soc. Am., 77, 2125–2134, 1987. Utsu, T.: Relationships between Magnitude scales, in: International Handbook of Earthquake and Engineering Seismology, part A, edited by: Lee, W. H. K., Kanamori, H., Jennings, P. C., and Kisslinger, C., Academic Press, Amsterdam, 733–746, 2002. Venisti, N., Calcagnile, G., Del Gaudio, V., and Pierri, P.: Combined analysis of seismic and gravimetric data in the Adriatic Plate, Phys. Earth Planet. In., 142, 89–100, 2004. Venisti, N., Calcagnile, G., Panza, G. F., and Pontevivo, A.: Surface-wave and body-wave-tomography combined study of the Apulian Plate, Pure Appl. Geophys., 162, 1–19, 2005. Wessel., P. and Smith, W. H. F.: New, improved version of Generic Mapping Tools released, EOS T. Am. Geophys. Un., 79(47), 579~pp., 1998.