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Bulletin of the Seismological Society of America; February 2006; v. 96; no. 1; p. 107-132; DOI: 10.1785/0120040231
© 2006 Seismological Society of America
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Layered Seismogenic Source Model and Probabilistic Seismic-Hazard Analyses in Central Italy

Bruno Pace1, Laura Peruzza2, Giusy Lavecchia1 and Paolo Boncio1

1 GeoSisLab
Dipartimento di Scienze della Terra
Università degli Studi "G. D'Annunzio"
Via dei Vestini, 30
66013 Chieti Scalo (CH) Italy
b.pace{at}unich.it
g.lavecchia{at}unich.it
p.boncio{at}unich.it
 (B.P., G.L., P.B.)

2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS)
Borgo Grotta Gigante 42/c
34010 Sgonico (TS) Italy
lperuzza{at}inogs.it
 (L.P.)


Figure 001
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  		Figure 1. Shaded relief of central Italy with tectonic elements and focal mechanisms. Selected earthquakes with M ≥4.0 from 1915 to 2002 (from Montone et al., 1999, implemented). Black focal mechanisms taken from Harvard centroid moment tensor (CMT) catalog at www.seismology.harvard.edu and MedNet regional CMT at www.mednet.ingv.it. The source codes are as follows: R89, Riguzzi et al. (1989); WV89, Ward and Valensise (1989); FA97, Frepoli and Amato (1997); D97, Di Luccio et al. (1997); S03, Santini (2003); B04, Boncio et al. (2004b).

 

Figure 002
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  		Figure 2. Map of the seismotectonic provinces (SPs) of central Italy and epicenters of instrumental earthquakes recorded in the period 1981–1996 (data from CSTI catalog, Working Group CSTI [2001], after removal of aftershocks as explained in the text). The epicenters of the mainshocks of the 1997–1998 Umbria-Marche seismic crisis (26 September 1997 00:33, Mw 5.7; 26 September 1997 09:40, Mw 6.0; 14 October 1997 Mw 5.6; Amato et al., 1998) are plotted with stars.

 

Figure 003
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  		Figure 3. Map of the seismogenic boxes (SBs) of central Italy and epicenters of historical earthquakes with epicentral intensity larger than VII–VIII MCS (corresponding roughly to M >5.1, Working Group CPTI, 2004). The dates refer to the earthquakes outside the SP B with epicentral intensity larger than VIII MCS. The stars are the epicenters of the mainshocks of the 1997–1998 Umbria-Marche seismic crisis (Amato et al., 1998); SBs are marked with numbers (see Tables 1 and 2).

 

Figure 004
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  		Figure 4. Maximum expected magnitude (Mmax) calculated for the seismogenic boxes in central Italy. Legend is as follows: M1 is Mw from master fault length (L, see Table 1) by using the Wells and Coppersmith (1994) empirical relations on subsurface rupture length (RLD); M2 is Mw as before, using the master fault rupture area (RA); M3 is Mw from the scalar seismic moment of equation (3), assuming a constant strain drop. The most important historical and instrumental earthquakes associated with the SBs (see Table 2) are marked by crosses; the source code is underlined for boxes with paleoseismologically observed earthquakes (Table 2).

 

Figure 005
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  		Figure 5. Mean recurrence times (Figure 005) associated with the maximum expected earthquake for SBs in central Italy; values T1–T3 are obtained by T = SE/SR (slip- ratio method) using, respectively, M1, M2, and M3 and the related SE from the Wells and Coppersmith (1994) relationships. Values T4–T6 use equation (4) (seismic- moment conservation method) as before; Tmean are the mean values (see Table 3).

 

Figure 007
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  		Figure 7. Seismicity rates associated to the four seismotectonic provinces in central Italy. Instrumental and historical catalogs (CSTI for Working Group CSTI, 2001; CSI for Castello et al., 2005; CPTI for Working Group CPTI, 2004) have been used; G-R relationship is interpolated by using the least-squares (lsq) method. Bars in SP B represent the summation of the rates of the 28 seismogenic boxes computed using the geological information; plusses are the CPTI rates aggregated with the same magnitude step (0.3).

 

Figure 006
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  		Figure 6. Examples of seismicity rates of some seismogenic boxes in central Italy. Observed rates (triangles and circles) represent the annual cumulative number of events located inside the box, according to the instrumental database CSTI (Working Group CSTI, 2001) and CSI (Castello et al., 2005). The computed rates (gray dots and squares) derive from the a priori selected model, combined with the geological constraints (see the text); plusses represent time-dependent rates (null if plotted on the x axis).

 

Figure 008
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  		Figure 8. Maps of G-R coefficients in central Italy, using the filtered CSTI catalog. Grid node of 0.1° spacing, search radius of 20 km, least-squares fitting; the a values refer to one year and unit area of 100 km2. The circles indicate the location of data sets shown in Figure 9.

 

Figure 009
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  		Figure 9. Examples G-R fitting in four nodes in central Italy (location map in Fig. 8). Circles are the earthquakes extracted for the node; full line indicates the relationship obtained using the maximum likelihood (mlk) algorithm and the dashed line indicates the relationship using the least-squares (lsq) method.

 

Figure 010
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  		Figure 10. Maps of PGA expected not to be exceeded in 50 years at 90% probability level in central Italy, with standard deviation in attenuation (Ambraseys et al., 1996), using the low-level background seismicity modeling (a) and the models of the seismotectonic provinces and of the seismogenic boxes (b).

 

Figure 011
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  		Figure 11. Map of PGA expected not to be exceeded in 50 years at 90% probability level in central Italy, with standard deviation in attenuation (Ambraseys et al., 1996), using the layered-source model.

 

Figure 012
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  		Figure 12. Quantities related to the renewal process adopted for some seismogenic boxes. In the lower part of the graph (axis on the left) mean recurrence time and time elapsed since the last event (known or inferred) are shown. In the upper part (axis on the right), conditional probability of occurrence of a Mmax event in the next 50 years (from 2004) using BPT distribution is shown.

 

Figure 013
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  		Figure 13. Map of PGA expected not to be exceeded in the next 50 years (from 2004) at 90% probability level in central Italy, with standard deviation in attenuation (Ambraseys et al., 1996), using the layered and time-dependent source model.

 

Figure 014
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  		Figure 14. Comparison of Poisson (see Fig. 11) and time-dependent (see Fig. 13) results: (a) relative differences expressed in percentage of PGA; (b) absolute differences in g.

 




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