GNGTS 2017 - 36° Convegno Nazionale

62 GNGTS 2017 S essione 1.1 DISTRIBUTION OF SECONDARY RUPTURES FOR THRUST EARTHQUAKES: IMPLICATIONS FOR EARTHQUAKE FAULT ZONING F.C. Nurminen 1 , P. Boncio 2 , F. Liberi 2 , M. Caldarella 2 1 Oulu Mining School, University of Oulu, Finland 2 CRUST-DISPUTer, Università “G. d’Annunzio”, Chieti, Italy Coseismic surface ruptures during large earthquakes might produce damage to buildings and facilities located on or close to the trace of active seismogenic faults. This is known as Sur- face Fault Rupture Hazard (SFRH). The mitigation of SFRH can be faced by strategies of fault zoning and avoidance or, alternatively, by probabilistic estimates of fault displacement hazard (e.g. Petersen et al. , 2011). Both strategies need to know the location of the active fault trace, the expected displacement on the main fault, the deformation close to the main fault, and the distribution of secondary faulting away from the main fault. While the general fault geometry and the expected displacement on the main fault can be obtained through a detailed geological study and the application of empirical relationships (e.g. Wells and Coppersmith, 1994), the oc- currence of secondary faulting close to and away from the main fault is particularly difficult to predict, and only direct observations from well-documented case studies may help (e.g. shape and size of rupture zones, attenuation relationships for secondary faulting). Worldwide, the distribution of secondary ruptures and the width of the rupture zone (WRZ) for normal and strike-slip earthquakes (e.g. Youngs et al. , 2003; Petersen et al. , 2011; Boncio et al. , 2012; Livio et al. , 2016) is much more studied than for thrust earthquakes (e.g., Zhou et al. , 2010). To our knowledge, a global data compilation from well-documented thrust faulting earthquakes aimed at analysing the characteristics of the surface rupture zone is lacking in the scientific literature. The objectives of this work are: 1) to compile the data from well-studied surface faulting thrust earthquakes globally; 2) to analyse statistically the distribution of surface ruptures com- pared to the main fault; and 3) to discuss the implications for earthquake fault zoning. Surface faulting data were compiled from the literature for 11 well-studied historic thrust earthquake ruptures occurred globally from 1971 to 2014 (5.4 ≤ M ≤ 7.9). Several different types of coseismic fault scarps characterise the analysed earthquakes, depending on the topo- graphy, fault geometry and near-surface materials (simple and hanging wall collapse scarps; pressure ridges; fold scarps and thrust or pressure ridges with bending-moment or flexural-slip secondary faults due to large-scale folding). For all the earthquakes, the distance of secondary ruptures from the main fault (r) and the width of the rupture zone (WRZ) were measured syste- matically in GIS-georeferenced published maps or compiled directly from the literature. Overall, surface ruptures can occur up to very large distances from the main fault (~2,000 m on the footwall and ~3,000 m on the hanging wall). The farthest ruptures (>900 m on the footwall and >2,000 m on the hanging wall) are ruptures isolated from the main fault, interpre- ted here as triggered slip on pre-existing faults not directly connected with the primary fault (sympathetic ruptures). Sympathetic ruptures were observed for the San Fernando 1971 (Mw 6.6), Al Asnam 1980 (Mw 7.1), Tennant Creek 1988 (Mw 6.3, Mw 6.4 and Mw 6.6), Chi Chi 1999 (Mw 7.6), and Kashmir 2005 (Mw 7.6) earthquakes, and represent a small percentage of the total secondary ruptures. Apart from sympathetic ones, secondary ruptures occur within ~750 m on the footwall and ~1,600 m on the hanging wall. Most of them occur on the hanging wall, preferentially in the vicinity of the main fault trace (< 50 m). The widest WRZ are re- corded where bending-moment (B-M) or flexural-slip (F-S) secondary faults, associated with large-scale folds (hundreds of meters to kilometres in wavelength), are present. The distribution of surface ruptures is fitted with probability density functions, in order to define a criterion to remove outliers and define the zone where the likelihood of having surface ruptures is the highest. This might help in sizing the zones of SFRH and in defining the attenua- tion relationships for secondary faulting away from the main fault.