Width of surface rupture zone for thrust earthquakes. Implications for earthquake fault zoning

Paolo Boncio, Francesca Liberi, Martina Caldarella, Fiia C. Nurminen
2017 NHESSD  
The characteristics of the zones of coseismic surface faulting along thrust faults are analysed in order to define the criteria for zoning the Surface Fault Rupture Hazard (SFRH) along thrust faults. Normal and strike-slip faults have been deeply studied in the past concerning SFRH, while thrust faults have not been studied with comparable attention. <br><br> Surface faulting data were collected from 10 well-studied historic thrust earthquakes occurred globally
more » ... sp;M&amp;thinsp;&amp;leq;&amp;thinsp;7.9). Several different types of coseismic fault scarps characterise the analysed earthquakes, depending on the topography, 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 collected directly from the literature or measured systematically in GIS-georeferenced published maps. <br><br> Overall, surface ruptures can occur up to large distances from the main fault (~&amp;thinsp;750&amp;thinsp;m on the footwall and ~&amp;thinsp;1600&amp;thinsp;m on the hanging wall). Most of them occur on the hanging wall, preferentially in the vicinity of the main fault trace (<&amp;thinsp;50&amp;thinsp;m). The widest WRZ are recorded where bending-moment (B-M) or flexural-slip (F-S) secondary faults, associated to large-scale folds (hundreds of meters to kilometres in wavelength), are present. <br><br> The distribution of surface ruptures is fitted with probability density functions, in order to define a criterion to remove outliers (e.g. 90&amp;thinsp;% probability of the cumulative distribution function) and define the zone where the likelihood of having surface ruptures is the highest. This might help in sizing the zones of SFRH during seismic microzonation (SM) mapping. <br><br> In order to shape zones of SFRH, a very detailed earthquake geologic study of the fault is necessary (the highest level of SM, i.e., Level 3 SM according to Italian guidelines). In the absence of such a very detailed study (basic SM, i.e., Level 1 SM of Italian guidelines) a width of ~&amp;thinsp;465&amp;thinsp;m (90&amp;thinsp;% probability) seems to be adequate. For more detailed level SM, where the fault is carefully mapped, one must consider that the highest SFRH is concentrated in a narrow zone, only 50&amp;ndash;70 in width, that should be considered as a fault avoidance zone (40&amp;ndash;50&amp;thinsp;% of the total ruptures are expected to occur within this zone). <br><br> A broad positive relation between the displacement on the main fault and the total width of the rupture zone is found only close to the main fault (total WRZ&amp;thinsp;&amp;leq;&amp;thinsp;60&amp;thinsp;m). The total WRZ appears to increase with displacement, from a minimum of nearly 20&amp;ndash;30&amp;thinsp;m for decimetric vertical displacement up to 50&amp;ndash;60&amp;thinsp;m for vertical displacement close to 2&amp;thinsp;m. <br><br> The fault zones should be asymmetric compared to the trace of the main fault. The average footwall to hanging wall ratio (FW&amp;thinsp;:&amp;thinsp;HW) is close to 1&amp;thinsp;:&amp;thinsp;2. <br><br> These criteria are applicable to <q>simple thrust faults</q>, without B-M or F-S secondary faults on large-scale folds. Zones potentially susceptible to B-M or F-S secondary faults can be inferred by detailed knowledge of the structural setting of the area (geometry, wavelength and lithology of the thrust-related large-scale folds) and by geomorphic evidence of past secondary faulting.
doi:10.5194/nhess-2017-123 fatcat:b3yfnmo77vg7zjvndqoyejqetq