Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

part of the rupture zone deeper incised stream channels expose alternating steeply south- dipping (up to 85°S) Paleogene brecciated gypsum and limestone units that occur topographically both above and below the fault zone. The units exposed farther downslope on the valley flank of the Nura River dip up 65°N forming an asymmetric syncline, with a north-dipping southern flank of a fold. Surface rupture kinematics and fault model Our study builds on observations from previous work indicating that the surface rupture zone is linked to uplift, dominated by thrust and reverse faulting mechanisms (Sippl et al., 2014; Teshebaeva et al., 2014). Interestingly, we observed extensional features in the resulting rupture zone, displaying en echelon fractures, shallow graben, and half-graben in some areas, while in others, there are nearly vertical offsets and obliquely oriented graben and fissures (Figure 2). In areas, characterized by Paleogene gypsum- bearing layers, extensional features with maximum offsets were observed. The important observation is the apparent relationship of the 2008 surface rupture with local topographic highs (see Discussion below). Our observations further show parallels to regions characterized by shortening, specifically noting similarities with flexural-slip faulting. Flexural-slip faulting involves sliding along mechanically weaker bedding surfaces within folded sequences, causing surface rupture. We reference for instance the 1980 El Asnam earthquake in Algeria and the 1994 Sefidabeh earthquake in Iran to illustrate how slip on subvertical bedding planes within a fold can lead to surface faulting along crest of topographic highs, similar to what we observe here (Berberian et al., 2000; Philip & Meghraoui, 1983). Considering the regional geological context in our study area, we infer that the surface rupture along the IrkF within Quaternary till results from flexural-slip faulting in folded Cenozoic strata. These strata rest unconformably upon folded Paleozoic rocks, which are, in turn, bounded by the PFT. We posit that the movement along the IrkF associated with the 2008 Nura earthquake can be ascribed to internal deformation of Cenozoic fold, which may be a result of either (1) movement along the PFT, (2) the transmission of dynamic stresses from seismic waves, (3) slip on a buried thrust triggered by the main shock or a combination of these factors. Fig. 3: Field photographs of geomorphic features and surface deformation along the 2008 Nura surface rupture. Mo Modified from Patyniak et al. in press.