ALBERT

Description: The 2011 October 23 M W 7.1 Van earthquake in eastern Turkey caused ~600 deaths and caused widespread damage and economic loss. The seismogenic rupture was restricted to 10–25 km in depth, but aseismic surface creep, coincident with outcrop fault exposures, was observed in the hours to months after the earthquake. We combine observations from radar interferometry, seismology, geomorphology and Quaternary dating to investigate the geological slip rate and seismotectonic context of the Van earthquake, and assess the implications for continuing seismic hazard in the region. Transient post-seismic slip on the upper Van fault started immediately following the earthquake, and decayed over a period of weeks; it may not fully account for our long-term surface slip-rate estimate of ≥0.5 mm yr –1 . Post-seismic slip on the Bostanici splay fault initiated several days to weeks after the main shock, and we infer that it may have followed the M W  5.9 aftershock on the 9th November. The Van earthquake shows that updip segmentation can be important in arresting seismic ruptures on dip-slip faults. Two large, shallow aftershocks show that the upper 10 km of crust can sustain significant earthquakes, and significant slip is observed to have reached the surface in the late Quaternary, so there may be a continuing seismic hazard from the upper Van fault and the associated splay. The wavelength of folding in the hanging wall of the Van fault is dominated by the structure in the upper 10 km of the crust, masking the effect of deeper seismogenic structures. Thus, models of subsurface faulting based solely on surface folding and faulting in regions of reverse faulting may underestimate the full depth extent of seismogenic structures in the region. In measuring the cumulative post-seismic offsets to anthropogenic structures, we show that Structure-from-Motion can be rapidly deployed to create snapshots of post-seismic displacement. We also demonstrate the utility of declassified Corona mission imagery (1960s–1970s) for geomorphic mapping in areas where recent urbanization has concealed the geomorphic markers.

Description: The intrusion mechanism and internal structure of sills are still under debate. We present a detailed magnetic study, including anisotropy of magnetic susceptibility and rock magnetic analyses of a Cretaceous (94 Ma), 7-m-thick sill from the Lusitanian Basin in Portugal, the Foz da Fonte sill. The results, from both the top surface and a vertical profile, allow us to propose a model for the magmatic flow pattern and sense of flow. According to their location in the vertical profile, three magnetic fabric domains are identified: (1) at the borders, qualified as chilled margins (~0–50 cm), low anisotropies suggest that low velocity gradients and heterogeneous flow paths occurred during the initial emplacement stages; (2) in the centre of the sill, where low anisotropies are observed, low shear gradients and magma displacement close to pure translation is inferred and (3) in the intermediate zones, high anisotropy values are ascribed to zones having undergone high shear gradients. The mean magnetic lineations from the top surface and basal contact indicate an almost horizontal and NW–SE orientation (azimuth: 310°) which agrees with the preferred orientation of iron oxide grain clusters and with the elongation of vesicles considered as coaxial with the magma flow direction. Moreover, the magnetic foliation planes and the lineations show both a mirror imbrication relative to the average upper and lower border surfaces of the sill, pointing to a flow direction towards the SE. Based on these results and on the interpretation of two seismic reflection lines, we show that the Cabo Raso magnetic anomaly, located 25 km to NW of the FF-sill, is associated to Cretaceous magmatic intrusions from which the sill likely originated. This tectono-magmatic setting is discussed with respect to the West Iberia Late Cretaceous magmatism, integrating magnetic anomalies, isotope chronology and tectonics.