S U M M A R YWe studied fault scarps along the northern sector of the Celano-L'Aquila fault system in the Abruzzi region (central Apennines). Up to ∼9.5 km long, 3 m high, fault scarp traces mark the slope foot of ridgetop valleys at Mt Ocre range. In order to provide direct evidence of the deformation history of these scarps, we initiated geomorphic, ground-penetrating radar (GPR) and trenching investigations. GPR investigations yielded subsurface stratigraphic features of the scarp zones, and determined the locations for trenching sites. A total of five trenches were excavated at two different sites. Structural and stratigraphic analysis of the trench exposures combined with historical considerations, showed three faulting events between 5620 BC and 1300 AD; the most recent of them occurred after 1690 BC. Each of these events produced an estimated minimum vertical displacement ranging between 0.3 and 0.5 m. Our interpretation is that the Mt Ocre fault branch represents the northernmost surface expression of a single 35 km long seismogenic structure associated with M ∼ 7 earthquakes. Any attempt to estimate the seismic hazard in the area must consider the presence of this important source.
A paleoseismic study conducted along the Cholame segment of the San Andreas fault provides evidence for three earthquakes and the amount of lateral offset for the most recent event (1857 Fort Tejon earthquake). Excavations at the Las Yeguas (LY4) site include five fault-perpendicular, two parallel, and several handdug trenches. Abruptly truncated sand and silt layers that are not correlative across the fault zone constrain the timing of the penultimate event (L2) between cal. A.D. 1030-1300 and 1390-1460. Vertical offset, shearing, and fracturing of silty sand and gravel units that overlie L2 and historical artifacts that bracket the timing of the MRE (L1) provide evidence that the most recent ground-rupturing event, L1, occurred between cal. A.D. 1390-1460 and ϳ1865. L1 is likely the 1857 Fort Tejon earthquake. Tectonic silt-filled fractures that dissect historic gray-tan silt and sand suggest a ground shaking or a triggered slip event (L0), which occurred after L1. Threedimensional excavation of an alluvial fan edge (unit 4) indicates that 3.0 ע 0.70 m of near-fault brittle slip occurred during the 1857 earthquake at this site.
Abstract. In this paper we present the geological effects induced by the 2012 Emilia seismic sequence in the Po Plain. Extensive liquefaction phenomena were observed over an area of ~ 1200 km2 following the 20 May, ML 5.9 and 29 May, ML 5.8 mainshocks; both occurred on about E–W trending, S dipping blind thrust faults. We collected the coseismic geological evidence through field and aerial surveys, reports from local people and Web-based survey. On the basis of their morphologic and structural characteristics, we grouped the 1362 effects surveyed into three main categories: liquefaction (485), fractures with liquefaction (768), and fractures (109). We show that the quite uneven distribution of liquefaction effects, which appear concentrated and aligned, is mostly controlled by the presence of paleo-riverbeds, out-flow channels and fans of the main rivers crossing the area; these terrains are characterised by the pervasive presence of sandy layers in the uppermost 5 m, a local feature that, along with the presence of a high water table, greatly favours liquefaction. We also find that the maximum distance of observed liquefaction from the earthquake epicentre is ~ 30 km, in agreement with the regional empirical relations available for the Italian Peninsula. Finally, we observe that the contour of the liquefaction observations has an elongated shape almost coinciding with the aftershock area, the InSAR deformation area, and the I ≥ 6 EMS area. This observation confirms the control of the earthquake source on the liquefaction distribution, and provides useful hints in the characterisation of the seismogenic source responsible for historical and pre-historical liquefactions.
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