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Agata Siniscalchi, Università di Bari (Italy)
Marianna Balasco, CNR (Italy)
Ida Diaferia, Università di Bari (Italy)
Daniela Di Bucci, Dipartimento della Protezione Civile (Italy)
Umberto Fracassi, Istituto Nazionale di Geofisica e Vulcanologia (Italy)
Mariano Loddo, Università di Bari (Italy)
Cosimo Magrì, Università di Bari (Italy)
Gerardo Romano, CNR (Italy)
Domenico Schiavone, Università di Bari (Italy)
Simona Tripaldi, Università di Bari (Italy)
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On 23 July 1930, the Irpinia region in southern Italy experienced a destructive (M 6.7) earthquake that struck the eastern sector of the southern Apennines moutain belt. Previous studies suggest that this earthquake was caused by a seismogenic source having oblique right-lateral kinematics and striking at an angle between the general trend of NE-verging large dip-slip faults in the southern Apennines (~ NW-SE) and the E-W near-vertical, strike-slip right lateral faults that have been recently discovered in the foreland, east of the main extensional axis. Also, the ∼14 km hypocentral depth of the 1930 earthquake that has been calculated in previous studies is likely located within the basement below the Apula carbonate platform succession. This puts the source of the 1930 earthquake not only in an intermediate region between pure normal (NW-SE) and strike-slip right-lateral (E-W) large seismogenic faults in the southern Apennines, but also at an hypocentral depth between the 12-13 km depth of the earthquakes caused by normal faulting (like the Irpinia 23 Nov. 1980, M 6.9 one) and the 15-20 km depth of the earthquakes caused by strike-slip faulting in the foreland (like the 31 Oct.-1 Nov. 2002, M 5.8 Molise ones).
In this framework, we performed a magnetotelluric (MT) study to investigate the evidence of preferential direction in resistivity anisotropy and to compare it with the strike of the 1930 seismogenic fault. Fifteen sites within a 1000 km2 area were investigated by means of remote referenced MT soundings in the 0.009-4000 s period range. We obtained good quality data that are in agreement with well-log data too. MT tensor analysis, following the scheme of Weaver et al. (2000), was accomplished. The dimensional study showed that the 75% of the involved structures is three-dimensional. Also, for each sounding at each period, the electric strike has been recovered and reported as a function of depth using the Niblett?Bostick transformation.
Taking into account the results in the 8-16 km depth range, thus below the resistive layer that can be associated with the Apula Platform successions, we infer promising considerations on the geometry of the seismogenic source. The map of the electric heterogeneity trends shows a preferential E-W strike in the eastern part of the investigated area. Westwards, approaching the extensional axis of the Apennine chain and the 1930 earthquake epicentral zone, the geoelectrical strike is driven by rotation toward NW-SE. This trend could suggest the evidence of a regional weakness in the upper crust, which in turn may reflect the displacement transfer due to the intersection between regional strike-slip (E-W) and extensional (NW-SE) fault systems.
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