Seismotectonics study of the valle umbra area at different space and time scales: an integration of geomorphological, geophysical and remote sensing data

Abstract

The Valle Umbra is a NW-SE 20 km long and 10 km wide Quaternary alluvial basin located in the internal sector of the active extensional tectonic Apennine chain, central Italy. This area historically suffered major earthquakes such as in 1832 (ME=6.3; I0=X) and 1854 (Me=5.6; I=VIII), both localized in the valley. It was marginally affected by the seismic event localized in the Colfiorito basin (M 6.0; September 1997) and more recently suffered from damages to buildings caused by the last seismic sequence of Amatrice-Visso-Norcia (main shock, Norcia, PG, Mw 6.5, 30 October 2016). We investigate the presence of active segments that may be hidden under the unconsolidated deposits filling the basin. We integrate single station ambient noise measurements with previously acquired seismic reflection data and surface geology in order to reconstruct the subsurface geometry of the basin and localize buried faults. Preliminary results from the ambient noise spectral ratios show, in general, one resonance peak at around 0.8 - 1 Hz for stations installed on the lower Pleistocene deposits overlapping the Miocenic bedrock and two resonance peaks for stations installed in the alluvial infill deposits in the central portion of the valley. An accelerometric station of the national civil protection provided us additional constraints about the level of amplification of the site, (connected to the presence of impedance contrasts in the subsoil) as recovered by spectral ratio analysis of several earthquakes recorded from the 1986 up to date. Moreover, the availability of more than 20 years of SAR data archives for the studied area, allowed us the observation of a differential ground subsidence, detected by means of the application of Differential InSAR techniques. The subsidence can be due to either human induced aquifers withdrawal or to tectonic of the area, and we interpret that these can be co-responsible for the observed subsidence pattern. Through the correlation of the time-series of observed recent ground deformations with the buried geological structures identified using geophysical techniques, we made an attempt of detecting and characterizing the geologic heterogeneities in the subsurface, defining the geometry and kinematics of the buried faults and eventually addressing the seismogenic potential of the area.