High precision leveling survey following the Md 4.0 Casamicciola earthquake of August 21, 2017 (Ischia, Southern Italy): field data and preliminary interpretation

The aim of this paper is the presentation of the results obtained from the high precision leveling survey carried out from November 6 to 11, 2017, in the area hit by the Md 4.0 Casamicciola earthquake of August 21, 2017, and critical discussion of these results in the light of the possible seismic source models. The measurements have been carried out on benchmarks of the northwestern sector of the altimetric network of the Ischia island, in the epicentral area. The heights of each benchmark have been obtained considering the historical reference benchmark n.1 located at Ischia Porto, and have been compensated over the whole stretch of the measured network; such heights have also been compared with those derived from the previous leveling campaign that was performed along the entire network in June 2010. The results obtained from the specific investigated leveling lines, which are considered representative of the kinematics of the northwestern area of the island, show conspicuous anomalies of ground deformation in the epicentral area of the August 21, 2017 earthquake. In particular, a significant ground subsidence of about 3.5 centimeters has been found between the benchmarks n. 92 and n. 98A along the “Borbonica Line”, quite anomalous compared to the previous measurements. Such subsidence located between the Piazza Maio (Casamicciola) and Fango (Lacco Ameno) localities, results spatially correlated with the deformation detected by the InSAR data analysis and coincides with the greatest macroseismic damages and with the coseismic surface fracture systems E-W oriented. Future leveling campaign in the area would be very crucial in order to follow and define the rate of postseismic motion of the seismogenetic source, also contributing with independent and valuable data to the seismic hazard knowledge of the Ischia island