GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.3 199 Macroseismic observations have shown that historical earthquakes are mainly located at Casamicciola Terme and, in general, in the northern sector of the island. This seismic activity occurred with short recurrence times, displaying magnitudes up to 4.3 in the case of the 1883 earthquake, which released the maximum energy among the historical events. Macroseismic data also show that earthquakes are characterized by very shallow hypocenters depths (1-2 km), as demonstrated by relatively high intensities rapidly decreasing with distance, local increase of the damage due to amplification effects, and strong directivity of the shaking attenuation. In particular, the damage distribution of the catastrophic 1883 event has been studied in detail to reconstruct the macroseismic field, to locate the seismogenic fault and to assess the local amplification effects (Carlino, 2012). This event is a rare example of earthquake occurred in an active volcanic area, which produced catastrophic damages, with more than 2300 fatalities as a result of the total destruction of the Casamicciola Terme. After the 1883 earthquake, a period of seismic quiet was observed on the island; a few events felt by the population occurred at the beginning of the last century and in 1980 (an offshore event instrumentally recorded), and very occasional low magnitudes earthquakes have been recorded in the last 30 years mainly located in the northern part of the island. The seismic event here analyzed represents the first damaging earthquake instrumentally recorded in the island. The M D 4.0 main shock, localized on the northern rim of the Mt. Epomeo block at a depth of about 1.2 (± 0.2) km, was followed by a seismic sequence of almost 20 lower magnitude earthquakes. All of them, except for the main shock, were detected only by the seismic stations (by the INGV – Osservatorio Vesuvian) located on the island, and most of them by the two stations co-located in the Casamicciola Terme. The distribution of the epicenters show an E-W preferential direction. Six continuous GPS (cGPS) stations, managed by the INGV – Osservatorio Vesuviano, are operating at Ischia Island. Following the main shock seismic event some cGPS stations experienced a coseismic deformation. The cGPS station on Mt. Epomeo (MEPO) shows the maximum horizontal displacement of about 1.5 cm in NNW direction, followed by the cGPS station of Casamicciola Terme (OSCM) station which moved about 1 cm toward NNE. Only MEPO station seems to be affected by a vertical coseismic displacement with a subsidence of about 1 cm. In addition, a large dataset of SAR images measurements was analyzed to investigate the ground displacement field. By exploiting the radar data acquired by the European Sentinel-1 satellites of the Copernicus programme, as well as those of the Italian COSMO-SkyMed (CSK) constellation, a subsidence up to 3 cm was detected in a region close to Casamicciola Terme, the area characterized by the heaviest building damages. In order to retrieve the causative source of the main shock and define its geometry and kinematics, four Sentinel-1 displacement maps, acquired on both ascending and descending orbits, were modeled by finite dislocation faults in an elastic and homogeneous half-space. In particular all the parameters were searched for by using a nonlinear inversion based on the Levenberg-Marquardt least-squares approach; the DInSAR data were subsampled over a mesh of about 5000 points for each map. We found that the best fit solution consists of a normal fault plane, about 2000 m long, 900 m wide E-W striking. We remark that, the analysis carried out does not allow to discriminate the fault immersion, because the achieved results are compatible with both a northern and southern dipping fault configuration. Hence, further investigations are still needed to identify the configuration that simulates at best the surface displacement pattern. Finally, in order to obtain a more accurate estimate of the slip along the fault plane, a distributed slip model was computed by partitioning the plane into 20 x 9 patches, fixing the parameters of the non-linear inversion and searching for the differential slip on each patch. The obtained results highlight that the maximum slip is of about 15 cm and it is concentrated in the range of 900 m depth.