More than 100 observations have been collected on the fractures attributable to the surface propagation of the fault responsible for the earthquake, of magnitude Md=4.0 (Lat. 40.74°, Long. 13.90°), which struck the 'Island of Ischia, in the upper part of the municipality of Casamicciola Terme at a depth of about 21 km. To detect the geological effects on the ground produced by the seismic event, starting from the day following the earthquake, the EMERGEO geological emergency group (http://emergeo.ingv.it/) of the National Institute of Geophysics and Volcanology (INGV) in collaboration with the Institute for the Coastal Marine Environment of the National Research Council (IAMC-CNR). The results of the work "The 21 August 2017 Md 4.0 Casamicciola Earthquake: First Evidence of Coseismic Normal Surface Faulting at the Ischia Volcanic Island", have been published in the journal Seismological Research Letters
(It hurts: https://doi.org/10.1785/0220180063). The mapping of surface ruptures, directly connected to the fault source of the earthquake, and the analysis of secondary effects, such as landslides and collapse of dry stone walls, allowed to formulate an earthquake source model.
Casamicciola Terme has been hit in recent centuries by volcano-tectonic earthquakes of 1762, 1796, 1828, 1881 and 1883 with an epicentral area similar to that of August 21, 2017. After the 1883 earthquake, the most destructive one with 2343 deaths, and a intensity equal to degree XI on the Mercalli-Cancani-Sieberg scale, the event of August 21, 2017 (Md=4.0) made it possible to integrate historical seismicity data, macroseismic observations, and instrumental information with the detailed map of the observed cosismic geological effects.
“The INGV Emergeo group studies the geological cosismic effects produced by strong earthquakes in the Apennine area (M>= 5.5) and by moderate earthquakes in volcanic areas. For this event, the survey was carried out in the epicentral area, i.e. in the northern sector of the island of Ischia, where the greatest macroseismic damage was concentrated, collecting observations relating to the various geological cosismic effects, such as fractures and breakages attributable to the propagation in surface of the fault in depth, and landslides in volcaniclastic deposits, as well as collapses of dry stone walls as secondary effects due to ground shaking", explains Giuliana Alessio researcher of the Naples Section - Vesuvius Observatory of the National Institute of Geophysics and Volcanology (INGV) .
"The geometry and regularity of the trend of the fractures", continues Rosa Nappi, researcher of the Naples Section - Vesuvius Observatory of the National Institute of Geophysics and Volcanology (INGV), "allowed us to state that the fault responsible for the earthquake of 21 August, located at the foot of the northern slope of Mount Epomeo, is oriented in an East-West direction, with a length of about 2 km, and could have the same characteristics and location as those that generated the catastrophic earthquakes of 1881 and 1883. data collected show that the block lowered by the fault is precisely the area north of Mount Epomeo”.
"This research highlights not only the work of the EMERGEO group but also the importance of collaboration between research institutions, favoring the collection, analysis and interpretation of cosismic geological data, never detected before in the volcanic areas of Campania". concludes Sabina Porfido of the IAMC – CNR, Naples.
The publication of these results is part of a lively scientific debate which sees various proposals for the reconstruction of the source mechanism. The multiplication of scientific articles on the recent Casamicciola earthquake testifies to the interest of an attentive and varied scientific community, which uses different methodological approaches to understand the dynamics of a complex geological context such as that of Ischia.
Image 1 - Map of cosismic ruptures in the relief area with the rose diagram (top left) showing the prevailing East-West direction of the cosismic ruptures, superimposed on the geological map by Vezzoli (1988) with the traces of the geological sections AA', BB', extracted from the work of Tibaldi and Vezzoli, (1998), modified by the authors, and shown below.
Image 2 - Co-seismic ruptures shown by the red arrows, which affected both the ground and the building, in via Crateca
Image 3 - Secondary cosismic effects observed: (a,c) collapse of dry stone wall and collapse of an embankment, (b,d) small landslides from collapse and small landslides in volcaniclastic deposits.
Image 4 - Co-seismic fractures and collapses in the dry stone walls observable in the same place in via Campomanno, on the left following the historic earthquake of July 28, 1883, on the right following the earthquake of August 21, 2017.