Constraints on the source mechanism of the damaging seismic event of August 21, 2017, on Ischia island (southern Italy)

Abstract

On Aug. 21, 2017, a MD4.0 earthquake struck Ischia island, claiming two lives and provoking severe building damages, disproportionate to the moderate magnitude of the event. Hypocenter locations based on arrival times were afflicted by a large uncertainty and the proposed moment tensor solutions resulted inconsistent among themselves. These contradictory observations gave motivation to recalculate earthquake parameters by alternative approaches. P-phase particle motion, the evaluation of the rotated spectra, and ts-tp lead to a hypocentral depth of 2 km, in the same epicentral area as the devastating seismic event that struck Ischia in 1883. The full moment tensor solution reveals non-double couple contributions of 62%—fitting best for a shallower source depth and estimating a higher magnitude of Mw4.2. To fit the first motion polarity mismatch observed at the closest station IOCA we suggest a complex rupture process, where a first shallow normal faulting event activates a subsequent collapse.