Seismological Constraints on the Source Mechanism of the Damaging Seismic Event of 21 August 2017 on Ischia Island (Southern Italy)

Abstract

On 21 August 2017, an M D 4.0 earthquake struck Ischia island in the Tyrrhenian Sea off the coast of Naples, Italy. In spite of its modest magnitude, the earthquake caused two deaths and severe building damage on the northern side of the island. Initial hypocenter locations based on arrival times were highly uncertain and several proposed moment tensor solutions were inconsistent. These contradictory observations prompted a new calculation of the earthquake parameters using alternative methods. Our new approach, based on the determination of P-wave particle motion, evaluation of rotated spectra, and accurate calculation of S-minus-P travel time, yields a hypocentral depth of 2 km and a location in the same area as the devastating seismic event that struck Ischia in 1883. We invert the moment tensor for a best-fitting double couple (DC), obtaining an M w 3.9 with a normal mechanism and an optimal depth of 8 km. Calculation of the full moment tensor results in (1) 36% negative isotropic component and 26% negative compensated linear vector dipole (CLVD) components, (2) a better fit at a shallower source depth than for the corresponding DC, and (3) a magnitude estimate of M w 4.1. Modeling of the waveform and the first motion recorded in Ischia's station IOCA predicts, however, a negative polarity, which is in disagreement with the observation. We therefore suggest a complex rupture process, with an initial shallow normal-faulting event that triggered a subsequent shallow underground collapse.

Characterization of the seismic dynamical state through joint analysis of earthquakes and seismic noise: the example of Ischia Volcanic Island (Italy)

“…Indeed, the basic structure oscillates with normal mode at 1-2 Hz and an abrupt increase (above a certain threshold) of the fluid pressure may have triggered the IE17. This is in agreement with the recent findings of Calderoni et al (2019), who associate the recent seismicity of Ischia to a fault-valve type mechanism consistent with the periodic pressurization of the hydrothermal reservoir induced by self-sealing processes. The pressurization 26 P. Cusano et al: Characterization of the seismic dynamical state cycles are then followed by depressurization episodes associated with ruptures and reactivation of the faults that generates seismicity.…”

“…The earthquake (hereinafter referred to IE17) triggered a significant ground displacement (Devoti et al, 2018) and tilting (Ricco et al, 2019) and it was followed by a sequence of about 20 aftershocks (Nappi et al, 2018) located at shallow depths (< 2.5 km b.s.l.). The interpretation of the source process of the IE17 is still controversial: re-activation caused by tectonic stress of an E-W striking fault with south (De Novellis et al, 2018) or north (Nappi et al, 2018) dipping, or complex rupture involving a normal faulting event triggering a shallow underground collapse (Braun et al, 2018), are some of the possi-ble generation mechanisms that have been proposed. Interestingly, Calderoni et al (2019) have highlighted the significant role of the hydrothermal fluids in triggering seismicity through the pressurization/depressurization cycles of the hydrothermal reservoir.…”

“…The interpretation of the source process of the IE17 is still controversial: re-activation caused by tectonic stress of an E-W striking fault with south (De Novellis et al, 2018) or north (Nappi et al, 2018) dipping, or complex rupture involving a normal faulting event triggering a shallow underground collapse (Braun et al, 2018), are some of the possi-ble generation mechanisms that have been proposed. Interestingly, Calderoni et al (2019) have highlighted the significant role of the hydrothermal fluids in triggering seismicity through the pressurization/depressurization cycles of the hydrothermal reservoir. The role of the hydrothermal system in the generation of the background seismic signal has been furtherly investigated by Cusano et al (2020).…”

Seismic and Geodetic Evidences of a Hydrothermal Source in the Md 4.0, 2017, Ischia Earthquake (Italy)

“…Therefore, we collect and analyze seismic and satellite data of a shallow (1.24‐ to 1.7‐km depth; Braun et al, ; D'Auria et al, ), M d 4.0 earthquake that occurred on 21 August 2017 in the northern sector of the Ischia volcanic island, Italy (Figure a). This event damaged the towns of Casamicciola Terme and Lacco Ameno (Figures a–c), where some buildings collapsed and resulted in two deaths and approximately 50 injuries.…”

“…These studies interpret this earthquake due to the reactivation of faults caused by tectonic stress. Braun et al () suggest a more complex mechanism, with a normal faulting event triggering a consequent shallow underground collapse. However, none of these studies merges the seismic and geodetic data to the existing geological and hydrothermal knowledge.…”