The 2017, MD = 4.0, Casamicciola Earthquake: ESI-07 Scale Evaluation and Implications for the Source Model

Abstract

On 21 August 2017 at 20:57 (local time) a very shallow (H = 1.2 km), moderate (Md = 4.0), earthquake hit the volcanic island of Ischia (Southern Italy), causing the death of two people. The study of the damage to the buildings with the European Macroseismic Scale 98 (EMS-98), carried out immediately after the earthquake, highlighted that hilly area of Casamicciola Terme, on the northern side of the Mt. Epomeo, was the most damaged part of the island with locally quite relevant damage (I = VIII EMS). This seismic event is the first damaging earthquake in Ischia during the instrumental era. In fact, this provides, for the first time, the opportunity to integrate historical seismicity, macroseismic observations, instrumental information, and detailed mapping of the geological coseismic effects. In this work we evaluate the effects induced by the 2017 Casamicciola earthquake on the environment using the Environmental Seismic Intensity 2007 (ESI-07) macroseismic scale. This macroseismic analysis, together with the superficial coseismic faulting characteristics and the available geophysical information, allows us to reconsider the source model for the 2017 earthquake and the previous damaging historical earthquakes in the Casamicciola Terme area. The application of the ESI scale to the Casamicciola Terme earthquake of 21 August 2017 and the assignment of seismic intensity offers better spatial resolution, as well as an increase of the time window for the assessment of the seismic hazard, allowing to reduce the implicit uncertainty in the intensity attenuation laws in this peculiar volcano-tectonic setting. Since intensity is linked to the direct measure of damage, and it is commonly used in hazard assessment, we argue that building damage at Casamicciola Terme is strongly influenced by earthquake surface faulting and near field effects, and therefore controlled by the geometry of the seismic source.

BEGEOSCIENTIST a young network - I Congresso Nazionale dei Giovani Geoscienziati

“…Etna volcano, where damage is concentrated along the trace of surface faulting. As already observed at Ischia (Nappi et al, 2021), the ESI scale seems to provide a more authentic description of the effects of shallow-focus volcanic earthquakes, allowing a consistent comparison of other local seismic events occurred in the last centuries.…”

The Seismicity of Ischia Island, Italy: An Integrated Earthquake Catalogue From 8th Century BC to 2019 and Its Statistical Properties

“…• The intensity distributions of the largest events (1828, 1881, 1883, and 2017) show an elongated trend in the E-W direction and a rapid decrease with distance indicating a shallow sources. The E-W trend coincides with local fault system, probably indicating the potential contribution of source directivity effects in the spatial distribution of intensities (Vezzoli, 1988;Alessio et al, 1996;Funiciello, 1999, 2006;Carlino et al, 2010;Nappi et al, 2018Nappi et al, , 2021Trasatti et al, 2019;Cubellis et al, 2020). • The observed Ischian seismicity significantly deviates from a stationary process, also taking into account the uncertainty in data.…”

“…Seismicity is mainly concentrated on the E-W structures that limit to the north the resurgent block of Mt. Epomeo (Carlino et al, 2006;Cubellis and Luongo, 2018;Trasatti et al, 2019;Cubellis et al, 2020;Carlino et al, 2021;Nappi et al, 2021;and references therein). Local seismicity may be connected to a relevant seismic hazard (Cubellis et al, 2004;Selva et al, 2019) and, in a multi-hazard view, it can be seen as one of the possible events in a multi-hazard chain that may include other phenomena like landslides or tsunami (e.g., Violante et al, 2003;Della Seta et al, 2012;Paparo and Tinti, 2017;Selva et al, 2019).…”

“…However, the same area was strongly hit by a series of heavily damaging earthquakes in the XIX century (Cubellis and Luongo, 2018;Guidoboni et al, 2018;Cubellis et al, 2020), the most devastating of which occurred on July 28 1883, causing more than 2000 casualties and heavy destruction in a large part of the island and following by only two years another destructive event occurred in 1881 (Cubellis, 1985;Alessio et al, 1996;Cubellis andLuongo, 1998, 2018;Cubellis et al, 2004Cubellis et al, , 2020Luongo et al, 2006;Guidoboni et al, 2018). Therefore, the 2017 event renewed the attention on the evaluation of the seismic hazard and related risk at Ischia (e.g., Avvisati et al, 2019;De Natale et al, 2019;Nappi et al, 2021), considering that this small island (less than 50 km 2 ) is densely inhabited with more than 60,000 steady inhabitants, incremented by tourists during the summer season.…”

Seismic microzonation in a complex volcano-tectonic setting: the case of northern and western Ischia Island (southern Italy)

“…Since this major event the Italian seismologist Mercalli (de rossi, 1884; Guidoboni , 2018) and, afterwards the two engineers, Giordano and Comotto, charged for a new building code definition (Marotta, 2019), recommended the construction of one storey new buildings, preferably located on flat sites or at least on gentle slopes to avoid local amplification effects (topographic and stratigraphic). The inhomogeneous distribution of the damages caused by the 1883 earthquake, in fact, was already observed where soft soils crop out (carlino, 2010), suggesting the role of site effects in influencing the damage patterns.…”