Rome, August 25 2017
As in any emergency, also following the earthquake that hit the island of Ischia on 21 August 2017, at 20:57 Italian time, the staff of the National Institute of Geophysics and Volcanology (INGV), organized in various teams, is carrying out both field activities and data processing in the INGV operations rooms of the National Earthquake Center of Rome and of the Vesuvius Observatory (OV) in Naples to deepen the understanding of the ongoing phenomenon and provide support to Civil Protection interventions
In all, three different INGV operational teams went to Ischia for the following research activities:
- macroseismic survey by the QUEST group (Quick Earthquake Survey Team)
- geological surveys by the Geological Emergency Service
- OV- geochemical analyzes by the geochemical group of Palermo
The earthquake that hit Ischia on the evening of August 21 had Local Magnitude ML 3.6 ± 0.2 and Duration Magnitude MD 4.0 ± 0.3.
The two magnitude estimates are visible at information page of the seismic event as well as the automatic estimation of Moment Magnitude (available on the page focal mechanism) still preliminary.
INGV uses different magnitude estimates. The Local Magnitude ML (or Richter Magnitude, from the name of the seismologist who proposed it) is the fastest to calculate and is the most widespread measurement for estimating the energy released by crustal earthquakes. In our volcanic areas the MD Duration Magnitude is often used, because it can be calculated quickly, even if it requires complete recording of the event, and because we have a specific calibration for it that takes into account the particular conditions of propagation of seismic waves within the earth's crust affected by volcanic phenomena. The Moment Magnitude Mw, on the other hand, provides an accurate and overall estimate of the energy released by the earthquake and is particularly suitable for estimating the energy of the strongest earthquakes.
As regards the Ischia event, since it occurred within a very superficial and heterogeneous portion of the crust, it was preferred to use the Magnitude Duration MD, which is equal to 4.0, as an estimate of the energy released by the earthquake. For this estimate, only the stations of the volcanic region of Campania were used, so that the characteristics of the rocks that make up the crust of Ischia in terms of seismic wave velocity and attenuation could be taken into consideration. This does not apply to the magnitude ML, calculated at all available stations of the National Seismic Network.
In order to be localized with precision, earthquakes in volcanic areas require specific velocity models of the area, both for the strong lithological variability and for the high geothermal gradient.
These models are available and well verified for the Vesuvian and Etna areas, but not for the island of Ischia because, in order to be developed and calibrated, the local seismicity itself must be used. Since 1999 in Ischia there have been on average less than 5 earthquakes per year (M<2.5), insufficient to elaborate a reliable reference velocity model. The use of non-specific models allows to obtain approximate results useful for the immediate needs of civil protection and represents the only procedure that can be implemented in the very short times required by the emergency.
Aware of this approximation, the INGV seismologists therefore started a path of refinement of the localization immediately after the event, using prototype velocity models. This allowed yesterday to obtain a first better result which was communicated to the Major Risks Commission in the meeting this morning, 25 August 2017, at the Civil Protection in Rome. This revaluation is to be considered preliminary, i.e. still subject to refinement once a three-dimensional velocity model of the area will be determined.
The hypocentral parameters presented to the CGR locate the earthquake 1 km SW of Casamicciola Terme (NA), with geographical coordinates (lat, lon) 40.74°, 13.90° at a depth of about 2 km. The strong damage detected in the upper area of Casamicciola with macroseismic intensity VIII, in addition to the poor resilience of the building, it is therefore attributable both to the superficiality of the event and to the local amplification of the soils which gave values of acceleration of the soil of about 0.28 g and speed of shaking of the soil of almost 18 cm/s.