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Earthquake in Croatia and its effects on the monitoring plants scattered throughout Italy

Earthquake in Croatia

With the disclosure of this paper we intend to inform the reader of the experimental results in our dynamic monitoring plants, settled in various sections of Italy, consequent the 6,2 Mw magnitude earthquake on 29/12/2020 with epicenter in Petrinja which is located southwest of Zagreb in Croatia.

All the accelerometers scattered in the many plants all over Italian territory (none excluded) recorded the seismic event with extreme level of definition. This is confirmed by the system installed in a building in via delle Medaglie d'Oro in Rome, in which the trigger was set to control the speed, obtained by single integration from acceleration (this is reasons deriving from the need to carry out a check according to the DIN 415O German standard which requires precisely the examination of the vibration speeds).  The threshold value, which in this case triggered the storage starting of the acquired data, was 2,5 mm/s. For the systems present in the pavilions number 13  and in the Service Center of the Fiera di Milano in Rho, the storage also began and took place for two less important earthquakes, with a lower magnitude, always with epicenter in Petrinja and on the occasion of the one with epicenter in Solizzole close to Verona. Considering that Milan is approximately the same distance as Rome is from the Croatian epicenter, this greater sensitivity is certainly due to our particular adaptive algorithm, on which the trigger mechanism is based, where the start-up threshold is always calibrated according to the environmental background noise.

The recordings available now allow us to have useful information about the behavior of the artifacts placed under observation. Many considerations can be made on the maximum values ​​of the measured kinematic quantities (acceleration) and calculated by numerical integration (speed and displacement). Furthermore, the frequency analysis of the recorded time histories will allow to evaluate how much the seismic action is aligned or not with the natural frequencies, determined ex ante with a necessary modal analysis, usually operational (output only or OMA).

As far as  we know, ESSEBI is perhaps the only company in Italy that at present can draw such a wealth of information from its instrumented sites. And this, let us repeat it, is very useful for evaluating, in truth, the behavior of the affected artifacts, when subjected to more or less strong shaking. INGV has a capillary network of seismographs and geophones, but it does not address the building, but the ground. The instrumental data must then be extrapolated, with the theoretical tools that the NTCs make available. It must be said that Civil Protection has several buildings of a certain value under control, but uses very high-end instrumentation, suitable for hit and run modal operational analysis, which therefore does not justify its use for seismic surveys as in the specific case.

In all the sites where a continuous seismic monitoring plant has been installed, ESSEBI has instead implemented accelerometric Dewesoft IOLITEd with a high performance MEMS transducer incorporated (the high performances consist mainly in an excellent signal-to-noise ratio, which in fact allows a higher dynamic up to 90 dB) easy to use, manageable with an excellent software platform and above all with a good performance-price ratio, such as to allow installations with an adequate number of measurement points compatible with the very frequent moderate budget availability.

On the basis of what is reported in this paper, which can be found in the accompanying annex, it is therefore desirable that the number of instrumented artifacts grow dramatically in order to have more and more useful and probative documentation for a correct anti-seismic design of the new building and to arrive at an effective method of conservative intervention on the old ones.

Hoping to have done something pleasant, we remain available for further information.

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