The ESI macroseismic field for the 1624 AD Argenta earthquake (Po Plain – N Italy): an integrated macroseismic approach to overcome MCS misfittings

The 2012 Emilia seismic sequence (two mainshocks, Mw = 5.86 and Mw = 5.66 on the 20th and 29th of May, respectively) overclouded the reliability of earthquake parameters derived from MCS Intensity data in the Po Plain. As highlighted for instance by Galli et al. (2012), Intensity-derived Magnitude, calculated through the BOXER 4 algorithm, pointed to a Me = 5.23, clearly underestimating the actual energy of the earthquake. Moreover the epicentral Intensity values (Io VII – VIII MCS) are not fitting well with the peak ground acceleration recorded during the sequence (ca. 0.23 – 0.28 g), consistent with a IX MCS instead. Livio et al. (2012) applied the same calculations on the ESI Intensity data for the 2012 Emilia seismic sequence (e.g., Di Manna et al., 2012), obtaining a better fitting between calculated and recorded Magnitude (Me = 5.86 ± 0.38) and concluding that best results could be obtained integrating both MCS and ESI databases. In this work we apply BOXER4 to a new dataset coming from an historical earthquake that hit the eastern Po Plain: the March 19th, 1624 Argenta earthquake. Earthquake parameters, recorded in the CPTI11, report for this earthquake a MCS Io = VII – VIII and a derived Magnitude Me = 5.47 ± 0.49 (recently revised by Caracciolo et al. 2012). The epicentral area of this earthquake is characterized by a geological and geomorphological framework similar to the one of the 2012 sequence, and by many environmental effects including liquefactions, collapse of terrains, hydrogeological anomalies and seiches. The ESI Intensity near field is characterized by a wider area occupied by high degrees (in particular by ESI VIII) but is similar to the MCS field in the far field. For our analysis we used 11 environmental effects, spread over an area of ca. 600 kmq. We integrated them with 17 MCS data points (DMBI catalogue). The calculated derived Magnitude Me = 5.86, obtained from the whole dataset, is in line with available empirical relations between epicentral intensity and magnitude in Italy (e.g. CPTI11). This would suggest that, in order to allow comparison between historical and modern strong earthquakes, coseismic environmental effects should be included in the intensity assessment, both in the Po Plain and in other similar areas of the Mediterranean region.