The 2012 Emilia (Northern Italy) earthquake sequence: an attempt of historical reading

Abstract

In May-June 2012, the Po Valley (Northern Italy) was struck by an earthquake sequence whose strongest event occurred on 20 May (Mw 5.9). The intensity values (Imax 7-8 EMS98) assessed through macroseismic field surveys seemed inappropriate to describe the whole range of effects observed, especially those to monumental heritage, which suffered very heavy damage and destruction. The observed intensities in fact were significantly lower than those we could have expected after a Mw 5.9 event for Italy. As magnitudeintensity regressions are mainly based on historical earthquake data, we handle this issue going back in time and debating the following hypotheses: (a) the 2012 Emilia earthquake sequence shows lower intensity values than expected because the affected urban context is more heterogeneous and much less vulnerable than that in the past; (b) some historical earthquakes, especially those that occurred centuries ago and are provided with little information, could show a tendency to be overestimated in intensity, and consequently in magnitude. In order to give consistency to such hypotheses, we have introduced, as a test, a dual historical reading of the 2012 Emilia earthquake sequence as if it had occurred in the past: the first reading refers to a period prior to the introduction of concrete in buildings assessing the intensity on traditional masonry buildings only. A further historical reading, assessed by using information on monumental buildings only, was performed, and it can be roughly referred to the XVI-XVII centuries. In both cases, intensity values tend to grow significantly. The results could have a relevant impact when considered for seismic hazard assessments if confirmed on a large scale.

Bayesian rupture imaging in a complex medium: The 29 May 2012 Emilia, Northern Italy, earthquake

“…Moderate to large earthquakes are infrequent in this area, which is characterized by a low‐to‐moderate level of seismicity as compared to Central and Southern Italy. It has remained almost silent since the M 5.5 1570 Ferrara earthquake, with the exception of sporadic small‐ to medium‐magnitude earthquakes [ Graziani et al , ]. This long return period results from a slow strain rate (north‐south shortening of ~1 mm/yr) [ Maesano et al , ].…”

Inhomogeneity of Macroseismic Intensities in Italy and Consequences for Macroseismic Magnitude Estimation

“…Even the comparison of magnitudes computed by BOXER (Gasperini et al, 1999(Gasperini et al, , 2010 using the intensity estimates made by the two scales shows a substantial coincidence (Table 2 and Table S1 of the supplemental material). This means that, apart from the data reported in the paper by Graziani et al (2015), EMS and MCS intensities estimated in Italy appear substantially equivalent.…”

“…If EMS and MCS were equivalent, intensities should coincide with each other, at least on average. Graziani et al (2015) found instead that EMS are smaller than MCS (OTC) by 0.6 degrees on average. Moreover, the macroseismic magnitude computed by BOXER (Gasperini et al, 1999(Gasperini et al, , 2010 using EMS is 0.28 m.u.…”

“…However, we can note that, contrary to Graziani et al (2015), none of papers providing MCS intensities in the last decade (Galli et al, 2012a, 2012b, Arcoraci et al, 2012, Rossi et al, 2019 mention the need of excluding from the computation of the percentage of damage the buildings made using modern building techniques like for example reinforced concrete.…”

“…A demonstration of the difference between the EMS and the MCS scale is given by the work of Graziani et al (2015) for the 2012 Pianura Emiliana earthquake sequence. They compared the cumulative (collected after both the strongest shocks of the sequence) EMS intensities estimated considering the whole building stock at the various localities by Tertulliani et al (2012) with the cumulative MCS intensities they estimated by only considering the old town centers (OTC) of the same localities where the building types were more similar to those existing at the time when Sieberg (1912Sieberg ( , 1932 Note that MCS, different from EMS and MSK, does not provide an explicit classification of building vulnerabilities.…”