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evidences from the preLiminary macroseismic surveyof the august 24, 2016 amatrice seismic sequence (centraL itaLy) M.A. Zanini, L. Hofer, F. Faleschini, P. Zampieri, N. Fabris, C. PellegrinoDipartimento di Ingegneria Civile, Edile e Ambientale, Universita’ degli Studi di Padova, Italy

Introduction. On August 24, 2016, at 3:36 local time (1:36 UTC) a Mw 6.0 earthquake occurred in Central Italy, at the boundaries of Lazio, Abruzzo, Marche and Umbria regions. The Italian Strong Motion Network of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) located the epicentre at 42.70°N and 13.24°E, between the Municipalities of Accumoli and Amatrice, in the Lazio region (ISIDe, 2016). The area hit by the seismic event is highly prone to seismic hazard and in the past twenty years two important earthquakes occurred. In 1997 a Mw 5.97 earthquake hit the Umbria region, while in 2009 L’Aquila Earthquake caused about 300 victims. Historically, the Parametric Catalogue of Italian Earthquakes (CPTI15) (Rovida et al., 2016) shows the high seismic activity of the examined zone in which from the eighteen century a mean annual rate of about 0.25 events with Mw greater than 4.0 was observed. First memories of the seismic activity in the area of Accumuli, date back to the Mw 5.3 Monti della Laga event, which occurred on July 1627 and related to a I0 7-8 MCS macroseismic level of damage (Monachesi and Castelli, 1992). On October 7, 1639, a Mw 6.21 (I0 8-9 MCS) earthquake hit Amatrice and its fractions (Castelli, 2013). The strongest event occurred in 1703 (January 14; I0 10 MCS, Mw 6.92), ca. 20 km from the 2016 epicenter and subsequently in 1730 the area was re-struck by a Mw 6.04 earthquake (May 12, I0 7 MCS) (Guidoboni et al., 2007). After about 150 years, on November 7, 1883, these territories were hit again by the Monti della Laga earthquake (I0 7 MCS, Mw 5.1) and finally in the twentieth century other three significant events occurred (1916, 1950 and 1979) (Tertulliani et al., 2006).

Survey methodology. After the 2016 Amatrice seismic sequence, teams of the University of Padova performed a field survey of the entire area involved by the earthquake, aiming to develop the macroseismic damage scenario of the event. The Macroseismic-Intensity assessment was performed according to the European Macroseismic Scale (EMS 98) (Grünthal, 1998) and the survey was continuously updated until September 6, 2016 to consider the effects of the cumulative damage caused by the occurred aftershocks. The teams performed detailed survey of 180 localities, assessing the intensity level on the basis of the structural damage observed and the vulnerability class of the buildings. These localities are distributed within the Provinces of Rieti (Lazio), Aquila and Teramo (Abruzzo), Perugia (Umbria) and Ascoli Piceno, Fermo and Macerata (Marche). In addition, short interviews on the earthquake perception were performed to better characterize especially the lowest grades of damage and macroseismic intensity levels. Most of the structures in the surveyed area, are one-or-two-story old stone-masonry residential buildings, with low quality mortar joints and with lack of connections (vulnerability class A and B).

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The building stock is also composed by recently retrofitted masonry structures (mostly renovated after the 1997 Umbria earthquake, to which a vulnerability class D was assigned) and seismically designed reinforced concrete frame buildings (vulnerability class C). Industrial warehouses are not included in the EMS98 scale since they are not considered representative for the intensity value assessment, anyway the surveyed area is mostly mountainous without the presence of significant industrial zones.

Intensity assessment and discussion of results. The 2016 Amatrice seismic sequence was characterized by one severe mainshock (1:36 UTC, Mw 6.0), and subsequently by only one relevant aftershock (2:33 UTC, Mw 5.3): as a consequence, the observed intensity map can reasonably be seen as the effects of a seismic scenario. According to the EMS98 scale and assessing method, the intensity values for each locality was established on the basis of the damage grades detected on the different vulnerability classes. In some localities, troubles on the evaluation of the IEMS intensity were detected, mainly due to the different grade of damage suffered by old stone-masonry residential buildings and reinforced concrete or retrofitted masonry structures. According to Galli et al. (2009), effects recorded by the building inside the old city centers were considered more reliable for a proper intensity assessment. Fig. 1 shows the surveyed localities with the observed macroseismic intensity IEMS.

Fig. 1 – Macroseismic IEMS intensities observed for the 2016 Amatrice seismic sequence (cumulative damage up to September 6, 2016).

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The major intensities were observed in Pescara del Tronto, Saletta and Amatrice (respectively IEMS 10-11, IEMS 10, IEMS 9-10), which were mainly justified by a diffuse grade 5 of damage to several masonry buildings (Fig. 2).

Fig. 2 – Diffused collapses in Amatrice (left) and aerial view of Pescara del Tronto (right, retrieved on www.meteoweb.eu).

Up to a radius of about 12 km far from the instrumental epicentre, damages compatible with a IEMS 8 level were observe. In these localities most of the residential masonry building (vulnerability class B) experienced a grade 3 of damage (Fig. 3). The villages of Vezzano, Spelonga and Colle d’Arquata (IEMS 6) showed a particular very slight grade of damage (Fig. 3), even if quite close to the instrumental epicentre zone, probably due to some beneficial site effects.

Fig. 3 – Large diagonal cracks in most walls (Preta IEMS 8-9, left) and slight damages in Spelonga (IEMS 6, right).

The intensity level IEMS evaluation was quite difficult for the centers located in the southeaster area, in the Abruzzo region, since many building were still presenting some effects induced by the 2009 L’Aquila earthquake. The low level of ground shaking experienced by this area, together with need of quantify the increment of damages, made the macroseismic intensity assessment quite challenging.

Conclusions. This work illustrates the first preliminary results of a macroseismic survey performed in the area hit by the August 24, 2016 Amatrice earthquake and provides the map of the intensity level experienced by 180 localities. The field observations and some of the preliminary results shows the significant influence of site effects for some localities. Finally the particularity of the area in which the earthquake occurred, introduced the problem of dealing with pre-damaged (2009 L’Aquila earthquake) and seismically retrofitted residential buildings (1997 Umbria earthquake) for the macroseismic intensity assessment.

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Acknowledgements. The authors would like to thank Prof. Alberto Bernardini for the helpful discussion and suggestions about these topics.

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