The MCS Intensity Distribution of the Devastating 24 August 2016 Earthquake in Central Italy (MW 6.2)
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ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7287 The MCS intensity distribution of the devastating 24 August 2016 earthquake in central Italy (MW 6.2) P. GALLI1*, E. PERONACE2, F. BRAMERINI1, S. CASTENETTO1, G. NASO1, F. CASSONE1, F. PALLONE1 1 Dipartimento della Protezione Civile Nazionale, Rome Italy; 2 Consiglio Nazionale Delle Ricerche, Istituto di Geologia Ambientale e Geoingegneria, Monterotondo Scalo, Rome, Italy * Corresponding author: [email protected] Abstract Here we describe the macroseismic survey of the 24 August 2016 earthquake in central Italy (MW 6.2). By apply- ing a revised version of the Mercalli-Cancani-Sieberg scale, we estimated the site intensity in more than 300 lo- calities of Lazio, Abruzzi, Umbria and Marche regions, providing the Civil Protection with a quick and robust snapshot of the earthquake. The most severe effects are focused south of the instrumental epicenter, in the Ama- trice intermountain basin, where intensity reached 10-11 MCS. Highest damage (area inside 9 MCS isoseismal) is focused in a NNW-SSE belt of the hangingwall of the causative faults, i.e. the southern segment of the Mount Vettore fault system and the northern segment of the Laga Mounts fault system, with northward damage propa- gation in the far-field. The intensity dataset allows to evaluate a MW 6.16±0.5, which is very close to the instru- mental magnitude, with a seismogenic box striking N161°, mimicking the geological active faults. Epicentral in- tensity is I0 10 MCS, IMAX 10-11. The elevated level of destruction is mainly due to the high vulnerability of build- ings, mostly made by cobblestone masonry. Integrating the macroseismic information with the geological, geo- detical and geophysical data it is possible to hypothesize a bidirectional rupture propagation (toward NNW and SSE) along the two different faults. It is also possible to attribute the 1639, MW 6.0 earthquake to the same source of the southern 2016 rupture (northernmost Laga Mounts faults). I. INTRODUCTION (Lazio Region), at a depth of approx. 8 km, and it was shortly followed by a foreshock (MW 5.5) n the night of 24 August 2016, at 3:36 local localized 10 km away, in the Norcia area (Um- time, a unexpected earthquake hit the bria Region). Both these events were clearly Iboundary area of four Regions of central It- felt in Rome, which is some 100 km far away, aly (Lazio, Abruzzi, Marche and Umbria), while the damage induced to the buildings es- rocking dozens of villages and burying under tate extended up to 40 km to the north. None- the rubbles hundreds of unaware, sleeping theless, the earthquake devastated mainly the people. The instrumental hypocenter of the localities south of the epicenter, namely the mainshock (ML 6.0; Mw 6.2) was below the vil- small town of Amatrice and its villages (locally lage of Accumoli, in the Tronto River valley known as ville; 3000 inhabitants that increase ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7287 tenfold during summer vacation). In October 30 October). The effects of these earthquakes the sequence was still ongoing, with more than are not discussed here. Suffice to say that al- a dozen of ML≥4 events scattered over 30 km most all villages surveyed during August- long area in a NNW-SSE direction, besides al- September suffered further damage, making most 800, ML≥2.5 [ISIDe 2016]. Almost all the new field work necessary. focal mechanisms indicate NW-SE normal faulting, i.e. extension in a NE-SW direction II. HINTS ON THE LOCAL HISTORICAL SEISMICITY [RCMT 2016]. Together with the nearby 2009 L'Aquila earth- The Amatrice area - besides the strong effects suffered during the catastrophic 1703 January- quake (MW 6.3), this is the strongest event that February seismic sequence [Uria de Llanos struck Italy since the 23 November 1980, MW 1703] (January 14, M 6.9 in the Norcia region; 6.9 Irpinia earthquake (southern Apennine, I0 W 10 MCS, almost 3000 casualties), with a death 2 February, MW 6.7 in the L'Aquila region) - toll reaching 297, i.e., quite the same sorrowful was the source region of a seismic period oc- th amount of L'Aquila in 2009. curred in the first half of the 17 century. On 7 October 1639 a first event (M 6.0, Io 9-10 Starting from 1997 Umbria-Marche event (MW W 6.0), just a few hours after every strong earth- MCS) hit many ville of Amatrice (Cantone, quake in Italy, different teams of experts from Casale, Colle Basso, Collemoresco, Corva, Fi- the Dipartimento della Protezione Civile letto, Forcelle, La Rocca, Roccasalli, San Gior- (DPC), and from the Istituto Nazionale di gio, San Martino, Scai, and Torrita: all 9-10 Geofisica e Vulcanologia (INGV) are dis- MCS), causing severe damage in Amatrice it- patched to the epicentral area in order to self (9 MCS) and Accumoli (8-9 MCS) [Tiberi launch the macroseismic survey. The main 1639], beside being felt in the far towns of purpose of this survey is to provide the Civil L'Aquila, Ascoli Piceno and Rieti, but not in Protection with 1) a univocal snapshot of the Rome [Castelli et al. 2002]. In Norcia, a pri- th heavy damage distribution within a couple of mary source [ASCN 17 cent.] suggests the ab- days, and 2) a reliable distribution of damage sence of any important effect, differently from in the far field within 20-30 days. Thus, before what happened in August 2016. On 28 April the end of September 2016, the teams from the 1646 a second event hit again some of the vil- DPC and INGV visited more than 300 localities lages around Amatrice, causing severe damage within 76 municipalities of the Provinces of in Scai and Torrita (9 MCS), and also in Ama- th Rieti (Lazio), Perugia (Umbria), Teramo and trice (8 MCS) [ASAQ 17 cent.]. All together, L'Aquila (Abruzzi), Ascoli Piceno, Macerata these two earthquakes had a smaller impact and Fermo (Marche), and assessed the site in- than the 2016 one, as they were mainly focused tensity (Is) applying the Mercalli-Cancani- in the Amatrice basin, without known effects Sieberg (MCS) scale [Sieberg, 1930]. The evalu- to the north. Indeed, according to Castelli et al. ated intensity datapoints distribution (IDD; [2002], the 1639 earthquake was caused by the Appendix 1) later will merge into the Italian rupture of a limited segment of the Laga catalogue of historical earthquakes and the as- Mounts fault, the total slip of which being in- sociated macroseismic intensities database (i.e., stead consistent with MW>6.5 earthquakes CPTI and DBMI. See Rovida et al. 2016, and [Galadini and Galli 2003]. Locati et al. 2016). It is worth noting that during the editing of III. BUILDING TYPOLOGY OF THE AREA this paper other two M >6 earthquakes oc- W As the earthquake struck a sector of the Apen- curred approx. 20 km NNW with respect to the nine chain mainly characterized by extensive 24 August event (Mw 6.1, 26 October; M 6.5, W outcrops of turbiditic sandstones (Laga ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7287 Flysch), these rocks constitute the preferred churches, castles and towers of the region. building material of all the houses, stables, Figure 1. A, Rounded, fluvial cobbles masonry in Sommati (Amatrice). B, Collapse of a stone masonry building with RC ring beans (San Lorenzo e Flaviano, Amatrice). C, Crashed RC roof over a stone masonry house; in the background, view of the destroyed village of Pescara del Tronto (10-11 MCS). D, Collapse of stone masonry house in Petrana; behind, note the part of the same building added interiorly with a steel moment-resisting frame. E, Collapse of a RC building in Amatrice. Indeed, buildings in the historical centers of all have been strengthened in the past decades the villages are mostly made of rough stone with reinforced concrete ring beams, iron tie masonry, often perfectly rounded fluvial cob- rods, while some were imprudently added with bles and boulders (Fig. 1A), with squared angu- heavy concrete roof that behaved like a ram lar stone and a very poor, dusty mortar. Secon- during the earth shaking, crushing the underly- darily, brick masonry (clay, hollow, and con- ing masonry (Fig. 1B-D). Relatively few rein- crete bricks) is diffused in most villages, as well forced-concrete frame buildings and villas were as Poroton® (i.e., load-bearing clay-brick with built in the outskirts of Amatrice (1E), and in low void percentage). Many of this buildings some other villages. At the same time, inside date back to the 15th-early 20th century; some the historical centers of almost all the villages, ANNALS OF GEOPHYSICS, 59, FAST TRACK 5, 2016; DOI: 10.4401/ag-7287 derelict and stables houses coexist close to in- The macrocosmic survey was carried out with habited structures the collaboration of the QUEST-INGV team, as in Galli et al. [2016]. It evidenced a huge vari- IV. THE MACROSEISMIC SURVEY ability of the building typologies within the same locality, with the presence of preexisting Also in this earthquake, the assessment of the crumbling structures, well reinforced masonry site intensity (IS) was achieved by applying the buildings, overloaded roofs, old and new RC MCS macroseismic scale, i.e., the scale adopted buildings. Sometime this variability makes the for the whole Italian macroseismic database intensity assessment difficult or not just easily (DBMI15). Indeed, as the European Macroseis- expressed by the usual uncertainness between mic scale (EMS) [Grünthal 1998] requires the two consecutive degrees (e.g., 6-7 MCS).