Piccardi, Luigi

We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting.

We present a 1:25,000 scale map of the coseismic surface ruptures following the 30 October 2016 M-w 6.5 Norcia normal-faulting earthquake, central Italy. Detailed rupture mapping is based on almost 11,000 oblique photographs taken from helicopter flights, that has been verified and integrated with field data (>7000 measurements). Thanks to the common efforts of the Open EMERGEO Working Group (130 people, 25 research institutions and universities from Europe), we were able to document a complex surface faulting pattern with a dominant strike of N135 degrees-160 degrees (SW-dipping) and a subordinate strike of N320 degrees-345 degrees (NE-dipping) along about 28km of the active Mt. Vettore-Mt. Bove fault system. Geometric and kinematic characteristics of the rupture were observed and recorded along closely spaced, parallel or subparallel, overlapping or step-like synthetic and antithetic fault splays of the activated fault systems, comprising a total surface rupture length of approximately 46km when all ruptures were considered.

In this study we analyse themain potential seismic sources in some axial and frontal sectors of the Northern Apennines, in Italy. This regionwas hit by a peculiar series of earthquakes that started in 1916 on the external thrust fronts near Rimini. Later, in 1917–1921, seismicity (up to Mw ≈ 6.5) shifted into the axial zone and clearly migrated north-westward, along the belt of active normal faults. The collection of fault-slip data focused on the active normal faults potentially involved in this earthquake series. The acquired data allowed us to better characterize the geometry and kinematics of the faults. In a few instances, the installation of local seismic networks during recent seismic sequences allowed the identification of the causative faults that are hinted to be also responsible for past earthquakes, particularly in the Romagna region and north-eastern Mugello. The Coulomb stress changes produced by the historical earthquakes generally brought closer to failure all the faults that supposedly caused themain seismic events of 1916–1921. However, the stress change magnitude is generally small and thus the static stress interaction among the main seismic sources is not supported by a significant seismic correlation. Significant stress change loading may be instead inferred for the triggering of a number of seismic events on neighbouring normal faults by the Garfagnana 1920 earthquake. In addition, the computation of the seismic stress changes suggests that seismic events with magnitude ≥ 6 may transmit stresses from the axial normal faults to specific external thrusts and vice versa. It is possible that a correlation may be made between loading applied by the major 1917–1920 extensional ruptures and the increased seismicity on the distal external thrusts.

This paper presents a discussion on some aspects of the active tectonics in Bulgaria based on available literature data as well as recent research mainly focused on the identification and characterization of active faults in selected key areas. In particular, through tectonic, GPS and remote sensing data, also coupling geological observations with seismological data, we present here 1) a comprehensive map of major active faults in Bulgaria, although still in progress, and 2) a detailed study of various active structures, in particular in the Sofia Graben, Struma valley and surrounding regions. Our research highlights the occurrence in south-west Bulgaria of several major active faults, the majority of which are normal faults accommodating a roughly N-S to NNE-SSW extension, in line with GPS data and analysis of the instrumental seismicity. The extensional stress field, with associated tectonic and seismic activity, is likely to be related to the northern extent of the Aegean extensional system and its recent evolution. In the Balkan mountains area the tectonic activity is somehow more complex, responding to the interplay of the northward propagation of the Aegean extension and the influence of a roughly E-W compression, which determines local shear zones.

We examine the tectonic evolution and structural characteristics of the Quaternary intermontane Mugello, Casentino, and Sansepolcro basins, in the Northern Apennines fold-andthrust belt. These basins have been classically interpreted to have developed under an extensional regime, and to mark the extension-compression transition. The results of our study have instead allowed framing the formation of these basins into a compressive setting tied to the activity of backthrust faults at their northeastern margin. Syndepositional activity of these structures is manifested by consistent architecture of sediments and outcrop-scale deformation. After this phase, the Mugello and Sansepolcro basins experienced a phase of normal faulting extending from the middle Pleistocene until Present. Basin evolution can be thus basically framed into a two-phase history, with extensional tectonics superposed onto compressional structures. Analysis of morphologic features has revealed the occurrence of fresh fault scarps and interaction of faulting with drainage systems, which have been interpreted as evidence for potential ongoing activity of normal faults. Extensional tectonics is also manifested by recent seismicity, and likely caused the strong historical earthquakes affecting the Mugello and Sansepolcro basins. Qualitative comparison of surface information with depth-converted seismic data suggests the basins to represent discrete subsiding areas within the seismic belt extending along the axial zone of the Apennines. The inferred chronology of deformation and the timing of activity of normal faults have an obvious impact on the elaboration of seismic hazard models.

The source of the famous 1 November, 1755 ‘‘Lisbon’’ earthquake has been constrained to be an up to 200 km long structure in the offshore west of Cabo de S. Vincente.The magnitude of this earthquake was estimated in the range of 8.5–9.4. The stress regime argued for this shock would have been characterised by an around NNW–SSE-oriented compression.Less well studied is the successive ‘‘Meknes’’ earthquake, which occurred a few days later in Morocco (27 November), and was erroneously confused by the European contemporary reports with a strong aftershock, occurred on 18 November, of the main seismic sequence of the 1 November earthquake.The Meknes earthquake had destructive effects in the region of Meknes and Fes and along the E–W trending Rides Pre´rifaines, the main frontal thrust of the Rif.Historical data indicate a macroseismic field closed around the towns of Meknes and Fes. Our structural–geological fieldwork and remote sensing analysis in the epicentral area of the Meknes earthquake, along the local major recent faults, indicate that the E–W-oriented thrusts of the Rides Pre´- rifaines are active.Throu gh a re-examination of historical sources compared with field work and air photo interpretation, we could individuate the traces of coseismic surface faulting of the 1755 Meknes earthquake in two areas of the Rides Pre´rifaines, both part of the local thrust front: the Jebel Zerhoun area and the Jebel Zalagh area.Tectonic data on the Quaternary stress fields derived from our fieldwork and from literature, consistently with the revised focal mechanisms in the region, indicate active shortening oriented NNW–SSE to N–S in northern Morocco.The data collected seem therefore to indicate the thrusts of the Rides Pre´rifaines, located within the macroseismic area of the Meknes earthquake, as the most probable seismic source of that event.As such, the activation of the thrusts of the Rides Pre´rifaines would be consistent with this stress regime, which in turn would be similar to the stress field maintained as responsible for the 1 November, 1755 Lisbon earthquake.W e also attempted an estimate of the change due to the Lisbon earthquake of the Coulomb Failure Function (CFF) on the Meknes structure, as identified in this paper, in order to evaluate if the Meknes earthquake could have been induced by the 1 November, 1755 Lisbon earthquake, or a local distinct earthquake.Our modelling suggests that the latter hypothesis is the more likely one.

The November 1st, 1755 Lisbon earthquake has been largely studied, its source constrained to be a 200 long structure, its magnitude estimated around 8.7, and the stress regime argued as a NNW-SSE directed compression. Less well knoll is the Meknes earthquake, occurred a few days later in Morocco and considered the effect of a strong aftershock in Portugal. It had destructive effects in the Meknes region, along the E-W trending Rides Prérifaines, the main frontal thrust of the Rif. Historical data indicate an E-W elongation of the epicentral macroseismic area, where we surveyed the major recent faults. Historical descriptions of the earthquake indicate ground ruptures in two areas of the Rides Prérifaines. Field work and air photo interpretation allowed us to relate these ruptures with the local thrust front, so that they may be assumed to correspond to coseismic surface faulting of the Meknes earthquake

The city of Florence possesses a concentration of cultural and artistic treasures which is unique in the world. In this sense it has a particularly high seismic exposure and a potentially high vulnerability. In order to better evaluate its seismic hazard and risk, we analyzed the seismic response of the urban area of Florence by performing a multidisciplinary study on the effects of earthquakes on the city. By a computer aided methodology we re-evaluated the seismic intensity reports of the May 18 and June 6, 1895 earthquakes in different parts of the city and compared these data with recent studies on surface geology, active tectonics and actual fault movements in the Florence basin. We concluded that more detailed studies of soil response are needed to form a basis for public policy.