Re-evaluation of minor events: The examples of the 1895 and 1909 Rome earthquakes

The city of Rome is subjected to moderate seismic risk due to both local and external seismicity. Up to now, the maximum intensity felt has never exceeded VIII MCS. The 1 November 1895 (I _o = VII) and 31 August 1909 (I _o = VI) earthquakes demonstrate that small local events can also cause damage in a large old city. In the present work, we have re-evaluated the intensity values of those two events by means of automatic processing. A comparison between the present results with geological evidence and previous studies is shown, especially for the historical centre of Rome. For the first time, the 1909 earthquake instrumental magnitudeM _L = 3.6 has been calculated from original recordings.     

The determination of the epicentre by a vectorial modelling of macroseismic intensity distribution

A vectorial modelling of observed macroseismic intensity aimed at the analytical determination of the epicentre is proposed here. The methodology is based on the determination of a plane system of vectors which characterises the macroseismic intensity distribution. The epicentre of each seismic event considered is determined as the centre of this vector system by an analytical expression which is independent from all possible directions of seismic energy propagation. The analysis of the intensity distribution is carried out by a new model called a macroseismic plane, different from the one known as macroseismic field, formed by a set of small areas built around the observed intensity points; hence its name.With the proposed methodology, some earthquakes in southern Italy and eastern Sicily are analysed calculating their epicentres, also for distributions of observed intensity which are particularly complex.     

An attempt to evaluate seismic hazard in Central-Southern Italy

A method for the evaluation of seismic hazard in a given zone, taking into account both the spreading of macroseismic effects and seismic catalogue information, is applied. A data-bank of some 500 digitized isoseisms of earthquakes having occurred in Italy between 1542 and 1986 is used. The isoseismical maps are digitized considering for each degree of intensity the length of 24 spreading rays starting from the macroseismic epicenter or barycentre of the megaseismic area. These rays are separated from each other by the same angle, i.e. every isoseism is divided into 24 equal circular sectors. The year 1542 is taken as the beginning of the time span, since this is when the first seismic event occurred for which reliable isoseismal maps are available. The epicentral intensities considered lie between theVI andXI degrees of the Mercalli-Cancani-Sieberg scale (MCS). This digitized data-bank is analyzed to achieve, for each homogeneous seismogenetic zone that has been recognized, the mean azimuthal spreadings of effects for each degree of intensity as a function of the epicentral intensity. Once a mean propagation model is obtained for each zone, this is applied to seismic events of the same zone, the isoseismal maps of which are not available. A geographic grid is defined to cover the analyzed area, and for each cell of this grid it is then possible to count the number of felt events and their degree. These effects have been evaluated either on the basis of the isoseismal maps (when available) or on the basis of the mean propagations of the zone in which the single event occurred. Moreover, an index summarizing the seismic information was computed for each cell of the previous grid. All the events producing effects and their provenance are stored on files, allowing the main seismogenic zones influencing this cell to be identified. This methodology has been applied to central and southern Italy in an area between the latitudes 40.6 and 43.3 N. In particular, attention is focussed on the sample areas of Rome (given the historical and political importance of the city) and of the Sannio-Matese and Irpinia zone (in which some of the strongest earthquakes of the Apennine chain have occurred). Finally, in order to evaluate the maximum expected magnitude, extreme value statistics (Gumbel III-type) are applied to the Colli Albani area, which represents the seismogenic zone nearest to Rome. For the Sannio-Matese and Irpinia area, considering the more dangerous zone as a ‘unicum’, theWeibull distribution has been hypothesized to determine the mean return time for events with an intensity greater than or equal to IX.     

Strong motion scenario of 25th November 2000 earthquake for Absheron peninsula (Azerbaijan)

Urban earthquake scenario requires compilation and interpretation of topographical, geological, geotechnical, macroseismic, and instrumental data, along with identification of proper ground motion prediction and site response analysis. Within the intensive city planning and infrastructure improvement of Baku city (the capital of Azerbaijan), and due to land and water instabilities, intensified landslides, and increasing seismic activity, Absheron peninsula has turned into one of the strategic earthquake case studies, representing exposure to earthquake hazard in the region. The last strongest 25th November 2000 earthquake revealed that the peninsula was severely vulnerable to seismic events, since there was a lack of public awareness of seismic disaster and its consequences, and there were not any preventive measures which might have been derived from the scenario-based simulations and prediction of strong motion distribution over the area. In the present work, integrated analysis of seismicity, engineering geology, geomorphology, topography, and site response is used to model strong motion dynamics in terms of peak ground acceleration distribution and intensity level for Absheron peninsula along with Baku city. The strong motion scenario of the 25th November 2000 earthquake shows that the larger area of the peninsula coincides with the VIII–IX intensity level, including Baku city. The scenario distribution can be valuable in all phases of the disaster management process.     

An attempt to evaluate seismic hazard in Central-Southern Italy

A method for the evaluation of seismic hazard in a given zone, taking into account both the spreading of macroseismic effects and seismic catalogue information, is applied. A data-bank of some 500 digitized isoseisms of earthquakes having occurred in Italy between 1542 and 1986 is used. The isoseismical maps are digitized considering for each degree of intensity the length of 24 spreading rays starting from the macroseismic epicenter or barycentre of the megaseismic area. These rays are separated from each other by the same angle, i.e. every isoseism is divided into 24 equal circular sectors. The year 1542 is taken as the beginning of the time span, since this is when the first seismic event occurred for which reliable isoseismal maps are available. The epicentral intensities considered lie between theVI andXI degrees of the Mercalli-Cancani-Sieberg scale (MCS). This digitized data-bank is analyzed to achieve, for each homogeneous seismogenetic zone that has been recognized, the mean azimuthal spreadings of effects for each degree of intensity as a function of the epicentral intensity. Once a mean propagation model is obtained for each zone, this is applied to seismic events of the same zone, the isoseismal maps of which are not available. A geographic grid is defined to cover the analyzed area, and for each cell of this grid it is then possible to count the number of felt events and their degree. These effects have been evaluated either on the basis of the isoseismal maps (when available) or on the basis of the mean propagations of the zone in which the single event occurred. Moreover, an index summarizing the seismic information was computed for each cell of the previous grid. All the events producing effects and their provenance are stored on files, allowing the main seismogenic zones influencing this cell to be identified. This methodology has been applied to central and southern Italy in an area between the latitudes 40.6 and 43.3 N. In particular, attention is focussed on the sample areas of Rome (given the historical and political importance of the city) and of the Sannio-Matese and Irpinia zone (in which some of the strongest earthquakes of the Apennine chain have occurred). Finally, in order to evaluate the maximum expected magnitude, extreme value statistics (Gumbel III-type) are applied to the Colli Albani area, which represents the seismogenic zone nearest to Rome. For the Sannio-Matese and Irpinia area, considering the more dangerous zone as a unicum, theWeibull distribution has been hypothesized to determine the mean return time for events with an intensity greater than or equal to IX.     

Regionalization of seismic hazard in Greece based on seismic sources

A semi-probabilistic approach to the seismic hazard assessment of Greece is presented. For this reason, a recent seismotectonic model for shallow and intermediate depth earthquake sources, based on historical as well as on instrumental data, was used. Different attenuation formulae were proposed for the macroseismic intensity and the strong ground motion parameters for the shallow and the intermediate focal depth shocks. The data were elaborated in terms of McGuire's computer program, which is based on the Cornell's method.A grid of equally spaced points at 20 km distance was made and the seismic hazard recurrence curves for various parameters of the seismic intensity was estimated for each point. Finally, seismic hazard maps for the area of Greece were compiled utilizing the entire range of recurrence curves. These maps depict areas of equal seismic hazard and for every area the analytical relations of the typeSI =f(Tm), whereSI is a seismic intensity parameter andTm is the mean return period, were determined.     

Historical seismicity on the southern margin of the Siberian craton: new data

We discuss historical evidence for seismicity on the southern margin of the Siberian craton collected from old local newspapers. The reported earthquakes vary in magnitude from M = 2.5 to 4.5, and their macroseismic locations agree well with the regional tectonic framework. The new data prove seismic activity in the area and can be used in seismic risk assessment.     

Vulnerability index and capacity spectrum based methods for urban seismic risk evaluation. A comparison

This article contributes to the development and application of two latest-generation methods of seismic risk analysis in urban areas. The first method, namely vulnerability index method (VIM), considers five non-null damage states, defines the action in terms of macroseismic intensity and the seismic quality of the building by means of a vulnerability index. The estimated damage degree is measured by semi-empirical functions. The second method, namely capacity spectrum based method (CSBM), considers four no damage states, defines the seismic action in terms of response spectra and the building vulnerability by means of its capacity spectrum. In order to apply both methods to Barcelona (Spain) and compare the results, a deterministic and a probabilistic hazard scenario with soil effects are used. The deterministic one corresponds to a historic earthquake, while the probabilistic seismic ground motion has a probability of exceedence of 10% in 50 years. Detailed information on the building design has been obtained along years by collecting, arranging, improving, and completing the database of the dwellings of the city. A Geographic Information System (GIS) has been customized allowing storing, analysing, and displaying this large amount of spatial and tabular data of dwellings. The obtained results are highly consistent with the historical and modern evolution of the populated area and show the validity and strength of both methods. Although Barcelona has a low to moderate seismic hazard, its expected seismic risk is significant because of the high vulnerability of its buildings. Cities such as Barcelona, located in a low to moderate seismic hazard region, are usually not aware of the seismic risk. The detailed risk maps obtained offer a great opportunity to guide the decision making in the field of seismic risk prevention and mitigation in Barcelona, and for emergency planning in the city.     

Macroseismic effects highlight site response in Rome and its geological signature

A detailed analysis of the earthquake effects on the urban area of Rome has been conducted for the L??Aquila sequence, which occurred in April 2009, by using an online macroseismic questionnaire. Intensity residuals calculated using the mainshock and four aftershocks are analyzed in light of a very accurate and original geological reconstruction of the subsoil of Rome based on a large amount of wells. The aim of this work is to highlight ground motion amplification areas and to find a correlation with the geological settings at a subregional scale, putting in evidence the extreme complexity of the phenomenon and the difficulty of making a simplified model. Correlations between amplification areas and both near-surface and deep geology were found. Moreover, the detailed scale of investigation has permitted us to find a correlation between seismic amplification in recent alluvial settings and subsiding zones, and between heard seismic sound and Tiber alluvial sediments.     

The record of historic earthquakes in lake sediments of Central Switzerland

Deformation structures in lake sediments in Central Switzerland can be attributed to strong historic earthquakes. The type and spatial distribution of the deformation structures reflect the historically documented macroseismic intensities thus providing a useful calibration tool for paleoseismic investigations in prehistoric lake sediments.The Swiss historical earthquake catalogue shows four moderate to strong earthquakes with moment magnitudes of M_w=5.7 to M_w=6.9 and epicentral intensities of I₀=VII to I₀=IX that affected the area of Central Switzerland during the last 1000 years. These are the 1964 Alpnach, 1774 Altdorf, 1601 Unterwalden, and 1356 Basel earthquakes. In order to understand the effect of these earthquakes on lacustrine sediments, four lakes in Central Switzerland (Sarner See, Lungerer See, Baldegger See, and Seelisberg Seeli) were investigated using high-resolution seismic data and sediment cores. The sediments consist of organic- and carbonate-rich clayey to sandy silts that display fine bedding on the centimeter to millimeter scale. The sediments are dated by historic climate and environmental records, ¹³⁷Cs activity, and radiocarbon ages. Deformation structures occur within distinct zones and include large-scale slumps and rockfalls, as well as small-scale features like disturbed and contorted lamination and liquefaction structures. These deformations are attributed to three of the abovementioned earthquakes. The spatial distribution of deformation structures in the different lakes clearly reflects the historical macroseismic dataset: Lake sediments are only affected if they are situated within an area that underwent groundshaking not smaller than intensity VI to VII. We estimate earthquake size by relating the epicentral distance of the farthest liquefaction structure to earthquake magnitude. This relationship is in agreement with earthquake size estimations based on the historical dataset.     

Lessons learned from two case histories of seismic microzonation in Italy

The prediction of the variability of the seismic ground motion in a given built-up area is considered an effective tool to plan appropriate urban development, to undertake actions on seismic risk mitigation and to understand the damage pattern caused by a strong-motion event. The procedures for studying the seismic response and the seismic microzonation of an urban area are well established; nevertheless, some controversial points still exists and are discussed here. In this paper, the selection of a reference input motion, the construction of a subsoil model and the seismic response analysis procedures are discussed in detail, based on the authors’ experience in two Italian case histories: the seismic microzonation of the city of Benevento, which was a predictive study, and the simulation of seismic response and damage distribution in the village of San Giuliano di Puglia, which was a retrospective analysis.     

Constraints on the location and mechanism of the 1511 Western-Slovenia earthquake from active tectonics and modeling of macroseismic data

The 1511 Western Slovenia earthquake (M = 6.9) is the largest event occurred so far in the region of the Alps–Dinarides junction. Though it strongly influences the regional seismic hazard assessment, the epicenter and mechanism are still under debate. The complexity of the active tectonics of the Alps–Dinarides junction is reflected by the presence of both compressional and transpressional deformations. This complexity is witnessed by the recent occurrence of three main earthquake sequences, the 1976 Friuli thrust faulting events, the 1998 Bovec–Krn Mountain and the 2004 Kobarid strike-slip events. The epicenters of the 1998 and 2004 strike-slip earthquakes (M_s = 5.7 and M_s = 4.9, respectively) lie only 50 km far from the 1976 thrust earthquake (M_s = 6.5).We use the available macroseismic data and recent active tectonics studies, to assess a possible epicenter and mechanism for the 1511 earthquake and causative fault. According with previous works reported in the literature, we analyze both a two-and a single-event case, defining several input fault models. We compute synthetic seismograms up to 1 Hz in an extended-source approximation, testing different rupture propagations and applying a uniform seismic moment distribution on the fault segments. We extract the maximum horizontal velocities from the synthetics and we convert them into intensities by means of an empirical relation. A rounded-to-integer misfit between observed and computed intensities is performed, considering both a minimized and a maximized databases, built to avoid the use of half-degree macroseismic intensity data points. Our results are consistent with a 6.9 magnitude single event rupturing 50 km of the Idrija right-lateral strike-slip fault with bilateral rupture propagation.     

Studies on the contribution of some geological factors to seismic hazard

The present study is primarily meant to make a separation of the amplification effects in the intensity distribution within the macroseismic field function of their different causes, i.e. the activation of some regional tectonic lines during the seismic motion and local seismogeological conditions.With this end in view, the methodology used consists of filtering the direct macroseismic observations by means of the weighted mean-values method and outlining the regional and local field anomalies.The regional anomaly map of the macroseismic field of the 4 March 1977 Vrancea earthquake, including the southern and south-eastern part of the Outer Carpathian Zone, has been analyzed. Some dislocations, located in the platform basement, which functioned as seismic energy amplifiers during the event, have been identified.The results obtained can be used in seismotectonic and microzoning studies.Paper presented at the 21st General Assembly of the European Seismological Commission, Sofia, 1988.     

Atlas of macroseismic maps for French earthquakes with their principal characteristics

The SIRENE macroseismic database has been utilized to draw isoseismal maps for the 140 best-documented French earthquakes, characterized by epicentral intensities of at least V (MSK) and located in all parts of the country. A study of focal depths derived from available local intensity data using an intensity versus distance decay law (Sponheuer) shows that the focal depths of most of the events considered do not exceed about 10 km. Their distribution correlates fairly well with regional dynamic geology features. A relationship is then computed between magnitude, intensity and focal distance, based on 73 instrumenta]ly recorded earthquakes (M _L between 3.3 and 6.3) and on 217 mean radius values (from 2 to 380 km) for isoseismals of intensity VIII to III (MSK). This relationship is applied to historical earthquakes contained in the database SIRENE which are characterised by their intensity only. These results are used in the evaluation as well deterministic as probabilistic of the seismic hazard on the national territory.     

Intensity field of the 19 June 1975 Gargano (Southern Italy) earthquake

Many moderate events reported by Italian earthquake catalogues (either historical or recent) are listed with an epicentral intensity derived from intensitymagnitude relationships or evaluated based on preliminary sources. Contradictions may arise among different catalogues when the effects of a given earthquake are not assessed through a specific macroseismic study as each catalogue generally uses its own criteria for evaluating the intensity. In this paper we present the case of the June 19 1975 earthquake, a M_L = 5.1 (ING seismological bulletin) event that occurred in the Gargano area (southern Italy). The intensity reported by the ING catalogue is VIII MCS (estimated from magnitude), that reported by the NT4.1 catalogue is VI MCS, while the PFG catalogue does not report an intensity. The case of this event is well representative of a period during which macroseismic studies were not undertaken systematically in Italy. In this paper we reassess the macroseismic intensity of this event using procedures implemented and routinely used at ING.     

Verification of macroseismic methods on five ML > 5 instrumental earthquakes in France

Macroseismic data available for five of the most recent M_L > 5 earthquakes that occurred in the Pyrenees and in the Alps, were analyzed using the Sponheuer and the Levret relationship to estimate depth and magnitude respectively. The aim of this paper is to verify if simple and robust macroseismic methods used on recent instrumental earthquakes may provide a good tool to calibrate historical events in France. The excellent agreement found between macroseismic and instrumental estimates shows that macroseismic data of historical events may provide the means to lengthen the instrumental catalogue and better constrain the recurrence rates of earthquakes in moderate seismic rate regions.     

Reassessing intensity of some Friuli earthquakes at the turn of the eighteenth century

During 1770–1820 Northeastern Italy was hit by a series of high intensity earthquakes affecting the Piedmont area of Friuli from Maniago to Tolmezzo. Greater knowledge of these events, which seem to be extremely circumscribed and to have damaged only a small number of localities (1776, 1789, 1794: Tramonti; 1788, 1790: Tolmezzo; 1812: Cavasso), could make a significant contribution towards defining better the potential seismic hazard in Northern Friuli. A review of these shocks has been undertaken within the framework of activities organized by the macroseismic working group of the National Group for Protection against Earthquakes (GNDT). The critical revision of the information gathered by the programme ‘analysis through the compilations’, has stressed the need for a new interpretative method and for great caution to be exercised when assessing reliable intensity degrees, to avoid possible inconsistencies in their values. This preliminary investigation allowed us to identify both doubtful and some misestimated shocks. A rough macroseismic intensity distribution pattern of each event, showing that the earthquakes were felt over an area (including Italian, Slovene and Austrian territories) coherent with the epicentre intensity, has been also delineated.     

Preliminary results of a macroseismic survey of the Colfiorito sequence (Central Italy)

Umbria-Marche region (Central Italy) has been interested by a seismic sequence, which caused a large amount of damage within an extensive area (around 5000 km²). The sequence produced eight shocks with magnitude higher than 5.0, the largest of them occurring on September 26, UTC 09:40, M₁ = 5.8. The incidence of many shocks with a magnitude higher than 4.0 contributed to the creation of a damage pattern that was in evolution for more than one month. Such seismic behavior motivated field operators to perform a real-time macroseismic survey to update the data set. One of the major objectives was to differentiate the effects pertaining to each of the largest shocks. In this paper we present the macroseismic survey performed during the sequence, along with some preliminary results inferred. Intensity points of seven of the main shocks are shown, together with the filtered macroseismic fields produced following the application of specific statistical methods. Finally, consideration is given to the comparison of these study results with corresponding tectonic and geological data.     

A study of the seismic response of the city of Benevento (Southern Italy) through a combined analysis of seismological and geological data

A previous analysis [Improta, L., G. Di Giulio, and A. Rovelli (2005). Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings, J. Seism. 9, 191–210.] of small magnitude earthquakes recorded at 12 sites within the city of Benevento has stressed the significant role played by near-surface geology in causing variability of the ground motion. In this paper, we extend the study of the seismic response from 12 sites to the entire urban area. Based on inferences from the comparison at the 12 sites between earthquake and ambient vibration results, we have collected ambient noise at about 100 sites within the city, intensifying measurements across the main shallow geological variations. We use borehole data to interpret ambient noise H/V spectral ratios in terms of near-surface geology comparing H/V curves to theoretical transfer functions of 1D models along five well-constrained profiles.

On the basis of geological, geotechnical, and seismic data, we identify three main typologies of seismic response in the city. Each type of response is associated to zones sharing common soil conditions and similar soil classes according to building codes for seismic design. Moreover, we find that the spatial variation of the seismic response in the ancient town area is consistent with the damage pattern produced by a very destructive, well-documented historical earthquake that struck the city in 1688, causing MCS intensity of IX–X in Benevento.

Finally, we use ground motions recorded during the experiment by Improta et al. [Improta, L., G. Di Giulio, and A. Rovelli (2005). Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings, J. Seism. 9, 191–210.] to generate synthetic seismograms of moderate to strong (Mw 5.7, Molise 2002 and Ms 6.9, 1980 Irpinia) earthquakes. We calibrate the random summation technique by Ordaz et al. [Ordaz, M., J. Arboleda, and S.K. Singh (1995). A scheme of random summation of an Empirical Green's Function to estimate ground motions for future large earthquakes, Bull. Seism. Soc. Am. 85, 1635–1647.] using recordings of these earthquakes available in Benevento. After a satisfactory fit between observed and synthetic seismograms, we compute response spectra at different sites and speculate on effects of the geology class at large level of shaking, including soil nonlinearity. We find that large discrepancies from design spectra prescribed by seismic codes can occur for a wide sector of Benevento, especially for periods < 0.5 s.