Estimation of seismic hazard with allowance for noncircular isoseismals of arbitrary orientation

A new mathematical model describing the field of macroseismic intensity has been elaborated. It is based on elliptic isoseismals. The orientation of the main axes of elliptic isoseismals depends on the direction of stretching of the main geological structures on the investigated territory.The new model of a macroseismic field was applied to the territory of Eastern Uzbekistan. Some results of macroseismic investigations of the effect of large regional earthquakes were used as initial data.A noncircular model of a macroseismic field was introduced into the integral of the seismic shakability of Riznichenko and, according to the model, a macroseismic shakability map for the territory of Eastern Uzbekistan was computed in isolines of the long-term mean return period of vibrations for the intensity I 8.Paper presented at the 21st General Assembly of the European Seismological Commission, Sofia, 1988.     

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.     

Web-based macroseismic survey in Italy: method validation and results

A new method of macroseismic surveys, based on voluntary collaboration through the Internet, has been running at the Istituto Nazionale di Geofisica e Vulcanologia (INGV) since July 2007. The macroseismic questionnaire is addressed to a single non-specialist; reported effects are statistically analysed to extrapolate a probabilistic estimate of Mercalli Cancani Sieberg and European Macroseismic Scale intensities for that observer. Maps of macroseismic intensity are displayed online in almost real time and are continuously updated when new data are made available. For densely inhabited zones, we have received reports of felt effects for even very small events (M = 2). Six earthquakes are presented here, showing the ability of the method to give fast and interesting results. The effects reported in questionnaires coming from three towns are carefully analysed and assigned intensities are compared with those derived from traditional macroseismic surveys, showing the reliability of our web-based method.     

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.     

On an anisotropic attenuation law of the macroseismic intensity

An anisotropic attenuation law of the macroseismic intensity has been deduced, congruent with an anisotropic modelling of macroseismic fields. The results, obtained by analysing a set of earthquakes in Eastern Sicily and Southern Calabria, show a greater adaptability to the observed data as compared with those deduced using isotropic attenuation laws modified to take the anisotropy into account.     

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.     

Anisotropic characterization of macroseismic fields

A methodology for the anisotropic characterization of macroseismic fields is proposed, in order to evaluate seismic hazard, based on the real geometry of the isoseismals of the field. The proposed methodology, independent of the macroseismic intensity attenuation law, allows both for a single field and for several fields in the same source zone, the determination of minimum and maximum attenuation values and of the relative directions.     

Macroseismic Surveys in Theory and Practice

Macroseismology is the part of seismology that collects and evaluates non-instrumental data on earthquakes, i.e., effects on people, objects, buildings and nature. The methods that seismologists use for collecting and evaluating the macroseismic data are often based on long (trial-and-error) experience more than on some formal procedure. Until very recently manuals or guidelines on how to do a macroseismic survey were rare and often superficial. After an earthquake is felt in some region, the data are usually collected by means of questionnaires. Field survey is an obligatory tool that complements the questionnaires in the case of a damaging earthquake. An overview of the approaches to deriving the earthquake parameters (epicentre and barycentre, epicentral intensity, magnitude, depth, source parametres) from macroseismic data, as well as a review of some existing practices is given.     

Magnitude Formulae and Intensity–Magnitude Relations for Early Instrumental Earthquakes in the Iberian Region

The magnitudes of early instrumentally recorded earthquakes in the Iberian region (1912–1962) have been studied through processing of digitized seismograms of Wiechert seismograph and analysis of macroseismic information. A magnitude system based on instrumental registrations and macroseismic observations has been proposed. It consists of two compatible magnitude formulae depending on the total duration of seismic oscillations and on the maximum ground amplitude/period ratio of surface waves and includes correspondent intensity–magnitude relationships.     

A macroseismic equation with a convergent solution

We obtained a macroseismic equation with a convergent solution at a hypocentral distance D ∼ 0 (independent of magnitude) for relative shaking intensity. The logarithmic distance dependence of intensity turned out to be piecewise linear no matter whether it is expressed in relative units of intensity degree or in logarithmic ground motion velocity. The macroseismic intensity shows high correlation with motion velocity. Another result is the magnitude dependence of dominant periods of ground motion velocity for large earthquakes.     

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.     

Macroseismic study of the Potenza (southern Italy), earthquake of 5 May 1990

The aim of this paper is to define the macroseismic field of the 5 May 1990 Potenza earthquake, analysing about 3000 questionnaire forms together with the results of the direct inquiries carried out in the field. The study evidences the seismic vulnerability of the Potenza district and of the whole of southern Italy in general, due to the low resistance characteristics of the old constructions and a lack of seismic prevention. Besides, geomorphological failure increases hazardous situations at the earthquake's occurrence. This earthquake provided a good test for checking and improving the ING macroseismic data collection procedures.     

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.     

The correlation between the macroseismic attenuation trend and the geo-structural framework; The calabropeloritan arc as an example

After describing attempts at perfecting a methodology for studying isotropic and anisotropic macroseismic fields in previous works, the authors here try to identify the causes of anisotropy in the context of the “new basement tectonics”.The seismic data are taken both from reconstructions of the macroseismic fields of historic events, by means of a critical analysis of the data, and from macroseismic fields of recent events surveyed by the authors. These data are correlated to the structural framework obtained through recent neotectonic studies and the lineament distribution traced on satellite images and using the “shadow” method. Generally the direction of elongation of the mesoseismic area is closely dependent on the source parameters and can be associated with recent and present-day systems outlined by the latest neotectonic studies.The best correlation is observed, however, with the lineament pattern obtained using the “shadow” method: the domains of the lineaments associated with the preferential trend of the macroseismic field show, in the rose diagram of cumulative number, values of prevalence and kurtosis higher than average; in the cumulative lengths diagram, on the other hand, they show prevalence maxima and, in particular, kurtosis maxima which are all the higher the more the anisotropic trend of field is accentuated. Using the “Giant Griffith Cracks” model for the lineaments, it can be deduced that the swarms refer to fracture systems with greater vertical development generated during the most recent tectonic phases.Finally, from a study of the dynamic characteristics of the elastic waves, that are the main agents responsible for macroseismic effects, it can observed that the wavelength order of magnitude is comparable with that of the linear parameters in the “warp” formed by the “Giant Cracks”. It can, thus, be deduced that the strong absorption of energy can be determined by the fracture swarm when the wave propagation occurs orthogonally to the swarm.     

Hazard mapping based on macroseismic data considering the influence of geological conditions

The object of this study is to consider directly the influence of regional geological conditions on the assessment of seismic hazard. It is assumed that macroseismic data at individual locations contain, in an average way, the influence of geological conditions.A Data Base referring to 199 historical (5) and instrumental (194, in the 1947–1993 period) events with macroseismic information in 1195 locations of Portugal was built. For any given seismic event, whenever macroseismic information was available at a location (town, village, etc.), an EMS-92 intensity value was estimated. To each one of those locations a geological unit, representing the most common type of soil, was assigned, based on the Geological Portuguese Map at a scale 1:500 000; the geological units were grouped into three categories: soft, intermediate and hard soils.The Data Base was used to determine the attenuation laws in terms of macroseismic intensity for the three different geological site conditions, using multiple linear regression analysis. The reasonability of the laws was tested by (i) checking residual distributions and (ii) comparing the map of isoseismals of important earthquakes with the isoseismals generated by the attenuation curves derived for each one of the three different soil classes, taking into consideration the soil class of each site. The main results of attenuation modeling are: high dispersion on macroseismic intensity data; all the models predict intensity values, for short hypocentral distances, lower than the ones observed; and for some important analyzed earthquakes and for the observed range of distances, the models confirm the expectancy that macroseismic intensity increases from hard to soft soil.The approach to obtain the hazard assessment at each location consisted in the use of the attenuation law specifically derived for the class of soil of that particular location. This method, which considers the influence of the regional geology, was illustrated with the mapping of hazard for the country for several return periods. Comparison with previous maps not taking into consideration the regional geological conditions emphasizes the importance of this new parameter. It can be concluded that (i) soil segmentation is clearly the cause for hazard increase in the region to the north of Lisbon, especially at sites with soft and intermediate soils as the ones in lower Tagus valley; the maximum increase on hazard is, in any case, less than one degree; (ii) when geological conditions are disregarded in the attenuation regression analysis, hazard pattern is similar to the one obtained for the case of hard soil everywhere.     

Atténuation de l'intensité macrosismique pour la France métropolitaine : importance de l'intensité épicentrale

The main result of this work is to show that macroseismic intensity decay with distance strongly depends on the epicentral intensity. An attenuation law that takes this parameter into account is proposed for Metropolitan France, from the analysis of SISFRANCE macroseismic database. Such a model significantly reduces the difference between observed and theoretical intensities. A map of the attenuation variations is also set up for Metropolitan France. No major site effects are observed, but at a broad scale, young Alpine regions display a stronger attenuation than old Hercynian regions. To cite this article: P. Arroucau et al., C. R. Geoscience 338 (2006).     

Relation between macroseismic intensity and instrumental parameters of strong motions — A statistical approach

One hundred and twenty-eight strong ground motion CALTECH (Earthquakes in the U.S.A., 1940–1971) records of five Californian earthquakes, recorded at ground level and in basements, for which the modified Mercalli macroseismic intensities are known, were statistically processed to calculate 165 wave parameters. Correlation of the peak values of particle acceleration, velocity and displacement, energy, impulse and root-mean-square amplitude of the vibrations, durations of vibrations for certain levels of the peak amplitude, and spectral energies of 10 frequency windows with macroseismic intensity are discussed from the viewpoint of classification of ground-motion records.Paper presented at the 21st General Assembly of the European Seismological Commission held in Sofia 1988.     

Distribution of effects in the urban area of Rome During the October 14, 1997 Umbria Marche earthquake

In the last years many studies were carried out in order to assess the correlation between nearsurface geology and seismic response of downtown Rome during local and regional earthquakes. Nevertheless, only historical macroseismic data and theoretical modelling could be used so far to estimate the ground motion in the historical part of the city. The occurrence of the October 14, 1997 Umbria Marche event (M_w = 5.7), located about 115 km far from Rome but largely felt in the city, allowed us to verify our knowledge of the seismic response on the basis of the present urban setting of Rome.A prompt macroseismic survey in the urban area within the ring-road (G.R.A.) was performed. 949 questionnaires were compiled by means of 1842 interviews: such a large amount of macroseismic information, for the first time available in Rome, was related to 669 observation points providing a picture of local intensity variations. The areal distribution of collected intensity points is quite homogeneous.The data analysis points out a preferential distribution of larger effects (65%) in correspondence of Holocene alluvium outcrops. Furthermore, a tight correspondence between remarkable intensities and alluvial sediments is pointed out in suburban areas characterized by the minor hydrographic network of the Tiber River. Macroseismic anomalies were outlined in sectors of the urban area where local hydrogeological conditions can be responsible for a significant deterioration of geomechanical properties.     

Croation macroseismic database

Compilation of the digital Croatian Macroseismic Database is an ongoing project that started in 1995. Final version of the database will contain all the macroseismic data from the archive of the Department of Geophysics (including data for neighboring countries) without time or intensity threshold. The database has two subsets: intensity point database and the digital archive. A total of 426 earthquakes felt in Croatia are currently being analyzed. The job is carried out through several stages, each of them corresponding to digitalization of a different input data format (intensity maps, intensity point lists, questionnaires, etc.). 69 intensity maps of the earthquakes felt in Croatia, published in the Catalogue of Earthquakes in the Balkan Region were digitized first. The coordinates of intensity points were assigned to geographic locations (villages, cities, …) by consulting the database of latitudes and longitudes of all settlements in Croatia (database of settlements). 330 existing intensity lists (intensity and name of the place without coordinates) were entered into the database during the second step. The intensity maps which were not published so far will be scanned and digitized during the third stage of the project. The last stage will be input of primary data to the digital archive and reassessing of intensities according to the EMS macroseismic scale. The main goal of this paper is to describe the way of database compilation. Preliminary felt intensity map and number of intensity points map illustrate possibilities of macroseismic field analyze.