Macroseismic Intensity Modelling of Earthquakes in Southern Spain: Attenuation and Tectonic Implications

A modelling of the observed macroseismic intensity of historical and instrumental earthquakes in southern Spain is proposed, with the aim of determining the macroseismic parameters for seismic hazard evaluation in a region in which the characterization of intensity distribution of seismic events shows different levels of difficulty referable to the complex faults system of the area in study. The adopted procedure allows an analytical determination of epicenters and principal attenuation directions of earthquakes with a double level of verification with reference to the maximum shaking area and structural lineaments of the region, respectively. The analyses, carried out on a suitable number of events, highlight, therefore, some elements for a preliminary characterization of a seismic zonation on the basis of the consistency between seismic intensity distribution of earthquakes and corresponding structural framework.     

On the Observed Intensity Filtering in the Anisotropic Distribution Modelling of Macroseismic Intensity

The anisotropic modelling of intensity distribution, affected by the construction of macroseismic planes, allows an analysis of the influence of each point of observed intensity on the analytical determination of epicenter and of the principal attenuation directions. Such a procedure is a vital aid in the cases in which the observed intensity points, that, for location or joined intensity level, are not consistent with an anisotropic model of intensity attenuation. A suitable filtering on intensity levels associated with the points of the intensity map, for a better modelling of observed intensity distribution, is proposed with the aim of a better seismic hazard evaluation.     

Seismic hazard assessment for Iceland in terms of macroseismic intensity using a site approach

We present the results of probabilistic seismic hazard assessment for Iceland in the framework of the EU project UPStrat-MAFA using the so-called site approach implemented in the SASHA computational code. This approach estimates seismic hazard in terms of macroseismic intensity by basically relying on local information about documented effects of past seismic events in the framework of a formally coherent and complete treatment of intensity data. In the case of Iceland, due to the lack of observed intensities for past earthquakes, local seismic histories were built using indirect macroseismic estimates deduced from epicentral information through an empirical attenuation relationship in probabilistic form. Seismic hazard was computed for four exceedance probabilities for an exposure time of 50 years, equivalent to average return periods of 50, 200, 475 and 975 years. For some localities, further return periods were examined and deaggregation analysis was performed. Results appear significantly different from previous seismic hazard maps, though just a semi-qualitative comparison is possible because of the different shaking measure considered (peak ground acceleration versus intensity), and the different computational methodology and input data used in these studies.     

Validation through HVSR measurements of a method for the quick detection of site amplification effects from intensity data: an application to a seismic area in Northern Italy

An instrumental validation is attempted of an innovative approach devoted to the quick individuation, from macroseismic data, of site amplification phenomena able to significantly modify seismic hazard levels expected on the basis of average propagation effects only. According to this methodology, two evaluations of hazard are performed at each investigated locality: the former, obtained by epicentral intensity data ‘reduced’ at the site through a probabilistic attenuation function and, the latter, computed by integrating such data with seismic effects actually observed at the site during past earthquakes. The comparison, for each locality, between these two hazard estimates allow to orientate the identification of those sites where local amplifications of earthquake ground motion could be significant. In order to check such methodology, indications obtained in this way from macroseismic data are compared with the estimates of transfer functions performed through the HVSR technique applied to microtremors. Results concerning municipalities located in a seismic area of Northern Italy indicate a good agreement between macroseismic and instrumental estimates.     

On the Intensity Virtual Area Characterization in the Intensity Distribution Modelling by Macroseismic Planes

A procedure is proposed for the reconfiguration of the macroseismic planes relative to earthquakes that, being characterized by a reduced number of points of observed intensity due to a lack of information, or having the epicenter very close to the coastline, are characterized by an incomplete distribution of observed intensity levels. The design of a plurality of virtual areas, through which a distribution of intensity consistent with an anisotropic model of attenuation is depicted, allows a reliable determination of macroseismic parameters of the same seismic event.     

Historical intermediate-depth earthquakes in the southern Aegean Sea Benioff zone: modeling their anomalous macroseismic patterns with stochastic ground-motion simulations

We model the macroseismic damage distribution of four important intermediate-depth earthquakes of the southern Aegean Sea subduction zone, namely the destructive 1926 M?=?7.7 Rhodes and 1935 M?=?6.9 Crete earthquakes, the unique 1956 M?=?6.9 Amorgos aftershock (recently proposed to be triggered by a shallow event), and the more recent 2002 M?=?5.9 Milos earthquake, which all exhibit spatially anomalous macroseismic patterns. Macroseismic data for these events are collected from published macroseismic databases and compared with the spatial distribution of seismic motions obtained from stochastic simulation, converted to macroseismic intensity (Modified Mercalli scale, I_MM). For this conversion, we present an updated correlation between macroseismic intensities and peak measures of seismic motions (PGA and PGV) for the intermediate-depth earthquakes of the southern Aegean Sea. Input model parameters for the simulations, such as fault dimensions, stress parameters, and attenuation parameters (e.g. back-arc/along anelastic attenuation) are adopted from previous work performed in the area. Site-effects on the observed seismic motions are approximated using generic transfer functions proposed for the broader Aegean Sea area on the basis of V_S30 values from topographic slope proxies. The results are in very good agreement with the observed anomalous damage patterns, for which the largest intensities are often observed at distances >?100 km from the earthquake epicenters. We also consider two additional “prediction” but realistic intermediate-depth earthquake scenarios, and model their macroseismic distributions, to assess their expected damage impact in the broader southern Aegean area. The results suggest that intermediate-depth events, especially north of central Crete, have a prominent effect on a wide area of the outer Hellenic arc, with a very important impact on modern urban centers along northern Crete coasts (e.g. city of Heraklion), in excellent agreement with the available historical information.     

The Grandori model: Anisotropic formulation and its application in the plotting of seismic hazard maps

The seismic hazard for the Calabro-Sicilian area is evaluated using an anisotropic formulation of the Grandori attenuation law. For each macroseismic field two main directions are identified: minimum and maximum attenuation of the macroseismic intensity. The results of the investigation show that the anisotropic formulation improves the compatibility level of the model (with respect to the isotropic one) with the intensities observed and produces probabilistic expected intensities which compare favourably with the values of seismic history in the investigated area when the zonation defined by the Messina University research group was used.     

Earthquake Hazard Parameters in Crete Island and its Surrounding Area Inferred from Bayes Statistics: An Integration of Morphology of the Seismically Active Structures and Seismological Data

-- The study addresses the evaluation of earthquake hazard parameters such as maximum regional magnitude (M_max) and the slope of Gutenberg-Richter law # (where b=# log e) for the Hellenic Wadati-Benioff zone and the overriding lithospheric plate in the area of Crete and its surroundings. The seismicity of the area is divided in a cellular (1.0° 2 1.0°) manner allowing analysis of the localized earthquake hazard parameters and graphical representation of their spatial variation. Our approach incorporates the recently updated earthquake catalogue for Greece and the adjacent areas, the consideration of the morphology of the deep seismically active structures in the studied area and use of a probabilistic procedure for estimating the earthquake hazard parameters.¶One of the main inconsistencies in the earthquake hazard assessment is the estimation of the maximum magnitude and the related uncertaint y. The Bayesian approach, applied in the present, is a straightforward technique for evaluating the earthquake hazard parameters and is based on the following assumptions: Poissonian character of seismic events flow, a frequency-magnitude law of Gutenberg-Richter's type with cutoff maximal value for estimated parameter and a seismic catalogue, having a rather sizeable number of events (i.e., 50 events at least per cell). For five cells in which the number of events is less than 50, an effort is made to produce synthetic data. The re-assessed parameters obtained from the synthetic data show no significant difference and the real data (of the five cells) are finally taken into account although the estimated uncertainty is high.¶For four random cells we constructed hazard curves showing the probabilities that a certain magnitude M will be exceeded in one year and the return periods (in years) that are expected for a given magnitude. These are particularly useful for the mapping of earthquake hazard in regions of either low or high seismic activity, as is Crete and the adjacent area.¶The obtained results show that the W and E parts of both subducting and overriding plates differ in the spatial distribution of all the estimated earthquake hazard parameters. The M_max distribution indicates strong coupling between the western portions of the interacting plates (M_max > 6.3) to the south of 36°N. The smaller values of M_max (M_max < 6.3) estimated in the SE part of the studied area indicate weak coupling between the eastern portions of the subducting and overriding plates.¶Values of b > 1.0 are found to the south and east of Crete for the Wadati-Benioff zone, and over the central part of the island and the area to the northeast of it (cell 11) for the continental wedge, which suggests nonuniform stress field and/or heterogeneous material.     

Estimates of site seismicity rates using ill-defined macroseismic data

A new approach to the problem of site seismic hazard analysis is proposed, based on intensity data affected by uncertainties. This approach takes into account the ordinal and discrete character of intensities, trying to avoid misleading results due to the assumption that intensity can be treated as a real number (continuous distribution estimators, attenuation relationships, etc.). The proposed formulation is based on the use of a distribution function describing, for each earthquake, the probability that site seismic effects can be described by each possible intensity value. In order to obtain site hazard estimates where local data are lacking, the dependence of this distribution function with the distance from the macroseismic epicenter and with epicentral intensity is examined. A methodology has been developed for the purpose of combining such probabilities and estimating site seismicity rates which takes into account the effect of uncertainties involved in this kind of analysis. An application of this approach is described and discussed.     

Further information on the macroseismic field in the Balkan area

Papazachos and Papaioannou (1997) (called PP97 hereinafter) studied the macroseismic field in the Balkan area (Greece, Albania, former Yugoslavia, Bulgaria and western Turkey) with the purpose of deriving attenuation and scaling relations useful for seismic hazard assessment and study of historical earthquakes. In his comment, Trifunac suggests that our analysis might exhibit certain bias for all countries except Greece due to problems mainly associated with the database (completeness, etc.), conversion of local intensity scales used in the Balkan countries, as well as to the local variations of the attenuation relation due to the variation of the geotectonic environment in this area. Specifically, his most important comments can be summarized as follows: a) The large participation of Greek data probably biased the scaling relations proposed in the study. b) The conversion relations used between local macroseismic scales are less accurate than their proposed such relations. c) The variation of attenuation (geometrical and anelastic) in different regions of the study area is important and local relations (instead of the proposed single relation) should be determined for seismic hazard assessment. In the following, we study in detail each of these possible bias sources. Additional work on the macroseismic field of the Balkan area shows that none of the previously described factors, suggested by Trifunac, introduces bias in the results presented by PP97. Specifically, it is shown that the database used by PP97 fulfills the basic requirements for a reliable determinations of attenuation and scaling relations proper for seismic hazard assessment in all five countries of this area. Evidence is presented that no strong geographical variation of the attenuation of macroseismic intensities of shallow earthquakes is observed. Relations between local version of intensity scales suggested by Shebalin et al. (1974) are shown to be reliable. Finally, it is demonstrated that national practices for estimation of macroseismic intensities may affect the results of seismic hazard assessment but proper formulation can be applied (PP97) which allows to take into account such differences in national practices. This formulation allows also to introduce and correct for anisotropic radiation at the seismic source as well as the incorporation of site effects.     

The 23 April 1909 Benavente earthquake (Portugal): macroseismic field revision

The 23 April 1909 earthquake, with epicentre near Benavente (Portugal), was the largest crustal earthquake in the Iberian Peninsula during the twentieth century (M _w = 6.0). Due to its importance, several studies were developed soon after its occurrence, in Portugal and in Spain. A perusal of the different studies on the macroseismic field of this earthquake showed some discrepancies, in particular on the abnormal patterns of the isoseismal curves in Spain. Besides, a complete list of intensity data points for the event is unavailable at present. Seismic moment, focal mechanism and other earthquake parameters obtained from the instrumental records have been recently reviewed and recalculated. Revision of the macroseismic field of this earthquake poses a unique opportunity to study macroseismic propagation and local effects in central Iberian Peninsula. For this reasons, a search to collect new macroseismic data for this earthquake has been carried out, and a re-evaluation of the whole set has been performed and it is presented here. Special attention is paid to the observed low attenuation of the macroseismic effects, heterogeneous propagation and the distortion introduced by local amplifications. Results of this study indicate, in general, an overestimation of the intensity degrees previously assigned to this earthquake in Spain; also it illustrates how difficult it is to assign an intensity degree to a large town, where local effects play an important role, and confirms the low attenuation of seismic propagation inside the Iberian Peninsula from west and southwest to east and northeast.     

Damage to residential buildings in Hveragerði during the 2008 Ölfus Earthquake: simulated and surveyed results

This study analyses the performance of residential buildings in the town of Hveragerði in South Iceland during the 29 May 2008 Mw 6.3 Ölfus Earthquake. The earthquake occurred very close to the town, approximately 3–4 km from it. Ground shaking caused by the earthquake was recorded by a dense strong-motion array in the town. The array provided high-quality three-component ground acceleration data which is used to quantify a hazard scenario. In addition, surveys conducted in the town in the aftermath of the earthquake have provided information on macroseismic intensity at various locations in the town. Detailed information regarding the building stock in the town is collected, and their seismic vulnerability models are created by using building damage data obtained from the June 2000 South Iceland earthquakes. Damage to buildings are then simulated by using the scenario hazard and vulnerability models. Damage estimates were also obtained by conducting a survey. Simulated damage based on the scenario macroseismic intensity is found to be similar to damage estimated from survey data. The buildings performed very well during the earthquake—damage suffered was only 5 % of the insured value on the average. Correlation between actual damage and recorded ground-motion parameters is found to be statistically insignificant. No significant correlation of damage was observed, even with macroseismic intensity. Whereas significant correlation was observed between peak ground velocity and macroseismic intensity, neither of them appear to be good indicators of damage to buildings in the study area. This lack of correlation is partly due to good seismic capacity of buildings and partly due to the ordinal nature of macroseismic intensity scale. Consistent with experience from many past earthquakes, the survey results indicate that seismic risk in South Iceland is not so much due to collapse of buildings but rather due to damage to non-structural components and building contents.     

Analysis of macroseismic fields using statistical data depth functions: considerations leading to attenuation probabilistic modelling

Modelling seismic attenuation is one of the most critical points in the hazard assessment process. In this article we consider the spatial distribution of the effects caused by an earthquake as expressed by the values of the macroseismic intensity recorded at various locations surrounding the epicentre. Considering the ordinal nature of the intensity, a way to show its decay with distance is to draw curves—isoseismal lines—on maps, which bound points of intensity not smaller than a fixed value. These lines usually take the form of closed and nested curves around the epicentre, with highly different shapes because of the effects of ground conditions and of complexities in rupture propagation. Forecasting seismic attenuation of future earthquakes requires stochastic modelling of the decay on the basis of a common spatial pattern. The aim of this study is to consider a statistical methodology that identifies a general shape, if it exists, for isoseismal lines of a set of macroseismic fields. Data depth is a general nonparametric method for analysis of probability distributions and datasets. It has arisen as a statistical method to order points of a multivariate space, e.g., Euclidean space \({\mathbb {R}}^{p}\), \(p \ge 1\), according to the centrality with respect to a distribution or a given data cloud. Recently, this method has been extended to the ordering of functions and trajectories. In our case, for a fixed intensity decay \(\varDelta I\), we build a set of convex hulls that enclose the sites of felt intensity \(I_s \ge I_0 -\varDelta I\), one for each macroseismic field of a set of earthquakes that are considered as similar from the attenuation point of view. By applying data depth functions to this functional dataset, it is possible to identify the most central curve, i.e., the attenuation pattern, and to consider other properties like variability, outlyingness, and possible clustering of such curves. Results are shown for earthquakes that occurred on the Central Po Plain in May 2012, and on the eastern flank of Mt. Etna since 1865.     

Microtremor Measurements for the Microzonation of Dinar

v--vThe geotechnical site conditions in Dinar town located in western Turkey were investigated after the 1995 Dinar earthquake based on borings, in situ penetration tests, seismic wave velocity measurements, and microtremor records. The variation of damage distribution within the town was evaluated with respect to 23 district damage ratios calculated, based on the detailed damage survey conducted by the General Directorate of Disaster Affairs. Site amplifications were estimated from microtremor spectral ratios and microzonation was performed using a GIS methodology. The results of in situ penetration tests and seismic wave velocity measurements as well as the damage distribution were compared with the amplification zonation obtained from microtremor records. The results indicate the applicability of microtremor spectral ratios for assessing the local site conditions and site amplifications.     

A method to estimate intensity occurrence probabilities in low seismic activity regions

The estimation of site intensity occurrence probabilities in low seismic activity regions has been studied from different points of view. However, no method has been definitively established because several problems arise when macroseismic historical data are incomplete and the active zones are not well determined. The purpose of this paper is to present a method that estimates site occurrence probabilities and at the same time measures the uncertainties inherent in these probabilities in low activity regions. The region to be studied is divided into very broad seismic zones. An exponential intensity probability law is adjusted for each zone and the degree of uncertainty in the assumed incompleteness of the catalogue is evaluated for each intensity. These probabilities are used to establish what may be termed ‘prior site occurrence models’. A Bayesian method is used to improve ‘prior models’ and to obtain the ‘posterior site occurrence models’. Epicentre locations are used to recover spatial information lost in the prior broad zoning. This Bayesian correction permits the use of specific attenuation for different events and may take into account, by means of conservative criteria, epicentre location errors. Following Bayesian methods, probabilities are assumed to be random variables and their distribution may be used to estimate the degree of uncertainty arising from (a) the statistical variance of estimators, (b) catalogue incompleteness and (c) mismatch of data to prior assumptions such as Poisson distribution for events and exponential distribution for intensities. The results are maps of probability and uncertainty for each intensity. These maps exhibit better spatial definition than those obtained by means of simple, broad zones. Some results for Catalonia (NE of Iberian Peninsula) are shown.     

Source model and the macroseismic effect of the 1991 Racha earthquake

Coherency of the source model of the 1991 Racha earthquake in the Greater Caucasus with different data types is analyzed. Authors, when interpreting macroseismic data, accept complex nature of macroseismic effects generation but, nevertheless, consider that its spatial distribution follows certain regularities. First time in the practice, method of evaluation of the mecroseismic material completeness is proposed based on the intensity attenuation along with distance. It is demonstrated the character of macroseismic intensity attenuation can be used for verification of the source model elements constructed based on other seismological data. Dependence of the macroseismic effect distribution on azimuth in near field of the 1991 Racha earthquake is recognized.     

Linking ground motion measurements and macroseismic observations in France: a case study based on accelerometric and macroseismic databases

Comparison between accelerometric and macroseismic observations is made for three M _w?=?4.5 earthquakes, which occurred in north-eastern France and south-western Germany in 2003 and 2004. Scalar and spectral instrumental parameters are processed from the accelerometric data recorded by nine accelerometric stations located between 29 and 180 km from the epicentres. Macroseismic data are based on French Internet reports. In addition to the single questionnaire intensity, analysis of the internal correlation between the encoded answers highlights four predominant fields of questions bearing different physical meanings: (1) “vibratory motions of small objects”, (2) “displacement and fall of objects”, (3) “acoustic noise” and (4) “personal feelings”. Best correlations between macroseismic and instrumental observations are obtained when the macroseismic parameters are averaged over 10-km-radius circles around each station. Macroseismic intensities predicted by published peak ground velocity (PGV)–intensity relationships agree with our observed intensities, contrary to those based on peak ground acceleration (PGA). Correlation between the macroseismic and instrumental data for intensities between II and V (EMS-98) is better for PGV than for PGA. Correlation with the response spectra exhibits clear frequency dependence for all macroseismic parameters. Horizontal and vertical components are significantly correlated with the macroseismic parameters between 1 and 10 Hz, a range corresponding to both natural frequencies of most buildings and high energy content in the seismic ground motion. Between 10 and 25 Hz, a clear lack of correlation between macroseismic and instrumental observations exists. It could be due to a combination of the decrease in the energy signal above 10 Hz, a high level of anthropogenic noise and an increase in variability in soil conditions. Above 25 Hz, the correlation coefficients between the acceleration response spectra and the macroseismic parameters are close to the PGA correlation level.     

Non-parametric Seismic Hazard in Mines

— Seismic hazard analysis methods in mines are reviewed for the purpose of selecting the best technique. To achieve this goal, the most often-used hazard analysis procedure, which is based on the classical frequency-magnitude Gutenberg-Richter relation, as well as alternative procedures are investigated.¶Since the maximum regional seismic event magnitude m _max is of paramount importance in seismic hazard analysis, this work provides a generic formula for the evaluation of this important parameter. The formula is capable of generating solutions in different forms, depending on the assumptions of the model of the magnitude distribution and/or the available information regarding past seismicity. It includes the cases (i) in which seismic event magnitudes are distributed according to the truncated frequency-magnitude Gutenberg-Richter relation, and (ii) in which no specific model of the magnitude distribution is assumed.¶Both synthetic, Monte-Carlo simulated seismic event catalogues, and actual data from the copper mine in Poland and gold mine in South Africa, are used to demonstrate the discussed hazard analysis techniques.¶Our studies show that the non-parametric technique, which is independent of the assumed model of the distribution of magnitude, provides an appropriate tool for seismic hazard assessment in mines where the magnitude distribution can be very complex.     

Anisotropic radiation modelling of macroseismic intensities for estimation of the attenuation structure of the upper crust in Greece

A method is suggested for the analysis of macroseismic intensity data in order to accurately determine an average attenuation structure of the upper part of the crust in an area. The method is based on a model which assumes that the observed intensities depend on source properties (radiation pattern, size, focal depth), geometrical spreading and anelastic attenuation. The method is applied to 13,008 intensity values, observed in corresponding sites of Greece and grouped (in 4228 groups), according to their spatial clustering in order to diminish observational errors and site effects. An average intensity attenuation coefficient,c=–0.0039±0.0016, corresponding to a quality factor, Q=350±140, is determined for the upper 20 km of the crust in this area. This value is relatively low, in good agreement with the relatively high heat flow and high seismic activity of this area. A byproduct of the present study is the determination, for each earthquake, of a macroseismic focal depth and of a macroseismic size, which is strongly correlatted with both the earthquake's magnitude and its seismic moment determined by independent methods.