Mean Return Period Research Articles

Disaggregation of probabilistic ground motions in two cities of Western Iran, Kermanshah and Sanandaj

This article presents the results of disaggregation of ground motion hazard obtained for two of Iran’s urban centers, Kermanshah and Sanandaj. Disaggregation of peak ground acceleration (PGA) and spectral acceleration hazard corresponding to mean return periods of 475 and 50 years is performed. 12 area seismic sources in the study region as well as 15 area seismic sources in a 150-km distance from the region are delineated. The scenario earthquakes are characterized by bins of magnitude, M, source-to-site distance, R, and number of standard deviations, ε, that the ground-motion parameter is away from its median value for that M-R pair as estimated by a prediction equation. In most cases, the sources closer to the site dominate. Larger, more distant earthquakes contribute more significantly to hazard for longer periods than for shorter periods. Disaggregation plots can provide useful information on the distance and magnitude of predominant sources, which can be used to generate scenario earthquakes and select corresponding time histories for seismic design.

Earthquake Risk Evaluation in Koyna Region of Maharastra, India

Gumbel’s extreme value theory has been applied to estimate the probability of occurrence and return period of the largest earthquakes in Koyna region India. The statistical model of Epstein and Lomnitz (1966) is discussed with reference to the Gumbel’s extreme value theory. The expected extremes based on 21-years (1964–1984) seismicity data of six-monthly extreme values of earthquakes for the region have been plotted. The mean line of expected extreme (LEE) is drawn to study the mean return periods of the largest possible earthquakes with their probability of occurrence. The yearly expected number, mean return period and earthquake risk have been estimated. The most probable medium size earthquakes that may occur in the region are predicted. The results presented in this study are only best long-term estimates subject to the limitation of the data.

Sandy berm and beach-ridge formation in relation to extreme sea-levels: A Danish example in a micro-tidal environment

Abstract The formation of berms and their transformation into beach ridges in a micro-tidal environment is coupled to wave run-up and overtopping during extreme sea levels. A straight-forward comparison between extreme sea levels due to storm-surges and active berm levels is impossible in the semi-enclosed bays along the Baltic Sea. Quite often, the maximum water levels do not coincide with the maximum intensity of the wave driven processes because of seiches in the Baltic. In this paper, we look into the joined distribution of extreme water levels and high-energetic wave conditions at Feddet, a sandy prograding spit on the south-eastern Baltic shores of Zealand, Denmark. The modern, sandy beach at this location consists of a beachface with a shallow incipient berm, a mature berm, and a dune-covered beach ridge. It borders a beach-ridge plain to the west, where more than 20 N–S oriented beach ridges and swales are present. Measured water-level data from 1991 to 2012 and topographical observations, carried out during fair weather period and during a storm event, provided the basis for a conceptual model exhibiting berm formation and transformation into the local beach-ridge system. The character of extreme sea level events is identified using thirty-three well described extreme events throughout a period of 15 years. Analysis of the meteorological conditions during these events revealed that berm formation only occurred during 20% of all extreme events when onshore winds, high-energy wave action and elevated water coincided. These berm-forming events had a mean return period of once every three years. The average age between successive beach ridges on the beach-ridge plain was about 180 years, which means that the transformation of a berm into a single beach ridge is the result of a large number of storm episodes with high-energy waves and extreme water levels.

New design chart for basic wind speeds in Brazil

The design charts for basic wind speeds currently in use in Brazil [1] were created in 1977, based on wind speed records from 1950 to 1974. The database covered up to 25 years of wind records obtained in 49 weather stations, totaling 919 stationyears of data. Since 1974, additional 37 years of wind records became available, but have not been incorporated in the design code. Hence, the updated wind charts proposed in this paper are long overdue. In this paper, 62 years of wind records in Brazil are gathered in order to construct updated and more reliable wind charts. The original data by Padaratz [2] are complemented by data from 104 airport weather stations, made available through the Wolfram Research Database [3]. In total, 4142 stationyears of data are used in the present study, hence providing much better cover in terms of space and time span. Maximum annual wind speeds for the individual stations are fitted to Gumbel distributions, from which basic wind speeds are evaluated (mean return period of 50 years). Basic wind speeds are used to build a non-linear regression model, using the p-value of the Anderson-Darling goodness-of-fit test as regression weight. This ensures that extreme value wind distributions for which a good fit is obtained are given more importance in the regression model, reducing the influence of spurious data. The regression model developed herein is used to plot a new design chart for basic wind speeds in Brazil. It is proposed that this new wind chart be incorporated in the Brazilian code for design of structures subject to wind [1]. In this regard, it is observed that basic wind speeds obtained herein are higher than the wind speeds of the current chart for significant parts of Brazil (notably the center, northeast and north), and are the same or smaller in some parts of the south and southeast. It is shown herein that the main differences between the current and the proposed charts are not due to different modeling assumptions, but are mainly due to the new data. The proposed chart is updated, and is more reliable than the chart currently in use, as it reflects 4142 stationyears of data, and covers up to 62 years of wind records.

Ground-motion scenarios consistent with PSH deaggregation for Tehran, capital city of Iran

Abstract. This study presents the results of probabilistic seismic hazard (PSH) deaggregation for 5%-damped 0.2 and 1.0 s spectral accelerations, corresponding to mean return periods (MRPs) of 50, and 475 yr for Tehran city. The aim of this paper is to quantify the dominant events that have the most contribution on ground-motion exceedance from the above mentioned hazard levels. The scenario earthquakes are characterized by bins of magnitude (M), source-to-site distance (R), and epsilon (ε). The results reveal that for Tehran city, the hazard is mainly controlled by local seismicity. Generally, as the spectral acceleration period increase, the contribution of larger and more distant scenario earthquakes to the overall seismic hazard increase.

Probabilities of earthquake occurrences in Mainland Southeast Asia

The frequency–magnitude distributions of earthquakes are used in this study to estimate the earthquake hazard parameters for individual earthquake source zones within the Mainland Southeast Asia. For this purpose, 13 earthquake source zones are newly defined based on the most recent geological, tectonic, and seismicity data. A homogeneous and complete seismicity database covering the period from 1964 to 2010 is prepared for this region and then used for the estimation of the constants, a and b, of the frequency–magnitude distributions. These constants are then applied to evaluate the most probable largest magnitude, the mean return period, and the probability of earthquake of different magnitudes in different time spans. The results clearly show that zones A, B, and E have the high probability for the earthquake occurrence comparing with the other seismic zones. All seismic source zones have 100 % probability that the earthquake with magnitude ≤6.0 generates in the next 25 years. For the Sagaing Fault Zone (zones C), the next Mw 7.2–7.5 earthquake may generate in this zone within the next two decades and should be aware of the prospective Mw 8.0 earthquake. Meanwhile, in Sumatra-Andaman Interplate (zone A), an earthquake with a magnitude of Mw 9.0 can possibly occur in every 50 years. Since an earthquake of magnitude Mw 9.0 was recorded in this region in 2004, there is a possibility of another Mw 9.0 earthquake within the next 50 years.

Quantification of wind damage to New Zealand's planted forests

Wind has a significant impact on planted forests in New Zealand, but the extent of loss and its spatial and temporal variability have not been quantified. Therefore, a formal analysis was undertaken using a database containing 78 records of wind damage in planted forests that spanned the period between 1945 and 2010. Approximately 63 000 ha of damage was recorded during this period. The damage that occurred in a single event ranged from 2.8 to 26 000 ha, with a median of 90 ha, and in most cases was confined to a single wood supply region. The relative risk of wind damage for each wood supply region was estimated by calculating the percentage of total net stocked area (NSA) that has been damaged by wind. Across all wood supply regions, an average of 0.21 per cent of NSA per annum has been damaged by wind; however, in the most affected region, the average level of damage was 0.94 per cent of NSA per annum, whereas in the least affected region it was 0.03 per cent. The probability of different levels of annual damage was modelled along with the level of damage associated with different return periods using a generalized Pareto distribution. Using this model, the mean return period for 500 ha of damage was estimated to be 5.0 years with 95% confidence interval (4.5, 6.2) years, whereas for 1000 ha of damage it was estimated to be 6.4 (4.8, 9.9) years.

Seven centuries of avalanche activity at Echalp (Queyras massif, southern French Alps) as inferred from tree rings

The purpose of this study was to reconstruct spatiotemporal patterns of avalanche events in a forested avalanche path of the Queyras massif (Echalp avalanche path, southeast French Alps). Analysis of past events was based on tree-ring series from 163 heavily affected multicentennial larch trees ( Larix decidua Mill.) growing near or next to the avalanche path. A total of 514 growth disturbances, such as tangential rows of traumatic resin ducts, the onset of compression wood as well as abrupt growth suppression or release, were identified in the samples indicating 38 destructive snow avalanches between 1338 and 2010. The mean return period of snow avalanches was 22 years with a 4% probability that an avalanche occurs in a particular year. On a temporal plan, three maxima in snow avalanche frequency were reconstructed at the beginning of the 16th and 19th centuries and around 1850, correlating with below-average winter temperatures and glacier advances. Analysis of the spatial distribution of disturbed trees contributed to the determination of four preferential patterns of avalanche events. The comparison of dendrogeomorphic data with historical records demonstrate that at least 18 events – six of which were undocumented – reached the hamlet of Echalp during the last seven centuries, but no significant temporal trend was detected concerning the frequency of these extreme events.

Numerical Investigation of Seismic Behavior of Spatial Asymmetric Multi- Storey Reinforced Concrete Buildings with Masonry Infill Walls

In order to insure the validity of the seismic performance matrix of the Eurocode EN 1998 for irregular in-plan, torsionally-flexible, spatial, asymmetric, multi-storey reinforced concrete (r/c) buildings with masonry infill walls, an ex- tended parametric numerical investigation has been performed, using non-linear response-history analysis. For this pur- pose, N representative asymmetric r/c buildings with torsional sensitivity, have been designed according to Eurocodes EN 1990, EN 1992 and EN 1998-1, for Ductility Class High (DCH), using design global behavior factor q equal to 3.00. Each of the masonry infill walls has been modeled with two nonlinear diagonal bars with hinges at their two ends and with one- sided behavior (in compression only). Three seismic levels of the seismic action have been considered with mean return period of 2475, 475 and 275 years, respectively. The above three earthquakes have been used for validity check of the states of Near Collapse, Significant and Damage Limitation, respectively. In order to apply the non-linear response-history analysis, suitable artificial accelerograms, which are compatible with the elastic response spectrum, for soil category D, of Eurocode EN 1998-1 on the one hand and with Hellenic geological and site-specific data on the other hand, have been used. In the present paper, important guidance on modelling plastic hinges and the masonry infill walls is presented, as well as, a numerical example of a three-storey r/c building is also presented for illustrative purposes

Peak Ground Acceleration Functions of Mean Return Period for Known Reliability Index of Hellenic Design Earthquakes

In order to design a structure according to Eurocode EN 1990, a uniform “reliability index” must be defined for all design parameters (strengths, loadings, etc.) that present an “uncertainty.” However, Design Earthquakes constitute exception of the above parameters; thus, the present article covers this gap. For various values of reliability index and using an ideal, fictitious, Standard Normal Probability Density Function, new Peak Ground Acceleration functions of the mean return period of the design earthquakes are proposed. As a pilot scheme, we chose the 82 most important seismic records of the last 40 years for the seismic area of Greece.

The New Slovak National Annex STN EN 1991-1-3/NA1 General Actions. Snow loads: 2012

Significant changes introduced into the new Slovak National Annex STN EN 1991-1-3/NA1 General actions. Snow loads: 2012 are based on results of two subsequent projects completed within the period 2006 to 2010. The projects aimed at retrieving, assembling and processing of long-term meteorological data related to snow loads, statistical analysis of the data, map applications and studies focusing on probabilistic design, verification and optimisation of safety factors of structures subjected mainly to snow loads. The new map of characteristic snow loads on the ground employs altitude functions rather than a set of contour lines in the antecedent National Annex (2004). The exceptional snow loads are analysed by an original approach assessing the accidental snow loads on the ground as fractiles of the mean return period of 10000 years within delimited four distinct regions in Slovakia. Approximate formulas for determination of frequent and quasi-permanent value of snow load are developed. Probabilistic optimisation of safety factors resulted in increased snow load partial factor for the design and verification of the frames of low-rise halls with light-weight roofs. Analogous feature of the Czechoslovakian Standard ČSN 73 0035 (1988) was in the preceding National Annex omitted. The maps of characteristic and accidental snow loads are provided in printed version as well as in an interactive form on CD.

Exceptional snowfalls and the assessment of accidental snow loads for structural design

Abstract Aiming at the assessment of accidental snow loads for the design of structures, a new approach to the analysis of exceptional snowfalls is suggested. The analysis incorporates both the attitudes of civil engineering and climatology on this natural phenomenon. Analysing the records of snow load annual maxima on the ground at measuring stations in Slovakia, the locations of occurrences of exceptional snow loads are identified. Then a climatological expertise is conducted resulting in the delimitation of three continuous lowland regions and a region of scattered mountain hollow-basins, in each of which similar climatological conditions for the occurrence of exceptional snowfalls can be expected. Within an individual region, an empirical probability distribution function based on annual snow load maxima at the observation stations is created, the upper tail of which is approximated by probability distribution functions of heavy and light tails. The accidental snow load on the ground corresponding to a mean return period of 10,000 years is assessed by the best fitting distribution function.

Seismic risk evaluation within the technology neutral framework

Abstract The NRC Office of Nuclear Regulatory Research has proposed a risk-informed and performance-based licensing process that is referred to as the technology neutral framework (TNF). In the TNF, licensing basis events (LBEs), determined using probabilistic risk assessment methods, take the place of design basis accidents. These LBEs are constructed by grouping together accident sequences with similar phenomenology. All event sequences with a mean frequency greater than 10−7 per reactor year are to be considered as part of the licensing basis. Imposing such a limit would require that earthquakes with a mean return period of ten million years be considered as part of the licensing basis. It is difficult to get seismic hazards (i.e., ground accelerations) from expert seismologists at such low frequencies. This is because it is difficult or impossible to confidently say what the seismic hazard might be at these extremely low frequencies. A linear extrapolation in log-log space of hazard curves at the Clinton site down to 10−7 per year leads to a peak ground acceleration of about 4.5 g. A Weibull distribution is also used to fit the curve leading to a peak ground acceleration of about 2.6 g. These extrapolations demonstrate the extreme nature of rare earthquakes. Even when seismic isolation is implemented, the TNF goal is not met. The problem appears to be that there is no limit on initiating event frequency in the TNF. Demonstrating that a design meets the goals of the TNF would be nearly impossible. A frequency limit for earthquakes could be imposed at a frequency of about 10−5 per year to focus on events that may be designed against. This type of initiating event frequency screening is consistent with current practice.

Development of comprehensive earthquake loss scenarios for a Greek and a Turkish city: seismic hazard, geotechnical and lifeline aspects

The development of reliable earthquake mitigation plans and seismic risk management procedures can only be based on the establishment of comprehensive earthquake hazard and loss scenarios. Two cities, Grevena (Greece) and Duzce (Turkey), were used as case studies in order to apply a comprehensive methodology for the vulnerability and loss assessment of lifelines. The methodology has the following distinctive phases: detailed inventory, identification of the typology of each component and system, evaluation of the probabilistic seismic hazard, geotechnical zonation, ground response analysis and estimation of the spatial distribution of seismic motion for different seismic scenarios, vulnerability analysis of the exposed elements at risk. Estimating adequate earthquake scenarios for different mean return periods, and selecting appropriate vulnerability functions, expected damages of the water and waste water systems in Duzce and of the roadway network and waste water system of Grevena are estimated and discussed; comparisons with observed earthquake damages are also made in the case of Duzce, proving the reliability and the efficiency of the proposed methodology. The results of the present study constitute a sound basis for the development of efficient loss scenarios for lifelines and infrastructure facilities in seismic prone areas. The first part of this paper, concerning the estimation of the seismic ground motions, has been utilized in the companion paper by Kappos et al. (2010) in the same journal.

Probability maps of landslide reactivation derived from tree-ring records: Pra Bellon landslide, southern French Alps

Probability maps of landslide reactivation are presented for the Pra Bellon landslide located in the southern French Alps based on results obtained with dendrogeomorphic analysis. Spatiotemporal patterns of past landslide activity was derived from tree-ring series of 403 disturbed mountain pine trees growing in the landslide body. In total, 704 growth disturbances were identified in the samples indicating 22 reactivation phases of the landslide body between 1910 and 2011. The mean return period was 4.5 years. Given the spatiotemporal completeness of the reconstruction, probabilities of landslide reactivation were computed and illustrated using a Poisson distribution model and for 5, 20, 50, and 100 years. Probability of landslide reactivation is highest in the central part of the landslide body and increases from 0.13 for a 5-year period to 0.94 for a 100-year period. Conversely, probabilities of reactivation are lower at its margins. The proposed method differs from conventional approaches based on statistical analyses or physical modeling that have demonstrated to have limitations in the prediction of spatiotemporal reactivation of landslides. Our approach is, in contrast, based on extensive data on past landslides and therefore allowed determination of quantitative probability maps of reactivation derived directly from the frequency of past events. This approach is considered a valuable tool for land managers in charge of protecting and forecasting people and their assets from the negative effects of landslides as well as for those responsible for land use planning and management. It demonstrates the reliability of dendrogeomorphic mapping that should be used systematically in forested shallow landslides.

Nonparametric seismic hazard estimation: A spatio-temporal application to the northwest of the Iberian Peninsula

Nonparametric techniques are frequently used in seismology studies to fit statistical models to earthquake data, as an alternative to classical parametric methods. In this paper, we apply nonparametric techniques to study some area-characteristic seismic hazard parameters (such as the mean return period or the expected number of earthquakes) in the northwest of the Iberian Peninsula, using a complete data catalog. Moreover, bootstrap techniques are used to develop simultaneous confidence bands for these functions. The previous nonparametric approaches are adapted to a spatial framework, obtaining maps of the probabilities of exceedance of the annual maximum magnitude in the zone of study. This procedure suitably describes the features of the area studied, being consistent with previous geological researches.

Assessment of seismic hazards for Hurghada, Red Sea, Egypt

The entrance of the southern Suez Gulf of the Red Sea is known to be an area of high seismic activity in Egypt. The high rate of seismic activity in this area is mainly related to the adjustment in motion at the triple junction between the African plate, the Arabian plate, and the Sinai microplate. The present study attempts to estimate the Probabilistic Seismic Hazard Analysis (PSHA) for Hurghada site. This was done in two steps; the first one is by estimating specific parameters for the site, such as the mean seismic activity, λ, the Gutenberg-Richter parameter, b, and the maximum regional magnitude, mmax. The second step is by selecting a ground motion parameter that is applicable to Hurghada site. The procedure permits the combination of both historical and recent instrumental data. The results of the hazard assessment, expressed as the worst case scenario, detect that Hurghada is exposed to the maximum credible earthquake event of magnitude mmax = 7.1 ± 0.31, at hypocentral distance of 31.6 ± 10 km. The possibility of the maximum Peak Ground Acceleration (PGA), which occurred in relation to this event at Hurghada site, is equal to 0.29 g. The mean return periods with the selected accelerations for Hurghada, a horizontal acceleration of 0.1 g, is expected to occur once every 74–106 years, with an average of one every 90 years. This result which obtained from the hazard assessment can be used as an input data for a seismic risk assessment.

Analysis of high level ozone concentrations using nonparametric methods

Controlling emissions of air pollutants and establishing air quality objectives to improve and protect ambient air quality are very important tasks of Governments. Ozone (O 3), one of those pollutants of concern, is not emitted directly into the atmosphere, but is a secondary pollutant produced by reaction between nitrogen dioxide (NO 2), hydrocarbons and sunlight. High levels of ozone can produce harmful effects on human health and the environment in general. Therefore, the study of extreme values of ozone represents an important topic of research in environmental problems. Classical extreme value theory has been usually used in air-pollution studies. It consists of fitting a parametric generalized extreme value (GEV) distribution to a data set of extreme values and using the estimated distribution to compute quantities like the probability of exceedance, the quantiles, the return levels or the mean return periods. In this paper, we propose nonparametric methods to estimate those quantities. Additionally, nonparametric estimators of the trends of very high values of ozone are proposed. The nonparametric estimators are applied to real samples of maximum ozone values obtained from several monitoring stations belonging to the Automatic Urban and Rural Network (AURN) from the UK. Results show that nonparametric estimators work satisfactorily, generally outperforming the behaviour of classical parametric estimators.

Seismic Response Spectra for Probabilistic Analysis of Nonlinear Systems

In the conventional seismic analysis of a system, the input motion is defined either by a probabilistic response spectrum or by ground-motion histories whose spectra “match” the probabilistic response spectrum. In both cases, it is implicitly assumed that the spectral values at different periods on a probabilistic response spectrum are fully correlated with one another. A nonlinear system changes its “effective” period during seismic shaking; hence, its response depends on spectral values at many natural periods. The assumption of complete correlation between spectral values at different periods results in an overestimation of the response of a nonlinear system. For the case analyzed in this paper, the conventional use of a 500-year mean return period (MRP) response spectrum in nonlinear analysis produces responses that have 20–30% longer MRPs. Also, the larger-component response acceleration was 11% higher than the geometric-mean response acceleration, and the larger component response deformation was 33...

Spatio-temporal reconstruction of snow avalanche activity using tree rings: Pierres Jean Jeanne avalanche talus, Massif de l'Oisans, France

Snow avalanches are a major threat in many parts of the Alps, where they periodically damage infrastructure, disrupt transportation corridors or even cause loss of life. Nonetheless, the spatial behavior of past avalanche activity and the analysis of areas affected during particular events remain often imprecise. It was therefore the purpose of this study to reconstruct spatio-temporal patterns of past avalanche activity on a forested avalanche talus in the French Alps (Pierres Jean-Jeanne talus, Massif de l'Oisans, France). A total of 232 European larches (Larix decidua Mill.) with clear signs of snow wasting events was analyzed and growth disturbances (GD) related to avalanche activity was assessed, such as tangential rows of traumatic resin ducts, the onset of compression wood or abrupt growth suppression and release. In total, 901 GD were identified in the tree-ring samples, indicating that 20 high-magnitude avalanches occurred between AD 1919 and 1994. The mean return period of snow avalanches was ~4 years with a ~26% probability that an avalanche occurs in any particular year. Interpolated maps allowed for explicit spatial estimates of return periods throughout the talus, showing a rapid increase of return frequency from 2.5 to 50 years with increasing distance from the talus apex. The distribution of avalanche years seems to be quite homogeneous in time with a gap between 1951 and 1959. Snowfall from a nearby meteorological station (Saint-Christophe en Oisans; 10 km from the study site) indicated that the five most recent high-magnitude events on record occurred due to above-average snowfall anomalies in December and January associated with abnormally low air temperatures. Findings suggest that a strong snow metamorphism under high temperature gradients in January could explain the occurrence of high-magnitude snow avalanches. 2010 Elsevier B.V. All rights reserved.