IC/2008/009. United Nations Educational, Scientific and Cultural Organization and International Atomic Energy Agency

Transcription

1 Available at: IC/2008/009 United Nations Educational, Scientific and Cultural Organization and International Atomic Energy Agency THE ABDUS SALAM INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS RECENT ACHIEVEMENTS OF THE NEO-DETERMINISTIC SEISMIC HAZARD ASSESSMENT IN THE CEI REGION G.F. Panza a,b, M. Kouteva 1,c,b, F. Vaccari a, A. Peresan a, C.O. Cioflan d, F. Romanelli a, I. Paskaleva c, M. Radulian d, K. Gribovszki e, M. Herak f, A. Zaichenco g, G. Marmureanu d, P. Varga e, M. Zivcic i a Earth Science Department, University of Trieste, Via E. Weiss 4, Trieste, Italy b The Abdus Salam International Centre for Theoretical Physics, SAND Group, Trieste, Italy c CLSMEE - BAS, 3 Acad. G. Bonchev str, 1113 Sofia, Bulgaria d NIEP - Magurele-Bucharest, 12 Calugareni str., Ilfov, Romania e Geodetic and Geophysical Research, Institute of HAS, Sopron, Hungary f Department of Geophysics, Faculty of Science, University of Zagreb, Horvatovac bb, Zagreb, Croatia g IGG - Institute of Geophysics and Geology, Moldavian Academy of Sciences, Chisinau, Republic of Moldova i ARSO - Seismology and Geology Office, Ljubljana, Slovenia MIRAMARE TRIESTE March Junior Associate of ICTP.

2 Abstract A review of the recent achievements of the innovative neo-deterministic approach for seismic hazard assessment through realistic earthquake scenarios has been performed. The procedure provides strong ground motion parameters for the purpose of earthquake engineering, based on the deterministic seismic wave propagation modelling at different scales - regional, national and metropolitan. The main advantage of this neo-deterministic procedure is the simultaneous treatment of the contribution of the earthquake source and seismic wave propagation media to the strong motion at the target site/region, as required by basic physical principles. The neo-deterministic seismic microzonation procedure has been successfully applied to numerous metropolitan areas all over the world in the framework of several international projects. In this study some examples focused on CEI region concerning both regional seismic hazard assessment and seismic microzonation of the selected metropolitan areas are shown. 1

3 INTRODUCTION Deterministic versus probabilistic approaches have differences, advantages, and disadvantages in assessing earthquake hazards and risks that often make use of one advantage over the other. The factors that influence the choice of seismic hazard assessment procedure including the decision to be made (i.e. the purpose of the hazard or risk assessment), the seismic environment (whether the location is in a high, moderate, or low seismic risk region), the available input data and the scope of the assessment (whether one is assessing a site risk, a multi-site risk, or risk to a region) have been recently discussed in the literature. THE NEO-DETERMINISTIC SEISMIC HAZARD ASSESSMENT (NDSHA) PROCEDURE - MAIN ADVANTAGES COMPARED TO THE PROBABILISTIC PROCEDURE McGuire [1] proposed a general rule stating the more qualitative the decision to be made, the more appropriate deterministic hazard assessment is. The classification of "highly qualitative" related to the deterministic assessments is debatable and in this paper, realistic and highly quantitative deterministic seismic hazard assessments at regional and local urban levels in the CEI region, are shown. The seismic environment plays also a strong role in the appropriateness of the deterministic assessments. For high seismic regions, where the largest earthquakes may occur every years, the 475-year shaking could be considered as design ground motion. This may correspond to the largest magnitude on the closest fault to the site, which is particularly relevant to a site located next to an active fault. A deterministic scenario for this event would allow to examine such details as ground motion effects caused by rupture propagation, leading to insights on the risk for a particular lifeline or city that might not be available from more encompassing probabilistic analyses. Regional assessments often benefit most from deterministic models, where the probability of occurrence of the scenario in, for example, one city is small, but is large for the region. The concept of multiple deterministic scenarios allows rational preparation, even if the details of the forecast earthquake may be wrong. The seismic hazard evaluation, which is based on the traditional Probabilistic Seismic Hazard Analysis (PSHA) relies on the probabilistic analysis of earthquake catalogues and of ground motion, macroseismic observations and instrumental recordings. Recently PSHA showed its limitation in providing a reliable seismic hazard assessment, possibly due to insufficient information about historical seismicity, which can introduce relevant errors in the purely statistical approach mainly based on the seismic history. The comparison between the observed peak ground accelerations (PGA) and the PGA predicted by PSHA (the GSHAP Project) for the recent examples of the Kobe, ; Bhuj, ; Boumerdes, and Bam, has shown significant disagreement [2]. The probabilistic map, for the 475 years return period, gives a maximum PGA in the range g, centered on the Vrancea region (GSHAP). Observations clearly indicate that Vrancea sits on a relative minimum. The major uncertainties and sources of errors of the PSHA have recently been analyzed by Klügel et al., [3, 4, 5]. The standard error of empirical attenuation equations (or the difference between observation and estimates of a physical predictive model) is one of the main sources of errors in the traditional PSHA, since it is interpreted as aleatory uncertainty and not understood as an (limited by our knowledge) estimate of total variability of the stochastic process attenuation equation. In view of the limited seismological data, it seems more appropriate to resort to a scenario-based deterministic approach, as it allows us to realistically 2

4 define hazard in scenario-like format accompanied by the determination of advanced hazard indicators as, for instance, damaging potential in terms of energy. In 2006 Klügel et al. [3] proposed a scenario-based procedure for seismic risk analysis. This scenario-based approach allows us to incorporate all available information collected in a geological, seismotectonic and geotechnical database of the site of interest as well as advanced physical modelling techniques providing a reliable and robust deterministic design basis for civil infrastructures. At the same time a scenario-based seismic hazard analysis allows to develop the required input for probabilistic risk assessment (PRA), as required by safety analysts and insurance companies. The scenario-based approach removes the ambiguity in the results of probabilistic seismic hazard analysis (PSHA) which relies on the projections of the Gutenberg Richter (G R) equation. The problems in the validity of G R projections, because of the incomplete to total absence of data for making the projections, are still unresolved. The scenario-based methodology is strictly based on observable facts and data and complemented by physical modelling techniques, which can be submitted to a formalized validation process. By means of sensitivity analysis, knowledge gaps related to lack of data can be dealt with easily, due to the limited amount of scenarios to be investigated [3]. The comparative analyses of the recently published results on regional seismic hazard assessments, obtained via PSHA and NDSHA [2; 6] have shown, as appropriate, the suggestion to limit the probabilistic analysis to the definition, for a given area, of the magnitude of the different scenario earthquakes: (a) disastrous (return period about 500 years); (b) very strong (return period about 250 years); (c) strong (return period about 125 years); (d) frequent (return period about 60 years) and to use them for deterministic computations [6]. TABLE 1. Summary of the discussed seismic hazard assessment procedures, PSHA, DSHA and NDSHA. Procedure description Step 1 Step 2 PSHA DSHA NDSHA Recurrence rate can be represented by a linear relation only if the size of the study area is large with respect to linear dimensions of sources. Seismic sources Identification of Seismogenic Zones and Capable Faults; Epicenters; Geometry and Focal mechanism; Fixed magnitude Fixed distance Choice of the Controlling Earthquake Scenario Earthquakes fixed magnitudes, distances and specific seismic source properties. Choice of the Controlling Earthquake Step 3 Step 4 Attenuation relations - they represent the functional dependency of the random spectral acceleration on the random variables, magnitude, distance and measurement error [3] and thus are source of systematic error in the seismic hazard assessment Seismic hazard assessment in terms of Probability of exceedance of a given ground motion measure Seismic hazard assessment in terms of Fixed Ground Motion Measure Synthetic ground motions. NO NEED OF ATTENUATION RELATIONS. Seismic hazard assessment Envelopes of PGA or other Ground Motion Measure The main advantage of the proposed neo-deterministic procedure is the simultaneous treatment of the contribution of the seismic source and seismic wave propagation media to the strong motion at the target site/region, as required by basic physical principles [7]. A brief comparative analysis of the traditionally used seismic hazard approaches, probabilistic 3

5 (PSHA) and deterministic (DSHA), and the neo-deterministic seismic hazard assessment (NDSHA) is provided in Table 1. NDSHA Applications for the CEI Region In the framework of the UNESCO-IUGS-IGCP Project 414 and taking the benefits of the existing CEI Network, neo-deterministic hazard computation for some CEI countries have been performed at national and regional scales. The peak values of the bedrock acceleration (frequency content 0-1 Hz), computed in Slovenia for the case of the Mt. Sneznik events were simulated and exhibited considerable similarity with the response spectra of the 1995 Mt. Sneznik [8]. Computation of realistic synthetic seismograms for the Croatian territory has yielded the also meaningful results, thus providing a powerful and economically valid scientific tool for seismic zonation and hazard assessment [9]. The results obtained for Hungary have shown that considerable seismic hazard can be expected in three major Hungarian cities, but for the significant part of the Great Hungarian Plain the seismic hazard can be practically neglected [10]. The deterministic seismic zoning of Romania [11] leads to effective peak acceleration (EPA) estimates in the range g. These values are shifted with respect to the hypocentral zone, in agreement with observations. Using numerically simulated ground motion, a first-order deterministic evaluation of the seismic hazard of Romania has been proposed [11]. Design Ground Accelerations (DGA) values greater than 0.3g have been obtained over an extended area. The distribution of the peak values numerically determined correlates with the values recorded in the areas situated eastwards and southward of the Carpathians arc. The results of the innovative integrated neo-deterministic seismic hazard assessment for the Italian territory have been recently discussed by Peresan et al. [6]. The obtained impending macroseismic intensities (ISG) [12], estimated in terms of peak ground velocities and displacements, and DGA as well, are consistent with the observed ones, showing intensities in the range of VII - X over the Italian territory. Maps of neo-determinstic seismic hazard (DGA, Peak Ground Velocities and Peak Ground Displacements) for some European countries have been published [13] and an example is shown in Figure 1. Shallow seismicity has been considered, limiting the computations to epicentral distances shorter than 90 km. The hypocentral depth considered is 10 km for events with magnitude Mw < 7 and 15 km for larger events. In the case of the Vrancea intermediate depth events spectral properties especially determined for the Romanian intermediate-depth earthquakes has been considered and the computations have been performed over the Romanian, Northeastern Croatian and Hungarian territory, within a circle of 350 km of radius centered on Vrancea. The hypocentral depths considered are 90 km for magnitude less than 7.4 and 150 km for larger quakes. According to this map DGA, up to 0.675g, can be expected in the investigated region. 4

6 FIGURE 1. Design Ground Acceleration computed for the CEI Region by Panza and Vaccari [2000]. NDSHA Applications for Seismic Microzonation Purposes The neo-deterministic approach has been successfully applied to numerous metropolitan areas e.g. Delphi (India), Beijin (China), Naples (Italy), Algiers (Algeria), Cairo (Egypt), Santiago de Cuba (Cuba), Thessaloniki (Greece) [14]. Complete synthetic seismograms in terms of displacement, velocity and acceleration are computed separately for the SH and P-SV waves in the frequency range of interest. For urban areas where the numerical modelling of ground motion has been successfully compared with strong motion records, the computations of synthetic seismograms permit a detailed microzoning based upon the set of possible scenario earthquakes. The results obtained for all discussed case studies have shown that for areas where very limited or no recordings are available the synthetic time series can be used to estimate the expected ground motion, thus leading to a pre-disaster microzonation without having to wait for an earthquake to occur. The use of synthetic computations is also necessary to overcome the fact that the local site response can be strongly dependent upon the properties of the seismic source generating the seismic input. The constructed data set of synthetic seismograms can be fruitfully used and analyzed by civil engineers for design and reinforcement actions, and therefore supply a particularly powerful and economical tool for the prevention aspects of Civil Defense. The well-documented distribution of damage in Rome, caused by the Fucino earthquake, is, in fact, successfully compared by Fäh et al. [15] with the results of a series of different numerical simulations, using PGA and Arias intensity. The good correlation between PGA and the damage statistics made it possible to extend the zoning to the entire city of Rome, thus providing a basis for the prediction of the expected damage from future strong events. The hybrid approach was successfully applied to compute P-SV (radial and vertical components) and SH (transversal component) synthetic displacement time series, velocity time series and accelerograms along the constructed eleven local models at Debrecen, Hungary, considering theoretically the macroseismic effects of intensity VI - VII (MSK) [16]. The obtained results are in agreement with the observed macroseismic intensities due to the devastating 1834 earthquake, which occurred in the Érmellék seismoactive region. The numerical synthesis of ground motion, simultaneously accounting for source, wave propagation path and local site geology, has been particularly useful for the microzonation of 5

7 Bucharest, the capital of Romania. The synthetic seismic signals (frequency range up to 1 Hz) have been computed for three recent strong intermediate-depth Vrancea earthquakes (1986 and 1990) along three representative profiles crossing the Bucharest area. The computed acceleration response spectra at the Magurele recording station have been successfully compared with the available registrations. The analysis of the characteristics of the computed response spectra, in correlation with the geological features of the shallow structure, results in a preliminary seismic zonation of the city [17]. Using the hybrid method, complete synthetic accelerograms (up to frequencies of 6 Hz) have been computed along a selected known geological profile crossing Zagreb, the capital and largest city of Croatia, considering the Kasina 1880 earthquake [18]. The computed PGA amplification along the investigated profile has been consistent with the reported distribution of intensities for this earthquake, which exhibited a uniform band of intensity I = VIII (MSK) stretching from the epicenter southwestwards along the investigated profile. Using a set of realistically chosen scaled sources [19] has shown that even variations of the order of commonly observed uncertainties of only dip and rake angles of the seismogenic fault cause the amplification to vary at some sites by more than a factor of two. They concluded that, especially for strongly laterally heterogeneous structures, local effects must be determined for each of the relevant sources considering all associated uncertainties as completely as possible. The city of Sofia, which is exposed to a high seismic risk, is a good example when a reliable definition of the seismic input for earthquake engineering purposes in the region are necessary in the lack of both instrumental data and the possibility to accurately quantify the magnitude scaling and the attenuation characteristics of the large magnitude earthquakes. The obtained results, applying the hybrid approach, are consistent with the few macroseismic data available for the city. The elastic displacement demand computed for Sofia has been consistent with the available near-field observations worldwide [20]. Synthetic time histories and site response along a selected geological profile crossing Russe, NE Bulgaria, for five recent strong intermediate-depth Vrancea earthquakes (1940, 1986, 1977 and 1990) have been computed applying the mode summation combined with the mode coupling technique [21]. The obtained results have been successfully validated against the available accelerograms and the derived acceleration response spectra. The analyses of the seismic record and the computed site response along the investigated profile at Russe have confirmed that urban areas located at large epicentral distances with respect to the seismic source may be prone to severe earthquake hazard and that all ground motion components give a significant contribution to the design seismic input. CONCLUSIONS The necessity of preventive action arises from the increasing vulnerability of the society in the processes of running globalization and urbanization due to the high and underestimated earthquake risk. The recently completed UNESCO - IUGS - IGCP Project 414, and the still running CEI Projects, the Central Europe Regional Geodynamics Project (CEGROP) and the Unification of Gravity Systems in Central and Eastern Europe (UNIGRACE) brought about, through their scientific conferences and publications, a new view on the dynamics of the Central European area. For areas where very limited or no recordings are available the synthetic time series can be used to estimate the expected ground motion, thus leading to a pre-disaster microzonation without having to wait for an earthquake to occur. The use of synthetic computations is also necessary to overcome the fact that the local site response can be strongly dependent upon the properties of the seismic source generating the seismic input. The scenario-based methodology is strictly based on observable facts and data and is complemented by physical modelling techniques, which can be submitted to a formalized validation process. By means of sensitivity analysis, knowledge gaps related to lack of data 6

8 can be dealt with easily, due to the limited amount of scenarios to be investigated. The applied neo-deterministic procedure provides realistic and reliable seismic input, which could be directly used in earthquake engineering practice, urban planning, land using, the insurance industry against natural disasters - e.g. design of new constructions, estimates of the earthquake resistant capacity of the existing built stock and metropolitan seismic microzonation. ACKNOWLEDGMENTS The Environment and Sustainable Development Programme, Contract number: EVK2-CT , the UNESCO - IGCP Project 414, the INTAS-Moldova Project Numerical Analysis of 3D seismic wave propagation using Modal Summation, Finite Elements and Finite Differences Method, the CEI university network and the CEI Project Geodynamical Model of Central Europe For Safe Development Of Ground Transportation Systems are gratefully acknowledged. This work was done within the framework of the Associateship Scheme of the Abdus Salam International Centre for Theoretical Physics, Trieste, Italy. REFERENCES 1. McGuire R., Deterministic vs Probabilistic Earthquake Hazards and Risks, Soil Dynamics and Earthquake Engineering 21 (2001), Panza G.F., Cioflan C.O., Kouteva-Guentcheva M., Paskaleva I., Marmureanu G., Radulian M. (2007) Preventive Actions to the Impactof Large Vrancea Events Carried on in the Framework of CEI; 3rd CEI Conference Science and Technology for Safe Development of Life Line Systems, October 24th 26th, 2007, Bucharest, Romania, 3. Klügel J.-U. Mualchin L., Panza G.F. (2006) A scenario-based procedure for seismic risk analysis, Engineering Geology 88 (2006) 1 22, available online at www. sciencedirect.com 4. Klügel, J. -U. (2007 a) Error inflation in Probabilistic Seismic Hazard Analysis, Engineering Geology 90 (2007) , available online at www. sciencedirect.com 5. Klügel, J. -U. (2007 b) Comment on Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates by Julian J. Bommer and Norman A. Abrahamson, BSSA, Vol. 97, No.6, pp.-, comment, doi: / Peresan A., Panza G.F., Romanelli F., Vaccari F. (2008) Deterministic seismic hazard assessment and earthquake prediction; 7. Panza, G.F., Romanelli, F., Vaccari, F. (2001) Seismic wave propagation in laterally heterogeneous anelastic media: theory and applications to the seismic zonation, Advances in Geophysics, Academic press; 43: Zivcic Ml., Suuhadolc P., Vaccari F. (2000) Seismic Zoning of Slovenia Based on Deterministic Hazard Computations. In "Seismic Hazard of the Circum-Pannonian Region (Editors: Panza, GF., Radulian, M., Trifu. C.), Pageoph Topical Volumes, Birkhauser Verlag: Markusic S., Suhadolc P., Herak M., Vaccari F. (2000) A Contribution to Seismic Hazard Assessment in Croatia from Deterministic Modeling. In "Seismic Hazard of the Circum-Pannonian Region (Editors: Panza, GF., Radulian, M., Trifu. C.), Pageoph Topical Volumes, Birkhauser Verlag: Bus Z., Szeidovitz G., Vaccari F. (2000) Synthetic Seismogram Based Deterministic Seismic ZOning for the Hungarian Part of the Pannonian Basin. In "Seismic Hazard of the Circum-Pannonian Region (Editors: Panza, GF., Radulian, M., Trifu. C.), Pageoph Topical Volumes, Birkhauser Verlag: Radulian M., Vaccari F., Manderscu N., Panza G.F., Moldoveanu C.L. (2000) Seismic Hazard of Romania: Deterministic Approach. In "Seismic Hazard of the Circum-Pannonian Region (Editors: Panza, GF., Radulian, M., Trifu. C.), Pageoph Topical Volumes, Birkhauser Verlag: Panza, G.F., Vaccari F., Cazzaro R. (1999) Deterministic seismic hazard assessment. In F. Wenzel et al. (Eds), Vrancea Earthquakes: Tectonics, Hazard and Risk Mitigation, Kluwer Academic Publishers, The Netherlands. 13. Panza, G.F., Vaccari F. (2000) Introduction in seismic hazard of the Circum-pannonian region (Editors: Panza, GF., Radulian, M., Trifu. C.), Pageoph Topical Volumes, Birkhauser Verlag: Panza, G.F., Alvarez, L., Aoudia, A., Ayadi, A., Benhallou, H., Benouar, D., Bus, Z., Chen, Y.-T., Cioflan, C., Ding, Z., El-Sayed, A., Garcia, J., Garofalo, B., Gorshkov, A., Gribovszki, K., Harbi, A., Hatzidimitriou, 7

9 P., Herak, M., Kouteva, M., Kuznetzov, I., Lokmer, I., Maouche, S., Marmureanu, G., Matova, M., Natale, M., Nunziata, C., Imtiyaz A. Parvez, Paskaleva, I., Pico, R., Radulian, M., Romanelli, F., Soloviev, A., Suhadolc, P., Szeidovitz, G., Triantafyllidis, P., Vaccari F., (2002). Realistic Modeling of Seismic Input for Megacities and Large Urban Areas (the UNESCO/IUGS/IGCP project 414). Episodes, 25(3), Fäh, D., Iodice, C., Suhadolc, P., Panza, G.F. (1993) A new method for the realistic estimation of seismic ground motion in megacities: The case of Rome, Earthquake Spectra 9: Gribovszki K., Panza G.F. (2004): Seismic microzonation with the use of GIS (Case study for Debrecen, Hungary). Acta Geod. Geoph. Hung., 39(2-3). pp Cioflan, C.O., Apostol, B.F., Moldoveanu, C.L., Panza, G.F., Marmureanu, Gh. (2004) Deterministic Approach for the Seismic Microzonation of Bucharest, PAGEOPH 161, Lokmer I, Herak M, Panza GF, Vaccari F. (2002): Amplification of strong-ground motion in the city of Zagreb, Croatia, estimated by computation of synthetic seismograms, Soil Dynamics and Earthquake Engineering, 22, Herak M, Lokmer I, Vaccari F, Panza GF (2004): Linear amplification of horizontal strong ground motion in Zagreb (Croatia) for a realistic range of scaled point sources, PAGEOPH, 161, Paskaleva, I., Kouteva, M., Vaccari, F., Panza, G.F. (2008) Characterization of the Elastic displacement demand: case STUDY - Sofia City, AGGH, (in print). 21. Kouteva, M., Panza, G.F., Romanelli, F., Paskaleva, I. (2004) Modelling of the ground motion at Russe site (NE Bulgaria) due to the Vrancea earthquakes. Journal of Earthquake Engineering, 8, 2,