Development of Probabilistic Seismic Hazard Analysis for International Sites, Challenges and Guidelines

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1 Development of Probabilistic Seismic Hazard Analysis for International Sites, Challenges and Guidelines ABSTRACT Dr. Antonio Fernandez Ares Paul C. Rizzo Associates, Inc. 500 Penn Center Boulevard, Suite 100 Pittsburgh, PA This article provides guidance to conduct a site-specific seismic hazard study, giving suggestions for overcoming those challenges that are inherent to the significant amounts of epistemic uncertainty for sites at remote locations. The text follows the general process of a seismic hazard study, describing both the deterministic and probabilistic approaches. Key and controversial items are identified in the areas of recorded seismicity, seismic sources, magnitude, ground motion models, and local site effects. A case history corresponding to a seismic hazard study in the Middle East for a Greenfield site in a remote location is incorporated along the development of the recommendations. Other examples of analysis case histories throughout the World are presented as well. 1 INTRODUCTION The use of building codes to establish seismic design basis parameters are applicable to regions where: 1. Sufficient earthquake data is available by means of both historical data and instrumentation, conditions typical of populated regions with recorded seismicity; 2. Extensive research is well documented; and 3. The performance criteria of the code in place are applicable and consistent with the expected lifespan and functionality of the structure. If any of the previous three conditions are not met, a site- specific seismic hazard analysis is either required or highly recommended. For example, a Nuclear Power Plant (NPP) located near a major urban region with moderate to high seismicity will meet the first and second criteria, but not necessarily the third, since high profile projects are expected to perform satisfactorily under higher seismic demand. This article provides guidance to conduct a site-specific seismic hazard study, giving suggestions for overcoming those challenges that are inherent to the significant amounts of epistemic uncertainty for sites at remote locations. The text follows the general process of a seismic hazard study, describing both the deterministic and probabilistic approaches. Key and controversial items are identified in the areas of recorded seismicity, seismic sources, magnitude, ground motion models, and local site effects. 2 DETERMINISTIC AND PROBABILISTIC ANALYSIS One of the following strategies or approaches may be undertaken when performing a seismic hazard analysis:

2 1. Deterministic Approach: uses a seismotectonic model and selects a source with a prescribed earthquake magnitude and distance to estimate the hazard at a given location. A deterministic approach may be thought of as a scenario-like description of earthquake hazard (Reiter 1999). 2. Probabilistic Approach: a Probabilistic Seismic Hazard Analysis (PSHA) determines the frequency for which a hazard parameter, such as acceleration or intensity, reaches some given threshold during some time in the future (McGuire 2004). The analysis incorporates all sources in the seismotectonic model assuming that they can act independently; 3. Hybrid Approach: a PSHA is conducted and a deaggregation analysis is performed to understand which distances and magnitudes contribute most to the hazard. A source with such magnitude-distance pair characteristic is selected as the controlling event and a deterministic analysis is performed. Figure 1 provides a graphic representation of the deterministic and probabilistic approaches. The deterministic analysis is simple in nature and it is ideal to perform scenariolike analysis. It helps the analyst gain insight on the effects of each source independently. It also has the advantage of allowing owners and engineers to select a controlling source that best addresses the design concerns at a particular site. If the controlling source is well defined, the deterministic approach will save time by allowing engineers to avoid the research of earthquake parameters of multiple sources that are not relevant to a site. The analysis may be geared towards the controlling source and its detailed characterization, and more reduction of epistemic uncertainty may be attained. A case-history for which the controlling event is well defined presents itself for a power plant site in the State of Missouri in the United States (Figure 2). The plant sits in a stable continental region located approximately 320 km from the New Madrid Fault Source Zone (NMFZ), where the largest earthquake cluster in the continental United States has been recorded. These events took place during the winter months. The magnitudes of the events ranged from M7.0 up to M8.1 (USGS 2008). Figure 1: Deterministic and Probabilistic Seismic Hazard Analysis For some cases in which a dominant controlling source is identified, a deterministic analysis is prudent, such as the NMFZ (Figure 2). Otherwise, a PSHA is recommended since

3 the results will show how earthquake hazard varies as a function of probability of exceedance, automatically giving an indication of the performance of the site under different demand earthquakes. In addition, hazard across the frequency spectrum is better assessed without the need of constructing a bounding spectra resulting from the independent hazard imposed by each source. A PSHA may be far superior to a deterministic approach. This is in general a legitimate statement, but one that needs to be taken with reservation. A PSHA must be tied to a comprehensive investigation of the seismic and tectonic conditions, incorporating expertise from experienced geologists, seismologists, and engineers. Failing to do so will result in unreliable results and wasted mathematical and computer simulations. A PSHA may disguise the effects of important controlling features so special attention needs to be placed to adequately represent sources and define characteristic events. Therefore, a PSHA is not recommended if resources for thorough research and analysis are not available. 3 PSHA, EARTHQUAKE SOURCES It is common practice to include all sources within a 320 km radial region and other sources that will contribute more than one percent of the hazard. Seismic sources are characterized as the following: General Area Sources The recurrence relationships are represented by the Guttenberg-Richter parameters in the form of the Truncated Exponential Distribution (TED). The probability of an earthquake of magnitude m occurring at the source is obtained with the TED (Eq. 1). If the activity of a source cannot be represented by the TED, it is necessary to develop a Characteristic Earthquake model. Where: M Magnitude; m min Minimum magnitude of earthquakes in source; m max Maximum magnitude of earthquakes in source; P(M>m) Probability of occurrence of an earthquake with magnitude M larger than m; and β β= β ln(10); slope of the exponential fit. Fault Sources These sources are directly tied to a well identified rupture zone that shows direct correlation with earthquake activity. Figure 2 shows the NMFZ, a prominent fault source in the Central United States. Background Activity not explained by area sources or faults. There is a significant amount of uncertainty in developing the geometry and recurrence of seismic sources. In doing so, it is important to account for historical seismicity, tectonic setting, and geologic studies. In many cases, historical seismicity will be the main driver for the definition of the properties of a seismic source. Development of seismic source models involves significant amounts of epistemic uncertainty. Common questions are: What is the maximum credible earthquake in a zone? Where are the boundaries that delineate the zone? What is the slip rate and length of a fault zone? What is the distribution of seismicity within a zone? (1)

4 Figure 2: New Madrid Fault Zone Controlling Feature To illustrate the fact, the Electrical Power Research Institute (EPRI) in the United States performed a high level study to define seismic sources in the United States involving up to six expert Earth Science Teams (EST) (EPRI 1986). In 2009 an updated seismic source model was developed in the Arab peninsula for a site in the Middle East (RIZZO, 2009). The model was used in a PSHA along with two independent models to account for epistemic uncertainty (Figure 3) (Musson, 2006, Aldama-Bustos, 2008). The development of seismic source models is a process that requires an in-depth analysis of existing extensive research in the fields of seismicity, paleoseismicity, geology, and tectonics. 4 EARTHQUAKE CATALOG AND RECURRENCE The recurrence laws that are assigned to each seismic source are derived from an earthquake catalog. The catalog is a list of recorded earthquakes that provides the following information: time of happening, location, and some measure of earthquake damage, such as intensity or magnitude. With the earthquake catalog, it is possible to obtain the logarithmic slopes and intercept of the recurrence model, which is conceptually shown in Figure 1. Figure 3: Alternate Seismic Source Models The least-squared regression and maximum likelihood method are the most common techniques to fit earthquake data. Catalogs must be developed with a compilation of historical accounts of seismicity and global and local earthquake records. Reliable catalog data may be obtained from organizations such as the International Seismological Center (ISC), UK or the US. Geological Survey Preliminary Determination of Epicenters (PDE). Earthquake catalogs need to be carefully compiled taking into account consistency of magnitude units, declustering of aftershocks and foreshocks, and completeness for periods of historical seismicity for which only large earthquakes are recorded. Once the seismic sources are defined and the earthquake catalog is built, it is possible to obtain the three key and critical parameters of an earthquake source: (1) Recurrence at low magnitude ( a Value), (2) b Value, and (3) Maximum Magnitude. Careful attention needs to be placed in the way in which

5 seismicity is distributed throughout the seismic source zone. It is important to account for the significant levels of aleatory uncertainty related to the location of future earthquakes. A common approach is to keep the recurrence constant throughout the zone, while smoothing the b value to each grid cell (USGS 2008). The selection of maximum magnitude has significant impact in the outcome of the PSHA. Both the a and b values will increase with increasing maximum magnitudes within a source zone. This parameter should not be based only on recorded seismicity, since the probability that the Maximum Credible Earthquake (MCE), which has already been recorded, is very low. The selection of the minimum magnitude also has a significant impact. The minimum magnitude can bias the hazard for higher response spectra frequencies. This bias results from incorporating close-in small magnitude earthquakes, whose recurrence rate is excessive relative to larger magnitude earthquakes of engineering significance due to the exponential recurrence relationship. It is common practice to adopt a value of m b 5.0, since it is well documented that earthquakes with lower magnitudes have a very low probability to cause damage to engineered facilities (USNRC 2007). 5 GROUND MOTION MODELS, PSHA COMPUTATION As previously stated, a PSHA determines the frequency for which a hazard parameter such as acceleration or intensity, reaches some given threshold during some time in the future. The analysis incorporates all sources in the seismotectonic model assuming that they can act independently; recurrence rates of earthquake events associated with distance and probability distributions of maximum magnitude are included to quantify the contribution to the hazard from each of the seismic sources. The PSHA is conducted in the steps indicated by Figure 1. The integration of seismic hazard for a particular location is given by Equation 2. P i (A>a m at r) Frequency with which a is exceeded Occurrence earthquakes at source i Source features Distance from source to receiver Probability of exceeding a at distance r from source, given magnitude m in source i Probability of m magnitude earthquake (2) Step 3 of Figure 1 relates to the Ground Motion Models (GMM). A GMM is an attenuation model that relates the energy released at the earthquake location to the energy experienced at the receiver location. In the context of the PSHA integration equation (Equation 3), the GMM are represented by the conditional probability factor that defines the probability of exceeding a ground motion level threshold, such as Peak Ground Acceleration (PGA), conditional to the occurrence of an earthquake at some distance away from the site: P(A>a m at r). Overall GMM are classified into one of the following categories: Empirical - Models primarily based on statistical analysis of earthquake data that consists of magnitude, distance, and soil properties. Stochastic Models that are primarily based on the physical properties of the energy released during an earthquake and the travel path of seismic waves. These models incorporate the characteristic of the earthquake mechanisms and therefore desirable for well investigated tectonic features. Stochastic models may be applied in regions for which not enough instrumented seismicity is available. Analytical Models based on numerical simulations of wave propagation.

6 Models built with a combination of the previous categories are denominated Hybrid Models. A comparison of the form of several ground motion models used for a PSHA in a remote region is provided in Figure 4. Naturally, two questions come to mind: (1) what attenuation model is the best fit for a particular problem; and (2) how reliable is the fit provided by the attenuation model? The answer to both questions is not trivial since there are significant amounts of uncertainty in the answers to both questions. Epistemic uncertainty is associated to the first question while aleatory uncertainty is associated with the second. Figure 4: Ground Motion Models Tables and figures should be placed close after their first reference in the text. All figures and tables should be numbered with Arabic numerals. Table headings should be centered above the tables. Figure captions should be centered below the figures. Aleatory uncertainty in the use of GMM originates from the scattering of earthquake data that is involved in the curve fitting process. The uncertainty is schematically indicated by the normal distribution in Step 3 of Figure 1 (P(Acc>a m). To account for epistemic uncertainty for a PSHA for a site project in the Middle East, three sets of GMM were selected based on the tectonic scenarios. The seismic source models shown in Figure 2 indicate that ground motion originates from three types of mechanisms and/or environments: (1) active zones in the Zagros Belt, (2) the Makran subduction zone, and (3) the stable region in the Arab Peninsula. For this PSHA the soil properties at the site location are not consistent with the regions from which data was reduced to develop the attenuation equations. This condition is common at locations in which marginal or null development has occurred so there is no soil site-specific data and there has been no particular interest to develop GMM. Careful analysis must be dedicated to gain reasonable compatibility between ground motion models and site-specific soil properties. It was therefore required to apply equations developed in other regions with similar geologic and tectonic environments. This problem is commonly denominated as the Target-Host consistency problem. The prevailing site-specific conditions considered for the development of the attenuation equations are referred to as the Target. The site-specific conditions at the location of interest are the Host. The steps involved in the development of the PSHA have been touched upon throughout the text. The outcome of the PSHA is a hazard curve, which provides ground motion levels for different probabilities. Figure 5 gives an example of hazard curves obtained for a site in the Central United States for Peak Ground Acceleration (PGA) for hard rock conditions. Equivalent curves are developed for spectral accelerations and a Uniform Hazard Response Spectra (UHRS) at hard rock conditions is built (Figure 6). This spectra represents the ground motion that is applied to a soil column to analyze local amplification effects.

7 Figure 5: Hazard Curves Figure 6: Hazard Curves 6 SITE AMPLIFICATION ANALYSIS The final step to assess earthquake hazard is site amplification. It is well documented that, as vertically propagated shear waves travel from the ground to the surface, the ground motion changes significantly, especially if major contrasts in shear wave velocity are present between layers of the soil model. The goal of the amplification analysis is to develop the Ground Motion Response Spectra (GMRS) (USNRC 2007). Other points of application of ground motion may also be required and these are denominated Foundation Input Response Spectra (FIRS), as shown by Figure 7, a conceptual plot showing the position of the GMRS and FIRS. Figure 7: GMRS and FIRS The subsurface conditions must be defined in detail since the shear wave velocity of the soil column is a key input to the amplification analysis. Therefore, a detailed geotechnical and geophysical exploration program is required. Figure 8 provides guidance to develop GMRS by considering the following steps: 1. PSHA Obtain Uniform Hazard Response Spectra (UHRS); 2. Deaggregation Controlling events and time histories; 3. Site Amplification Analysis Use shear wave velocity profile and develop the Amplification Function (AF); 4. Uniform Hazard Response at Soil Develop UHRS Soil with AF and UHRS Rock; 5. Performance Apply factors to soil spectra to account for site performance under different probabilities; and 6. V/H Ratio Develop vertical to horizontal ground motion ratios and obtain the GMRS or FIRS.

8 An example of GMRS is shown for a site in the Central United States and is provided in Figure 9. Figure 8: Steps involved in the development of GMRS Figure 9: GMRS for site in Central United States 7 SENIOR SEISMIC HAZARD COMMITTEE For a comprehensive PSHA, it is fundamental to incorporate the expertise of the scientific community. A PSHA analyst will encounter controversy in the selection of inputs and the development of calculations. Therefore, it is recommended to incorporate the opinion of the learned scientific community, which is represented by experts in the fields of geology, seismology, and engineering. Differences in opinion between experts are the rule rather than the exception, and therefore, it is not trivial to reach consensus on the selection of input and calculation approach. One of the strengths of the PSHA is that it has the mechanisms in place to handle uncertainty and incorporate different interpretations into the analysis. The SSHAC should promote the incorporation of differences of legitimate scientific opinion that exist in a seismic hazard study. Consensus should be reached on how to account for different views and interpretations. 8 CONCLUSIONS The development of a successful PSHA is a challenge that involves expertise from multiple disciplines in earth science and engineering. It also requires in-depth, site-specific investigations that include understanding the regional and site geology, seismicity, state-ofthe-art ground motion estimation methods, and geophysical conditions. Venturing into a PSHA without a serious and comprehensive site-specific investigation is a futile and meaningless exercise. It is imperative to reduce uncertainty with investigations that are meaningful, relevant, and implementable. More importantly, it is essential to understand the nature of uncertainty and what it means in the context of the outcome to the PSHA. A PSHA must always be updated as new data and new research becomes available. The most evident example is the occurrence of an earthquake of unexpected magnitude or unexpected location. 9 REFERENCES [1] Reiter L. Earthquake Hazard Analysis, Columbia Univ. Press, New York, [2] McGuire, R.K., Seismic Hazard and Risk Analysis, Earthquake Engineering and Research Institute, [3] United States Geological Survey (USGS), Documentation for the 2008 Update of the United States National Seismic Hazard Maps, Open File Report , 2008.

9 [4] EPRI, Electrical Power Research Institute, Guidelines for determining design basis ground motions, [5] Aldama-Bustos, G., Bommer J.J., C.H. Fenton, and P.J. Stafford. "Probabilistic Seismic Hazard Analysis for Rock Sites in the Cities of Abu Dhabi, Dubai and Ra's Al Khaymah, United Arab Emirates," (Georisk) 3, no. 2, 1-29 (2008). [6] RIZZO, PSHA Analysis, (Proprietary), [7] USNRC. Regulatory Guide 1.208, A Performance Based Approach to Define the Site Specific Earthquake Ground Motion, U.S. Nuclear Regulatory Commission, [8] Campbell, K.W, and Y. Bozorgnia. "NGA Ground Motion Model for the Geometric Mean Horizontal Component of PGA, PGV, PGD and 5% Damped Linear Elastic Response Spectra for Periods Ranging from 0.01 to 10 s," Earthquake Spectra, Volume 24, No. 1, pp , [9] Boore, D.M., and G.M. Atkinson. "Ground-Motion Prediction Equations for the Average Horizontal Component of PGA, PGV, and 5%-Damped PSA at Spectral Periods between 0.01 s and 10.0 s," Earthquake Spectra, Volume 24, No. 1, pp , [10] Akkar, S., and J. Bommer. "Prediction of elastic displacement response spectra in Europe and the Middle East," Earthquake Engineering and Structural Dynamic, Vol. 36: , 2007.