Christophe Martin

A Bayesian methodology to update the probabilistic seismic hazard assessment

This paper presents a Bayesian methodology for updating the seismic hazard curves. The methodology is based on the comparison of predictive exceedance rates of a fixed acceleration level (given by the seismic hazard curves) and the observed exceedance rates in some selected sites. The application of the methodology needs, firstly, the definition of a prior probabilistic seismic hazard assessment based in a logic tree. Each main branch corresponds to a probabilistic model of calculus of seismic hazard. The method considers that, initially (or a priori), the weights of all branches of the logic tree are equivalent. Secondly, the method needs to compile the observations in the region. They are introduced in a database containing the recorded acceleration data (during the instrumental period). Nevertheless, the instrumental period in stable zones (as France) shows only very low acceleration levels recorded during a short observation period. Then, a method to enlarge the REX (number of observations) is presented taking into account the historical data and defining “synthetic” accelerations in the sites of observation. The synthetic REX allows to expand the period of observation and to increase the acceleration thresholds used in the Bayesian updating process. The application of the Bayesian approach leads to a new and more objective definition of the weights of each branch of the logic tree and, therefore, to new seismic hazard curves (mean and centiles). The Bayesian approach doesn’t change the probabilistic models (seismic hazard curves). It only modifies the weights of each branch of the logic tree.

Seismic Hazard Computation

This section deals with the seismic hazard computation process. It should be noted that, in cases where the seismic hazard to be calculated includes site-specific soil amplifications, this process may include two steps: hazard calculation for reference rock conditions and subsequent site response analysis to combine the local soil response with the rock hazard (see Sect. 5.2).

General Concepts and PSHA Background

The first step in building the PSHA model is the collection of geological, geophysical, geotechnical and seismological data from published and unpublished documents, theses, and field investigations. These data are integrated to develop a coherent interpretation of a seismotectonic framework for the study region. Its size can vary depending on the purpose. The international practice for a site-specific study is to distinguish between the investigations at a regional, near regional and site vicinity level (e.g. 300 km, 25 km and 5 km radius in IAEA SSG-9, IAEA (2010)). In order to include all features and areas with significant potential contribution to the hazard, it may also be necessary to include information in a radius up to 500 km (e.g. for subduction zones). This framework provides the guiding philosophy for the identification of seismic sources. Furthermore, the framework should address the important issues that each expert expects to influence the identification and characterisation of seismic sources in the region. The main topics to be addressed in the seismotectonic framework include: Use of pre-existing geological structures to provide a basis for defining the present and future seismicity. Tectonic models that are applicable to contemporary processes, the observed seismicity, and are compatible with seismic sources. Spatial distribution of the seismicity in three dimensions, and associated focal mechanisms and their relation to potential seismic sources. Implications of contemporary stresses and strains (e.g. earthquake focal mechanisms, geodetics, other kinematic constraints) for defining sources. Use of historical and instrumental seismicity and seismic source delineation to provide a basis for defining the locations of future earthquake activity.

Seismic Source Characterization

Seismic sources characterization (SSC) relies on the interpretation and integration of a variety of parameters and data primarily collected in a geological, geophysical, geotechnical and seismological database (D4-41, Carbon et al. 2012). It has three fundamental objectives: The identification (location and geometry) of all seismic sources contributing to the total hazard at the site of interest. According to the seismotectonic context of the site, and to the considered hazard return period, this involves developing the database within a radius of a few hundred kilometres around the site and to consider different scales of analysis especially when individual faults are considered. The characterization of the seismic activity with a large number of uncertain parameters: the maximum magnitude of these seismic sources, their activity rate and the models for seismicity distribution. The consideration of all inherent epistemic and aleatory uncertainties and the influence of the different uncertainties in the hazard results.

Site Response Characterization

It is well recognized that the seismic response of a site is strongly dependent on the local geological and geotechnical features of the ground profile. Several approaches are available to include site response effects in the hazard assessment. They are detailed in the following paragraphs but all of them require more or less in-depth knowledge of the geotechnical characteristics. Such knowledge can only be acquired through sit investigations. Therefore, a considerable amount of efforts has been devoted in SIGMA to investigating the reliability of several site investigation techniques, whose level of complexity depends on the choice of the site effect evaluation method; characterization is also mandatory to get the minimum information to choose the method itself.

Interfaces Between Subprojects

Interface issues between the seismic source characterization and the ground motion characterization are usually implicitly handled by the hazard computation. Nevertheless, the most relevant interface topics, for example the depth distribution and the distance metric, should be discussed among experts from both sides of the interface. In the following sections, the interface issues identified as the most important for the hazard are discussed briefly.

Summary and Way Forward

During 5 years SIGMA has not only achieved significant steps forward in the methods for evaluating seismic hazard at a site, but it also contributed to establishing a strong network of academic institutions, researchers and engineering companies that will survive after the project ends. The overall organisation of the project fostered very lively and fruitful discussions between all participants, including with the members of the Scientific and Steering Committees.

Probabilistic Seismic Testing and Updating of Seismic Hazard Results

Considering the high uncertainties in PSHA and the importance of PSHA results for seismic design and retrofit, it is pertinent to focus on the issue of consistency checking of the PSHA results. In the last decade several approaches to test PSHA results have been published (e.g. Stirling and Petersen 2006; Stirling and Gerstenberger 2009, 2010; Stirling 2012; Mucciarelli et al. 2008; Humbert and Viallet 2008; Labbe 2010; Anderson et al. 2011; Mezcua et al. 2013; Selva and Sandri 2013; Gribovszki et al. 2013). In addition, several recent opinion papers are encouraging hazard analysts to carry out tests of PSHA results (e.g. Stein et al. 2011). During the SIGMA period an international workshop was held in Pavia (NEA 2015) on “Testing PSHA results and Benefit from Bayesian Techniques for Seismic Hazard Assessment” which concluded that a state-of-the-art PSHA should include a testing phase against any available observation, including any kind of observation and any period of observation, including instrumental seismicity, historical seismicity and paleoseismicity data if available. It should include testing not only against its median hazard estimates but also against their entire distribution (percentiles).

Rock Motion Characterization

As stated in Chap. 2, rock (as well as soil) ground motion characterization for PSHA requires that both the median response spectral acceleration and its standard deviation (aleatory variability) be estimated by appropriate algorithms, such as GMPEs or stochastic models. In Sect. 2.5 of Chap. 2 the logic tree approach was introduced for handling epistemic uncertainty, pointing out that the dynamic characteristics of the earthquake source and wave propagation near the site are typical sources of uncertainty in ground-motion prediction. Some implications for the logic tree treatment of epistemic uncertainty of rock ground motion will be discussed at the end of this chapter.

Guidelines for Site Investigation and Analysis of Nuclear Facilities: A Consultants Perspective

Planning site investigations for siting, licensing, or constructing nuclear power plants require implementation of appropriate technical guidelines. Guideline documents have been developed in the USA and internationally to provide standards for how geologic, geotechnical, hydrologic and seismic data are collected, analyzed and presented in safety analysis reports. The authors discuss how guidelines established by both the U.S. Nuclear Regulatory Commission (USNRC) and the International Atomic Energy Agency (IAEA) are used in the regulatory framework of nuclear power plants in France, the United Arab Emirates, Japan and the United States. The importance of the choice of methodologies engineering geologists use to successfully perform nuclear safety-related site characterization studies and analysis is also discussed. Lastly, new guidelines and approaches developed to address seismic and flooding risks in response to the 2011 Fukishima Daiichi Power Plant incident in Japan are presented.