In the framework of our project and in addition to the innovative scientific research proposed, one very original aspect, a scientific procedure to assess the reliability and the uncertainties linked to paleoseismic informations, is introduced.

Evaluation of the uncertainties in Paleoseismic investigations.

Quantification of uncertainties related to paleoseismological data in seismic hazard analysis is difficult. Unless documented in detail by the scientist providing such data, using it in seismic hazard analysis may result in misinterpretation of the true seismic hazard potential of the area of interest. There is therefore a need for a systematic treatment of uncertainties in paleoseismological data. Usually, uncertainties are inherent to the interpretation of geological phenomena based on field obervations which may satisfy several alternatives. Such interpretations become useless when alternative solutions exist but not documented in detail, and especially when the relative reliability of the favored interpretation with respect to the alternative interpretations is not known. In this project, a recently introduced method (Atakan, 1996), based on qualitative description of the uncertainties related to the paleoseismological data and especially in its interpretation, will be used. Any interpretation of a geological phenomenon involves scientist’s judgement of some diagnostic criteria. Once a classification of the diagnostic criteria is prepared, a priority can be given, based on the reliability of each. Weighted quality factors can then be assigned for each diagnostic feature used during the consecutive stages of the interpretation process and finally, a cumulative uncertainty can be attached to the final result. This will allow the user from the seismic hazard applications, to account for these uncertainties in a systematic way prior to its use in the analysis. The method will be applied to the four test areas described in the project proposal using logic-tree formalism in the paleoseismological data interpretation process

The main objective of the paleoseismological analysis is important to specify before the treatment of uncertainties. Once this is done, the consecutive stages of the analysis procedure can be identified. In general the paleoseismological interpretation process goes through the following stages:

1. Fault type
2. Trench site selection
3. Trench/fault ratio
4. Diagnostic criteria
5. Dating techniques
6. Earthquake size estimates

These different stages may then be integrated into a logic-tree as the different successive nodes with alternative branches. At each nod, the different alternatives can be described with their relative uncertainty attached to each. These uncertainties can be expressed in terms of probabilities assigned to each branch of the logic-tree. In the most simple case, at each nod a minimum of two alternatives can be given, one with the preferred solution, the other representing the sum of the other existing possible alternatives. At the end joint probability of the preferred alternatives will give a qualitative measure of uncertainty related to the analysis. In other words, the probability that the objectives set in the beginning of the analysis is achieved, will be obtained.

During the project, the four test areas will be used as a basis for investigating the potential for large earthquakes in regions of present-day low seismic activity in Europe. The results from these investigations may provide valuable data to the estimation of the future earthquake potential in these areas. It is therefore important to evaluate the possible uncertainties related to these paleoseismic studies. Each test area will be studied in detail to evaluate the uncertainties. The results will be presented in logic-tree formalism and in assigned weighted quality factors for each alternative solution. The end solutions for each combination of branches of the logic-tree will provide a probability which can later be used in the seismic analysis easily. This procedure will be computerized through software which will be developed partly through the project.

Reference

Atakan, K. (1996), Systematic treatment of uncertainties in the geological data for seismic hazard analysis. Abstract in the ESC General Assembly, Reykjavik, Iceland, September 1996.