Interpreting intraplate tectonics for seismic hazard : a UK historical perspective

It is notoriously difficult to construct seismic source models for probabilistic seismic hazard assessment in intraplate areas on the basis of geological information, and many practitioners have given up the task in favour of purely seismicity-based models. This risks losing potentially valuable information in regions where the earthquake catalogue is short compared to the seismic cycle. It is interesting to survey how attitudes to this issue have evolved over the past 30 years. This paper takes the UK as an example, and traces the evolution of seismic source models through generations of hazard studies. It is found that in the UK, while the earliest studies did not consider regional tectonics in any way, there has been a gradual evolution towards more tectonically based models. Experience in other countries, of course, may differ

Capturing geographically-varying uncertainty in earthquake ground motion models or what we think we know may change

Our knowledge of earthquake ground motions of engineering significance varies geographically. The prediction of earthquake shaking in parts of the globe with high seismicity and a long history of observations from dense strong-motion networks, such as coastal California, much of Japan and central Italy, should be associated with lower uncertainty than ground-motion models for use in much of the rest of the world, where moderate and large earthquakes occur infrequently and monitoring networks are sparse or only recently installed. This variation in uncertainty, however, is not often captured in the models currently used for seismic hazard assessments, particularly for national or continental-scale studies. In this theme lecture, firstly I review recent proposals for developing ground-motion logic trees and then I develop and test a new approach for application in Europe. The proposed procedure is based on the backbone approach with scale factors that are derived to account for potential differences between regions. Weights are proposed for each of the logic-tree branches to model large epistemic uncertainty in the absence of local data. When local data are available these weights are updated so that the epistemic uncertainty captured by the logic tree reduces. I argue that this approach is more defensible than a logic tree populated by previously published ground-motion models. It should lead to more stable and robust seismic hazard assessments that capture our doubt over future earthquake shaking

The comparison of macroseismic intensity scales

The number of different macroseismic scales that have been used to express earthquake shaking in the course of the last 200 years is not known; it may reach three figures. The number of important scales that have been widely adopted is much smaller, perhaps about eight, not counting minor variants. Where data sets exist that are expressed in different scales, it is often necessary to establish some sort of equivalence between them, although best practice would be to reassign intensity values rather than convert them. This is particularly true because difference between workers in assigning intensity is often greater than differences between the scales themselves, particularly in cases where one scale may not be very well defined. The extent to which a scale guides the user to arrive at a correct assessment of the intensity is a measure of the quality of the scale. There are a number of reasons why one should prefer one scale to another for routine use, and some of these tend in different directions. If a scale has many tests (diagnostics) for each degree, it is more likely that the scale can be applied in any case that comes to hand, but if the diagnostics are so numerous that they include ones that do not accurately indicate any one intensity level, then the use of the scale will tend to produce false values. The purpose of this paper is chiefly to discuss in a general way the principles involved in the analysis of intensity scales. Conversions from different scales to the European Macroseismic Scale are discussed

Objective assessment of source models for seismic hazard studies : with a worked example from UK data

Up to now, the search for increased reliability in probabilistic seismic hazard analysis (PSHA) has concentrated on ways of assessing expert opinion and subjective judgement. Although in some areas of PSHA subjective opinion is unavoidable, there is a danger that assessment procedures and review methods contribute further subjective judgements on top of those already elicited. It is helpful to find techniques for objectively assessing seismic source models that show what the interpretations physically mean in terms of seismicity. Experience shows that well-meaning but flawed design decisions can lead to source models that are incompatible with the seismic history that was used as input. In this paper a method is demonstrated in which large numbers of synthetic earthquake catalogues, that match the completeness thresholds of the historical catalogue, are generated. The study area can be divided into a grid of uniform cells, and the number of earthquakes in each cell in both the historical catalogue and each simulated catalogue are then counted. Comparison of the historical pattern and a set of 1,000 simulated patterns, using a X2 test, shows if the historical pattern is credibly a member of the set of outcomes obtainable from the seismic source model. A second method is to chart the distribution of a large sample of simulated catalogues in terms of magnitude frequency, and observe whether the historical catalogue is comfortably within this distribution, or an outlier. If it proves impossible to replicate the historical catalogue using the model, it casts doubt on whether the model is a valid depiction of the seismicity rates that will govern the future hazard. At the very least, the disparity needs careful investigation to ensure the model is error-free. A worked example is presented here for the UK, using a source model that was used in Global Seismic Hazard Map (GSHAP), compared to one that was artificially constructed to be credible but flawed. Two tests find the GSHAP model to be an acceptable representation of the pattern of seismicity in the UK, while the artificial model is conclusively rejected