Reducing the subjectivity in Modified Mercalli intensity analysis: drawing isoseismals

MM intensities are anathema to most engineers, who would naturally prefer instrumental measures of ground motion, but the fact remains that most of our large historical earthquakes were not recorded by modern equipment. The only clue we have to the severity of their ground motion is the intensity data, so we need to make the best use of it that we can. The traditional method of analysis is to plot the spot estimates of intensity on maps, and draw contours (isoseismals) to enclose, as far as possible, all the observations of the same intensity, while excluding lower values. A new development offers a more objective way of drawing isoseismals. This is important because the dimensions of the isoseismal contours are used to derive attenuation functions, which are in turn used in the modelling of hazard and risk. The technique uses an algorithm for fitting elliptical contours to the raw intensity observations. It is necessary to constrain the solution by (a) identifying a priori the centre of the pattern, and (b) requiring that the major axes of all the individual elliptical isoseismals for any one earthquake have the same orientation. With these constraints, the solution is usually stable.