Intensity attenuation in the UK

Intensity attenuation is relatively little studied compared with the attenuation of peak ground acceleration, due to the fact that the PGA can be used for engineering design, while intensity cannot. However, intensity has other uses, including the estimation of effects (including damage) of future earthquakes, and hence, at least in a general way, the study of earthquake risk. Knowledge of intensity attenuation is also useful in calibrating hazard models against historical experience. In this study, the attenuation of intensity in the U.K. is thoroughly evaluated from a data set comprising 727 isoseismals from 326 British earthquakes, including both modern and historical events. Best results are obtained by restricting the data set to events contributing at least two isoseismals. The preferred equation is $ I = 3.31 + 1.28{\rm ML} - 1.22\ln\!R $I=331+128ML−122lnR where I is intensity (European Macroseismic Scale), ML is local magnitude, and R is hypocentral distance. The sigma (uncertainty) value is 0.46. Some sample applications of this formula are demonstrated.