A Local Magnitude Scale ML for the United Kingdom

Abstract

We have developed a new local magnitude scale ML for the United Kingdom (UK) to replace the Hutton and Boore (1987) scale developed for southern California, which has been used in the UK until now. The new UK scale is developed from 1482 observations of 85 earthquakes on 50 stations located across the British Isles and Ireland. Most of the observations are from epicentral distances of less than 600 km and only few from greater distances up to 900 km. The distance range of the scale is, therefore, 0–600 km. The amplitude observations were used to invert for the parameters defining distance dependence in the ML scale and station corrections. Synthetic tests showed that the inversion was robust. The new ML scale for the UK is given by ML=logA+0.95logR+0.00183R−1.76, in which A is horizontal‐component ground displacement amplitude in nanometers. The amplitudes are measured on traces that are filtered to simulate the Wood–Anderson seismograph. R is the hypocentral distance (in km). The UK scale is intermediate between scales determined for California and those of other intraplate areas such as Norway or the northeastern United States. The absolute station corrections found are all less than 0.5. The scale derived for the UK helps to reduce the overall variance of the mean magnitude estimates by 30%. Much of this improvement is due to the use of station corrections. Applying the UK scale to the database of recorded earthquakes results in a reduction of magnitude for earthquakes above ML 2 and a slight increase in magnitude for earthquakes below ML 2. The biggest change to the ML computation is likely to be for small earthquakes with few amplitude readings, where the use of station corrections makes a significant difference.