Distance and station effects on UK shear- and Lg-wave amplitudes recorded in the range 0-600Km

The amplitudes of shear-waves and Lg-waves recorded at UK seismograph stations from local earthquakes in the range 0-600km have been subjected to an analysis of variance, allowing separation of the effects of earthquake size, distance and local attenuation near each station. The analysis of 385 amplitude readings at 28 stations from 39 earthquakes showed that the effects of both distance and station attenuation were statistically significant. Tables of corrections for both distance and station effects have been derived to allow local magnitude ML to be determined more accurately from horizontal and vertical component records. One set of tables allows the estimation of an ML which is consistent with the original Richter definition of ML, with a standard deviation which is smaller than that produced by the theoretical attenuation curve, defined according to attenuation in Southern California, which has customarily been used to calculate ML for seismic events in the UK. The improvement in accuracy is mainly due to the incorporation of station terms to correct for near-station attenuation, since the theoretical and observed variations of attenuation with distance are similar. This similarity implies that Southern California and the UK show a similar variation of distance-dependent attenuation, a surprising result in view of the differences in geology. Another set of tables will produce an ML which is consistent with the body wave magnitude mb determined by the International Data Centre (IDC) from station records of the Comprehensive Test-Ban Treaty Organisation’s global monitoring network. These local magnitude estimates are consistently closer to mb (IDC) estimates than the UK bulletin estimates of local magnitude which are made with a standardised amplitude-distance curve and no station correction.