A UK local seismic magnitude scale, MLP, using P-wave amplitudes

Abstract

SUMMARY A local seismic magnitude scale, MLP, has been developed for the United Kingdom (UK) using automated measurements of 8902 half peak-to-peak vertical component seismic P-wave displacement amplitudes from 630 earthquakes. The measurement time window increases with source-to-receiver range such that MLP is sensitive to the dominant phase within the P-wavetrain at a given distance. To avoid contamination due to low-frequency noise, the P-wave amplitude measurements are made in the 1.5–30 Hz passband. A least-squares inversion was undertaken to estimate source size, distance and station effects. The distance effect values suggest that P-wave amplitude attenuation across the UK is low when compared to other tectonically stable regions. The station effects are broadly consistent with UK geology, with signal amplification observed within the sediments towards the south-east of the country. MLP has been tied to the UK local magnitude scale routinely estimated by the British Geological Survey (BGS, determined using S waves, and here denoted MLBGS). For earthquakes with MLBGS > 3, MLP exhibits a closer correspondence to the moment magnitude than MLBGS (i.e. MLP≈Mw). It is tentatively suggested that this reduction in bias is caused by the P-wave scale being less affected by along-path attenuation. The difference with respect to physical source scaling helps explain the divergence of the MLBGS and MLP scales at ML > 3. MLP allows a robust estimate of event size to be made for small events which predominantly generate P waves, for example, near-surface explosions. MLP values have been calculated for 239 explosive events, mostly mining blasts and munitions disposal. Although there is significant scatter, explosive events exhibit elevated MLP values compared to MLBGS, consistent with explosions generating proportionally more compressional wave energy than earthquakes. For example, 33 explosions at sea exhibit a median MLP–MLBGS value of 0.50 mag units. Despite its sensitivity to P-wave amplitude, MLP is not a more consistent estimator of explosive source size than MLBGS; the magnitude residuals (station estimate − event estimate) are slightly less for MLBGS compared to MLP. This is primarily due to variability of the P-wave amplitudes that cannot be explained by a 1-D distance correction. MLP should be considered as an additional tool for characterizing small seismic events within the UK.