Attenuation of peak ground accelerations for recent UK earthquakes of magnitude ≤ 4.6 ML

I have measured empirical horizontal and vertical peak ground accelerations for __ earthquakes from between 1992 and 2001 with magnitudes between local magnitude (ML) -07 and 4.6, and for distances of 1 to 867 km. The majority of the ground accelerations were measured from short-period velocity records by removing the instrument response and differentiating the time series. All seismograms were first filtered between 1 and 30 Hz to reduce instabilities in the processing due to amplification of low and high frequency noise. Some higher frequency accelerations (f > 30 Hz) may have been filtered out from the records but only result in a small reduction in the peak acceleration values. Data recorded in the Scottish Borders was investigated preferentially due to the higher dynamic range of the 16-bit system installed there. The resulting empirical data are quite scattered for a given event, resulting from the varying effects of source radiation pattern, path and site effects. However, ground acceleration generally decays exponentially with distance. Peak ground accelerations are about an order of magnitude lower than predicted from published empirical attenuation relations currently used in the UK hazard studies, for distances greater than 100 km. The rate of amplitude decay at these distances is also much higher than predicted.
\nIn this study various other aspects of the decay of peak ground accelerations are investigated, such as the effect of source depth (some coalfield events were included in the analyses since these generate more surface waves than earthquakes from greater depths), site effects, path effects and underlying geology. A special study was made of unsaturated near-field peak ground accelerations, to describe this portion of the attenuation curve. The full attenuation curves for distances both less than and greater than 100 km were only achieved for small magnitude events (magnitude between 2 and 3 ML) due to the limited amplitude range available for the current instrumentation in the UK. These attenuation curves show a similar pattern of a slower rate of decay at distances less than 100 km, and a more rapid decay at distances greater than 100 km. This is similar to what has been observed for Canada (Mereu and Atkinson, 1992) for a range of magnitudes (mb 3.5 to5.0). Ratios between horizontal and vertical peak ground accelerations were also calculated where both readings were available. The average ratio found to be about 1.6 and no strong dependence on distance or magnitude was observed, despite the larger scatter in the near field, where the measured accelerations are most affected by the earthquake radiation pattern.
\nThe decay of the PGA with distance for larger events appears to be more rapid than for the much smaller earthquakes (magnitudes ≤ 2.5 ML), however this might be related to the observable portion of the attenuation curve available from on-scale UK measurements. It is hoped that by observing the decay of accelerations for small events at distances less than 100 km, the rate of decay for larger events at these distances might be estimated, for which few unsaturated measurements are available in the UK. However, one has to be careful in making such extrapolations, because ground motions from small earthquakes tend to attenuate much faster than those from larger earthquakes. This is because small earthquakes produce a larger proportion of high frequency ground motion, and high frequencies attenuate much faster than low frequencies. Also the duration of the highest ground accelerations is much less for smaller events.

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