Historical earthquakes noted in the written records of the South China region, including Hong Kong, are not well delineated along identi®ed prominent fault sources. Despite the lack of any de®nitive, localised trend in the spatial distribution of seismic activity in the region, there does appear to be some major disparity in the seismic activity rates (especially for large magnitude earthquakes) between the near-®eld and the far-®eld regions of Hong Kong. Despite this observation, previous studies of the regional seismic activity and seismic ground motion hazard (the latter using a probabilistic seismic hazard assessment, PSHA) have considered very broad source zone regions, in which uniform levels of seismic activity have been assumed. The present paper further scrutinises this broad source zone (BSZ) approach by adopting a novel expanding circular disc (ECD) method to determine the rates of earthquake recurrence. Such a method is intended to counter-check previously developed models by determining earthquake scenario events in terms of magnitude±distance (M±R) pairs or combinations, having de®ned values of average return period. Unlike the BSZ approach, the ECD method speci®cally accounts for the supposed variations in the seismic activity rates between events in the near-®eld and the far-®eld of Hong Kong.
The form of the developed method is particularly suited to the determination of design-level earthquake ground motions for bedrock sites, since it assumes a directionally-independent attenuation model as described in the companion paper. It is found that, whilst the BSZ approach may indicate the overall average levels of hazard that are representative of the South China region as a whole, it does not capture the large disparity in seismic activity rates between near-®eld and far-®eld events. This important feature is expected to have a signi®cant impact upon engineering assessments of the seismic safety of structures in Hong Kong and elsewhere in the South China region. For example, it is found that for events with M $ 6, the seismic activity rate (normalised by time and area) in the very far-®eld is around 3.5 times larger than in the near-®eld and medium-®eld of Hong Kong. The resulting design M-R combinations, covering a range of return periods from 70 to 2500 years, are limited, for very long return periods and for distant events, by the maximum credible earthquake (MCE) magnitude. Intensive research to determine this seismic hazard parameter is recommended, in order to re®ne further the results of the ECD analysis, which presently conservatively assumes the MCE to range between M 6 in the near-®eld of Hong Kong to M 8 in the very far-®eld, at distances greater than 280 km from Hong Kong.