Spatial bias in b-value of the frequency-magnitude relation for the Hong Kong region

The onshore and offshore regions in South Guangdong around Hong Kong exhibit different b-values of the G-R relation. The two regions have yielded b-values of 0.85 and 0.67, respectively, based on the most complete seismic database for the period 1970-1995. Tests on the completeness of the seismic catalogue and dominance of local source zones suggest a fundamentally complete record for M greater than or equal to 3 events and the spatial bias in the b-values cannot be explained entirely in terms of these two factors. The subcatalogue of onshore events with focal depth greater than or equal to 6 km has yielded an essentially identical b-value as that for the offshore region, thus revealing a weak upper crust with diffused seismicity overlying a relatively strong lower crust. The present study shows that spatial bias in the b-value can be resolved by a proper analysis of the seismic data. The interpretation is also consistent with the model that the relative motion between South China and the South China Sea is diffused over a broad fault zone along the coast, with a higher b-value in the upper crust and the lack of a single wrench fault. (C) 2001 Elsevier Science Ltd. All rights reserved.