Intensity attenuation relationship for the South China region and comparison with the component attenuation model Adrian M. Chandler a, * , Nelson T.K. Lam b a Department of Civil Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong, People's Republic of China b Department of Civil and Environmental Engineering, The University of Melbourne, Parkville, Vic. 3052, Australia Received 21 July 2000; revised 31 January 2001; accepted 18 April 2001 Abstract The paper utilises seismic intensity information from historical (pre-instrumental) earthquakes in the South China region, to evaluate the mean attenuation relations. The formulation of these attenuation relations accounts for the seismic waveguide effects, in delineating three parts of the geometrical attenuation relationship. The derived relations are then compared with the intensity attenuation predictions implied by the newly developed component attenuation model (CAM), which is considered to be particularly applicable in regions of low or moderate seismicity that typically have limited earthquake records. Recognising that the observed intensities from historical earthquake events may implicitly incorporate some site effects, the incremental difference between the two models (between 0 and 1 unit of intensity) is found to be accountable in terms of average regional site effects arising due to the near-surface geology or soils. It is therefore concluded that the CAM model gives remarkably accurate predictions of ground motion and implied seismic intensity across a large range of earthquake magnitudes and at site-source distances ranging up to 500 km. The larger site-source distances, representing far ®eld and very far ®eld events, are particularly important in the South China region, including Hong Kong (HK), due to the large disparity in seismic activity rates in the region surrounding HK (which is rather seismically stable) and at distances . 300 km from HK, where some relatively active seismic zones exist. q 2002 Elsevier Science Ltd. All rights reserved. 1. Introduction Earthquake resistant design requires ground motion parameters to be quanti®ed, which might be peak accel- eration, velocity, displacement, or the response spectrum amplitudes (Lam et al., 2000a,b). Any of the parameters can be established from the spatial and temporal charac- teristics of regional seismic activity. The authors have carried out a detailed analysis of the historical and instru- mentally-recorded earthquake catalogues in the South China region, including Hong Kong (Chandler and Lam, 2001), from which the important conclusion has been drawn that seismic activity rates for earthquakes of magnitude M . 6 in the very far ®eld from Hong Kong (horizontal distances R . 300 km) are around 3± 4 times higher than for near ®eld events, at R , 45 km: The ground motions arising from large magnitude, far ®eld earthquakes have been found to exceed those from moderate magnitude, near ®eld earthquakes, over a wide range of structural periods and for design return periods of up to 2,475 years (Lam et al., 2001a). Hence, the signi®cant threat from such far ®eld events must be considered as part of an on-going programme of seismic ground motion and structural assessments for the Hong Kong region (Kwong et al., 2000). Fig. 1 indicates the earthquake distribution in South China. Furthermore, the severe lack of both historical and instru- mental records in the near ®eld region of Hong Kong make the signi®cance of far ®eld events (for which the availability of earthquake records is much more plentiful) even greater, since it is considered that the reliability of seismic activity rate evaluation, leading to ground motion predictions and subsequent structural stability assessments, is much higher for far ®eld events. Thus, only tentative conclusions may be drawn concerning long return period, design-level earth- quake events in the near ®eld region (Chandler and Lam, 2001). The seismic activity assessment reported by the authors in Chandler and Lam (2001) has derived a set of design-level magnitude±distance (M±R) combinations, associated with a range of return periods. The return periods, T RP (years) correspond to various probabilities of exceedance (PE) over a typical structure exposure period of 50 years. The selected Journal of Asian Earth Sciences 20 (2002) 775±790 1367-9120/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(01)00054-2 www.elsevier.com/locate/jseaes * Corresponding author. Tel.: 1852-2859-1973; fax: 1852-2559-5337. E-mail address: amchandl@hkucc.hku.hk (A.M. Chandler).