SEISMIC HAZARD ASSESSMENT AND SITE RESPONSE EVALUATION IN HONG KONG

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1 Proceedings of the SECED Young Engineers Conference - March 00, University of Bath, Bath, UK SEISMIC HAZARD ASSESSMENT AND SITE RESPONSE EVALUATION IN HONG KONG Raymond Koo, Vickie Kong, Matthew Free,, Ove Arup & Partners Hong Kong Ltd. The Hong Kong Special Administrative Region is located in an area of low to moderate seismicity. The current codes of practice for building design do not require any seismic considerations. This paper describes the assessment of the potential seismic ground motion levels on rock in Hong Kong in conjunction with the evaluation of the potential site response effects in Hong Kong. A detailed catalogue of historical and recent seismicity within the South China region was compiled. A suite of published empirical and stochastic attenuation relationships were used for the hazard assessment. A logic tree with weight has been developed to make allowance for variability in input data. The seismic hazard results have been de-aggregated to investigate what earthquake magnitude and distance combinations are more significant to the different return periods and for different structural periods. These magnitude distance combinations were used in the determination of scenario earthquakes for site response analyses. The ground conditions in the Hong Kong have been classified using the current NEHRP site classes. Published geological maps and detailed ground investigation information have been used for the classification. One-dimensional site response analyses have been carried out to determine how various types of site profiles, representative of Hong Kong subsoil conditions, will potentially respond to earthquake ground motion. The response of a suite of soil profiles was investigated using three input earthquake ground-motion levels and the results are presented in terms of spectral ratios for the different Site Classes and at the three ground-motion hazard levels. Keywords: low to moderate seismicity, seismic hazard, site response, site classes, spectral ratios Introduction Hong Kong is located in an area of low to moderate seismicity. The current codes of practice for building design in Hong Kong do not require any seismic considerations. However, with Hong Kong being a densely populated city of high economic importance, the Government of the Hong Kong SAR commissioned a study to investigate the potential seismic hazards and to evaluate the risks. This paper describes the first two phases of the assignment, the assessment of the potential seismic hazards and the evaluation of site response in Hong Kong. Geology and Tectonics of the Hong Kong Region Hong Kong is situated in southeast China near the south-eastern margin of the Eurasian Continental Plate in a stable continental intraplate region about 00 km from the nearest plate boundary, which underlies Taiwan and trends south to the Philippines and northeast to Japan, see Sewell et al. [] and Fyfe et al. []. The regional tectonic setting is shown in Figure. More than three-quarters of the land area of Hong Kong is underlain by igneous rocks predominantly volcanic tuffs and granites of Late Jurassic to Early Cretaceous age (0 to 0Ma). Superficial deposits comprising Quaternary (less than Ma) alluvium and other unconsolidated deposits are also present throughout the Geotechnical Engineer, raymond.koo@arup.com Geotechnical Engineer, vickie.kong@arup.com Associate, matthew.free@arup.com

2 territory. Large areas of reclamation have been formed around the coastal areas of the territory. spatial pattern of seismicity and the understanding of the regional geology rather than the location of mapped faults. A number of previous studies have defined seismic source zone models for Southeast China (Pun and Pun & Ambraseys [8], Wong et al. [9] and Lee et al. [0]) and these models have been included in this study using the logic tree described in Figure. SOURCE ZONES Pun & Ambraseys (0.) ATTENUATION ACTIVITY & RELATIONSHIPS b VALUE MMAX Boore, Joyner and Fumal (99) (0.). (0.) Median & b at 0.9 (0.) Atkinson and Boore (99) (0.) Lee et al (0.). (0.) For general areas.9 (0.) GENQKE Lam et al (000) (0.) Median & b at 0.9 (0.) GENQKE Wong et al (00) (0.) Median & b at 0.98 (0.).0 (0.) Wong & Chau (0.) Figure : Tectonic setting of Hong Kong. Seismic Hazard Assessment Methodology The probabilistic seismic hazard assessment (PSHA) methodology, e.g. Cornell [], McGuire [], has been applied using Oasys SISMIC, the in-house PSHA program of Arup. The PSHA methodology used the following steps: i. Potential seismic sources have been defined on the basis of regional geology and seismicity, ii. Seismicity parameters defining the rate of earthquake activity has been derived for each of the potential seismic sources, iii. Ground motion attenuation relationships, considered to be appropriate for the region, have been defined, and iv. The frequency of specified ground motion levels being exceeded has been derived by first determining the likelihood that the ground motion will be exceeded if an earthquake of a certain magnitude at a certain distance occurs. Then multiplying this likelihood by the annual frequency of such an event occurring in any of the source zones. By summing the annual frequencies from all specified earthquake distances and magnitudes the overall frequency is established. Taiwan (.0) For areas in vicinity of Hong Kong.0 (0.) Arup (0.) Seismic Hazard Assessment. (0.) GENQKE for Taiwan (.0) Median & b at 0. (.0) 8. (.0) NOTE: (0.) = Weight : For the second period only the GENQKE attenuation relationships have been used. Figure : Logic tree In addition, the source zone model shown in Figure has been developed and is incorporated into the logic tree with higher weighting. In order to determine the recurrence parameters, it is necessary to define the magnitude and time ranges over which the earthquake catalogue is complete. For the onshore and near-shore seismic source zones three completeness ranges have been defined (00 to 00 for M.0, 80 to 00 for M.0, 9 to 00 for M.). For the offshore seismic source zones two completeness ranges have been defined (90 to 00 for M. and 9 to 00 for M.). The model extends out to a distance of 00km from Hong Kong. A more distant seismic source zone was also included for the region of Taiwan (not shown on Figure ). The epistemic uncertainty arising from differences in expert opinion on a range of modelling assumptions has been addressed through the use of a logic tree, Kulkarni et al. [], Coppersmith and Youngs [] and NRC []. The uncertainty arising from natural physical variability, has been addressed by allowing for the normal variation, represented by its standard deviation sigma, of the ground motion attenuation relationships in the hazard computation. Figure : Source zone model for Hong Kong region. Seismic Source Zones and Parameters Hong Kong is located within a stable continental intraplate region and the association of earthquakes with defined faults is not clear and as a result the identification of seismic sources is based upon the Attenuation Relationships Very few strong motion records have been recorded in the Southeast China region and consequently it is not possible to derive empirical attenuation equations for

3 the region. Relationships have been derived for macroseismic intensity in Southeast China, Lee et al. [0], and these show that isoseismal areas and attenuation of macroseismic intensity generally fall between those for Western and Eastern North America. Attenuation relationships for Southeast China for peak ground acceleration and response spectral values have been derived for this study based on stochastic simulations of a model developed by Lam et al. [, ]. The model, which is based upon the stochastic model of Boore [] and Atkinson and Boore []. In order to account for the uncertainty in the attenuation relationships appropriate for the region, a number of relationships have been used in the assessment and have been incorporated into the logic tree with different weightings. The attenuation relationships for Western North America, Boore et al. [], for Eastern North America, Atkinson and Boore [], and for Southeast China, Wong et al. [], Lam et al. [, ] and Chandler et al. [], have been used in this study. Bedrock in this study is defined as NEHRP Site Class A hard rock. Seismic Hazard Assessment Results. Uniform Hazard Response Spectra on Rock The calculated hazard levels, in terms of horizontal response spectral acceleration (for % damping) on rock, at the three probabilities of being exceeded, are summarized in Table. The assessment has been undertaken for periods of 0., 0., 0.,.0,.0 and.0 seconds. The uniform hazard response spectra (acceleration) at each probability of being exceeded are shown in Figures. Spectral Acceleration (m/s ) 8 % damping 0 0 Period (sec) Ground motion % in 0 year 0% in 0 year 0% in 0 year Figure : Horizontal acceleration UHRS for bedrock The % in 0-years ground motion acceleration response spectra are compared with the 0. and.0 second spectral values defined in IBC 000 [8] for New York City and the acceleration response spectral values determined for New York by Weidlinger Associates [9]. It can be seen that the % in 0-year hazard level for New York is very similar to the hazard level calculated for Hong Kong (Figure ). Spectral Acceleration (m/s ) 8 % in 0 Year Ground Motion % Damping Period (sec) New York, IBC (000) NYCDOT, Weidlinger Associates (000) This study Figure : Comparison of bedrock % in 0-year UHRS with spectra for New York [9, 0] Deaggregations The hazard results have been de-aggregated, in terms of magnitude and distance, to investigate earthquake occurrences that have contributed the most to resulting ground-motion hazard. The de-aggregation was undertaken in accordance with the procedure recommended by McGuire [0]. For a region with dispersed seismicity such as Hong Kong, it is considered that log of distance is more appropriate. The results of the de-aggregation at the 0. and second acceleration response spectral values shows that the seismic activity is similar to that observed in the Eastern North America and about 0 times less than that in highly seismic areas such as California, Japan, Taiwan or the Philippines (see Figure ). The statistics show that the return period for a large magnitude, M = or greater event, within 00km of Hong Kong, is greater than,00 years. For a moderate size event, with magnitude M = or greater, within 00km of Hong Kong, the return period reduces to between 00 to 800 years. Examples of such event occurred in 9 at Honjhai Bay (M=.0, within 00km) and in 99 at Yangjiang Xinyi (M=., within 00km). For a smaller size event, with a magnitude M = or greater, within 00km of Hong Kong, the return period is approximately 0 to 0 years. For a very small magnitude event, with M = or greater, within 00km of Hong Kong, the return period is between. to years. Ground investigation data The ground investigation information has been compiled from three main sources; from Geotechnical Information Unit (GIU) at the Geotechnical Engineering

4 Office (GEO); Arup in-house databases; and ground investigation contractors databases. Over,00 borehole logs have been compiled into a GIS and analysed for the purposes of ground conditions categorisation. In addition, shear-wave velocity profiles were collated at locations from published and unpublished studies (Wong et al. [, ], Kwong [], Lee et al. [], Chan & Bell [], Tam [] and Arup [8]). The shear-wave velocity profiles were obtained using a range of methods including downhole, crosshole, seismic cone, and spectral analysis of surface waves (SASW). 00 Figure : Extract of site classification map for Hong Kong NUMBER >M PER YEAR (per 0 km) Western USA, Greece 9 One-Dimensional Site Response Analysis Methodology 0 Philippines In order to investigate the seismic site response characteristics of the Hong Kong ground conditions, a series of one-dimensional site response analyses were undertaken for a wide range of soil profiles and input parameters. The site response effects were analysed using the following steps: Taiwan Hong Kong Japan 0. i. UK Eastern USA MAGNITUDE Figure : Comparison of seismicity of the Hong Kong region with other regions, Department of the Environment [] 8 Classification according to the NEHRP site classification system The NEHRP Site Class has been derived for, boreholes using SPT N data, shear wave velocities and/or undrained strength measurements. Figure shows the locations of the ground investigation stations where the site class values were evaluated. These results were then combined with the geological map to develop a site classification map. The site class boundaries were found to broadly coincide with geological boundaries across the territory. In particular, the boundary between Quaternary and pre-quaternary age materials tends to coincide with the boundary between Site Class C and Class D. The boundary between Site Class C and Class B is associated with the presence or not of a relatively thin layer of saprolite over bedrock, which was found to generally coincide with slope gradient. Site class E is typically in areas where there is a considerable thickness of reclamation material and/or reclamation material overlying a considerable thickness of soils such as marine or alluvial deposits. representative soil profiles were compiled to represent the range of ground conditions encountered in Hong Kong ( Site Class B, Site Class C, Site Class D and Site Class E). Each profile is defined in terms of the soil and rock material types encountered and the variation in small strain shear modulus (G0) versus depth. In each case the profile extends into bedrock. ii. The shear modulus degradation curve, representing the non-linear behaviour of the soil was defined for each soil and rock material type, as was the soil density. The main soil types encountered were reclamation fill sand, marine silt/clay, colluvium sand and gravel deposits, alluvium sand and silt/clay deposits and in situ weathered rock or saprolite. iii. Bedrock response spectra determined from the seismic hazard analysis, representative of seismic hazard levels in Hong Kong were used to define appropriate earthquake strong-motion records for input as reference bedrock ground motions. iv. One-dimensional site response analyses were carried out using Arup in-house program Oasys SIREN [9]. v. Spectral ratios were determined for each soil profile. vi. Design surface response spectra were obtained by multiplying the bedrock design response spectra by amplitude dependent spectral ratios for a range of periods. 0 Input ground motions In the absence of appropriate measured strongmotion records the South China region, time histories

5 from stable continental regions and elsewhere have been modified by adjusting them to a target spectrum in the frequency domain. The target design spectra were represented by the scenarios for earthquake motion level with 0%, 0% and % probability of being exceeded in 0 years. Site Amplification Factors The scaled time history is then used to determine ground-motion amplitude- and period-dependent factors for each of the site classes. The factors specific to Hong Kong in terms of the local soil conditions and the amplitude of the reference input motion. Due to its moderate level of seismicity, the input motions in Hong Kong are considerably lower than most of those used to derive the amplification factors for NEHRP [0]. Results of site response analysis For each input time history and each soil class, the mean spectral ratio was assessed at structural periods of 0.0, 0., 0., 0.,.0..0 and.0 seconds by calculating a running average spectral ratio, which was found to give a better representation of the site response effects and reduce the influence of minor perturbations of the spectral ratio with change in period. Figure 8 shows the calculated spectral ratios for Site Class D sites when subjected to the short period % in 0 year ground motion. Spectral Ratio Class D profiles, % in 0-yearground motion, short period + Sigma + Sigma Average - Sigma - Sigma Period (s) Figure 8: Spectral ratios for Site Class D sites (% in 0 year ground motion) For the short period 0. second ground motion, the predicted amplifications are between about and times, with greater amplifications for Site Class C than for Site Classes D and E. As the input ground motion increases, the amplification of the soft soil sites decreases, until eventually a reduction is shown. This result is consistent with the non-linear response expected for soft soil profiles. The long period second motion is not amplified for Site Classes B and C, irrespective of the amplitude of the input motion. Moderate amplification is found for Site Classes D and E of. to times. For Site Class E, the amplification continues to increase slightly as the amplitude of motion increases, contrary to expectations. The design ground motions for Hong Kong comprise very low energy long period motions, which may explain these results. At longer structural periods of.0 seconds, little or no amplification was found for all site classes. Space limitations preclude a detailed description of the work, which can be found in Free et al. [] and Pappin el al. [] Conclusions A probabilistic seismic hazard assessment has been carried out for the Hong Kong region. The results of the seismic hazard assessment are presented in terms of horizontal peak ground acceleration and uniform hazard response spectra for structural periods up to seconds for bedrock ground conditions. The results are presented for ground motions with 0%, 0 % and % probability of being exceeded in 0 years. To capture the uncertainty in the calculation input for the probabilistic assessment a range of seismic source models, variations of seismic activities and several different attenuation relationships have been incorporated using the logic tree method. The resulting ground motion seismic hazard is expressed in terms of peak horizontal ground acceleration and uniform hazard horizontal response spectra for structural periods up to seconds. The results show peak horizontal ground accelerations of %, % and % of gravity for the ground motion levels with 0%, 0% and % probability of being exceeded in 0 years. The response spectral values in the medium to long structural period range are found to be very similar to those published for New York City. The seismic hazard has been de-aggregated to determine scenario earthquakes for the three design levels. Site classification, using the system defined in the NEHRP Provisions, has been undertaken for the subsoil conditions in Hong Kong. Over,00 borehole logs have been examined and analysed to determine the site classification at individual locations. A site classification map has been produced, which shows the distribution of the different site classes across Hong Kong. Site response analyses have been undertaken for earthquake ground motion levels with 0%, 0% and % probability of being exceeded in 0 years. Spectral ratios have been presented for the different site classes with respect to the level of the input bedrock motion such that site response effects can be determined for a range of scenario input ground motion levels within appropriate limits. The site response effects are found to be dependent on both structural period and input amplitude with maximum mean spectral ratios up to about.

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