Abstract
A pilot zoning study has been carried out for ground shaking hazard in the city of Christchurch, using the grade-3 procedures of the draft manual on seismic zonation for geotechnical hazards, being developed by Technical Committee TC-4 of the ISSMFE. Because of limited site data, we were not able to produce a complete zoning map for the city, but ground motions were estimated at six distinct sites for which borehole data was available, and this provided a good test for the procedures of the manual. The city of Christchurch is situated on nearly 1 km of volcanic rock and alluvial sediments overlying greywacke basement, on the edge of the main seismic region of New Zealand. In addition to being an interesting site from the point of view of ground shaking, there are also liquefaction and slope stability hazards in the alluvial and beach sands found throughout the city, and in the recent loess deposits mantling the adjacent Port Hills. In the pilot study of ground shaking hazard, a seismicity model for the central South Island region published recently by Elder et al. (1991) was combined with the attenuation model of Kawashima et al. (1984) recommended in the draft manual, to estimate rock motion at Christchurch, described by its acceleration response spectrum. Transfer functions were computed for the site response to estimate motions at the six selected sites using the Thomson-Haskell method. The results were highly sensitive to details of. the upper 30 m or so of the soil profile. In the local context, the large disparity between our estimates of ground shaking at Christchurch and those implicit in the draft revision of NZS 4203 are disturbing. According to our study the draft code underestimates shaking by about a factor of two or more.
Highlights
The seismicity model of Elder et al (1991) has been used with a two modifications: firstly, the activity rate of the Alpine Fault has been increased to allow for the long period of quiescence geologists, the maximum magnitudes in two zones, the Canterbury Plains zone (CPS) and the Banks Peninsula zone (BPS) have been reduced to M6.5 Elder's model itself is a refinement for the central South Island region of an earlier seismicity model by Smith and Berryman (1983), and incorporates smaller source zones than the earlier model, corresponding more closely to the different geologic provinces
The Grade 3 procedures of the draft manual have been followed in this pilot study to estimate ground shaking at six distinct sites in the city of Christchurch
Since the methods of the TC4 manual are more refined than those of the draft code, and a more detailed seismicity model is employed in the pilot study, we are worried that the proposed code underestimates hazard in Christchurch
Summary
A pilot zoning study has been carried out for ground shaking hazard in the city of Christchurch, using the grade-3 procedures of the draft manual on seismic zonation for geotechnical hazards, being developed by Technical Committee TC-4 of the ISSMFE. Because of limited site data, we were not able to produce a complete zoning map for the city, but ground motions were estimated at six distinct sites for which borehole data was available, and this provided a good test for the procedures of the manual. In the pilot study of ground shaking hazard, a seismicity model for the central South Island region published recently by Elder et al (1991) was combined with the attenuation model of Kawashima et al (1984) recommended in the draft manual, to estimate rock motion at Christchurch, described by its acceleration response spectrum. According to our study the draft code underestimates shaking by about a factor of two or more
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