5. Seismic hazard
Until the end of the 20th century, seismic hazard in China was primarily expressed in terms of the probable maximum intensity of an earthquake event for either the next 100 years or with 10 percent probability of exceedance in 50 years. These parameters, however, do not indicate to builders and engineers the maximum force a structure will experience during an earthquake, a particularly critical parameter as China continues to witness unprecedented levels of building growth. The peak ground acceleration or PGA is a far more useful measure when building design is under consideration, and the new National Seismic Zoning Map of China, 2000, illustrates the expected ground acceleration estimated through probabilistic methods (figure 7). China is now divided into seven zones with zone seven indicating those areas of highest hazard with acceleration equal to or greater than 0.4g (where g is equivalent to the acceleration due to gravity or 981 cm s-2).

Other methods for determining seismic hazard involve, for example, taking the extreme values of magnitude or ground acceleration from a catalogue of past earthquake activity. This ‘part-process’ method has an advantage in that extreme values are more likely to have been listed than smaller events because these will be more widely documented and result in observable macroseismic damage (figures 8 and 9).

Figure 7. The new National Seismic Zoning map of China illustrating the expected ground acceleration with 10% probability of exceedance in 50 years: there is a one-in-ten chance of these values being exceeded locally in any 50 year period. Units are in g (see text).

Figure 8. Peak ground acceleration (PGA) in cm s-2 for

Figure 9. Peak ground acceleration (PGA) in cm s-2 for central-northern China from extreme value statistics
with 10% probability of exceedance in 50 years.

Seismic hazard analysis is a useful tool for identifying areas that have a greater or lesser probability of earthquake occurrence and also helps the building and engineering industry to determine the levels of strong ground shaking that a structure should withstand. It does not, however, comprehensively indicate the vulnerability or exposure of populations and infrastructure to the seismic threat. This is achieved through seismic risk analysis.

With China’s record growth, potential losses arising from destruction and damage of the building stock is a critical area of investigation.

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