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Earthquake Protection Systems_11

cung cấp cho các giá trị cường độ lớn vô hạn, là không thực tế. Đối với mỗi khu vực động đất, có hiệu lực một giới hạn về kích thước tối đa của động đất có thể xảy ra, bắt nguồn từ bản chất địa chất | SITE SELECTION AND SEISMIC HAZARD ASSESSMENT 243 Figure 7.4 Division of earthquake activity in southern Italy into seismic source zones Number of Events per Year per Km2 within the Seismic Source Zone of greater than or equal to Magnitude ML 10-4 10-5 10-6 10-7 10-8 Zone B Central Apennines Zone D Background Activity Zone C The Calabrian Arc 10-9 - 4.0 5.0 6.0 7.0 Magnitude ML Figure 7.5 Seismicity of the separate source zones shown in Figure 7.4 244 EARTHQUAKE PROTECTION give infinitely large magnitude values which is unrealistic. For each earthquake region there is in effect a limit on the maximum size of earthquakes which could occur deriving from the geological nature of the faulting. To deal with this various modifications of the Gutenberg-Richter formula have been proposed such as the use of a curved or truncated linear relationship. Figure 7.3 compares the Gutenberg-Richter formula with an alternative formula14 for the data for earthquakes in southern Italy showing that the curved relationship with a definite upper bound is much more useful for predicting the recurrence of earthquakes of magnitude greater than 7.0. Time Sequence Analysis These analyses generate expected return periods for events i.e. the average rates of occurrence of earthquake activity. It is clear from most earthquake catalogues that earthquake activity does not occur uniformly in time - it is sporadic and unevenly spread over the years. An administration responsible for a region or an organisation working across an area may well be concerned to estimate the numbers of earthquakes of different sizes likely to occur within that region in any given period of time. An example of a typical time sequence of earthquakes of different magnitudes occurring across a region is given in Figure 7.6. The number of earthquakes occurring within a given time period e.g. 10 years can be derived from the data for successive time intervals 1900-1910 1901-1911 etc. as presented in Table 7.1. Analyses like .