| Summary: | 碩士 === 國立成功大學 === 地球科學系專班 === 95 === The b-value of the frequency-magnitude relation, introduced by Gutenberg and Richter in 1944, is well known to be a proper index to reflect the ratio of large and small earthquakes on some region. A number of factors have been identified to be important to the b-values. These factors include geological structure, tectonic regimes, geothermal gradient, and stress level. In this study, the earthquake catalogs established by Central Weather Bureau Seismic Network (CWBSN) are used to estimate the b-values of shallow earthquakes in Taiwan, and to discuss the characters of the b-values as a function of locations, time, and depths, respectively. Another purpose of this research is to discuss the variation of b-values before- and after- the Chi-Chi earthquake.
The results show the b-values of middle and northern Taiwan are considerably higher than those of eastern Taiwan, and this is conformed with the GPS observations on the strain rate field in the Taiwan regions. It is seemed to show that there is larger stress in the rock formations where with larger strain accumulation, and then the lower b-values. For the issue of depth, the research results show in Central range, the b-values of lower crust (about 20 to 35 kilometers) are lower than of upper crust (0 to 20 kilometers). However, in most areas of Taiwan, the b-values of the depth between 0 and 10 kilometers are lower than of the depth between 10 and 20 kilometers.
At depth of about 15 kilometers and in the seismogenic zones of the epicenter of Chi-Chi earthquake, it is observed that b-values are quite low before the earthquake and then increased in and after the earthquake gradually. It is seemed to show before the Chi-Chi earthquake, there was larger stress accumulation at depth. When the Chelungpu fault was active, this stress was then released. It is noted that this depth is the location of decollement of these areas, so the accumulated stress can be released during aftershocks of Chi-Chi earthquake. On the other hand, near these seismogenic zones, the variations of b-values are different. Before Chi-Chi b-values are higher, during Chi-Chi, b-values are decreasing substantially, and after Chi-Chi b-values are then increasing gradually. This phenomenon shows although these areas did not accumulate considerable strain energy before Chi-Chi earthquake, this earthquake is so violent that it broke the equilibrium of stress system. The stress distribution of the crust will adjust, and it reflects the stress system of these areas.
The results of this study also show that before a large earthquake, there are no phenomena like b-values being decreasing or amount of small earthquakes being decreasing happened in the seismogenic zones of the epicenter or the nearby areas. This seemed that the variation of b-values in time series is still not a earthquake precursor.
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