A destructive earthquake occurred around Changde,Hunan Province,south-central China in 1631.The previous research of this earthquake yielded 4 different epicenter locations and isoseismal intensity maps.The authors replotted the isoseismals of this event based on checking historical earthquake records,in which the intensity value of the innermost isoseismal is Ⅷ.We concluded that the depth of this earthquake is from 15km to 18km.The basic considerations of our conclusion are as follows:a.This earthquake occurred in an area of lacustrine and fluvial deposits,with the magnitude of M 6¹/2;b.The geometrical center of the innermost isoseismal is the epicenter;c.The depth of epicenter is about 15 to 18km,which is based on the statistical relation between magnitude,depth of the earthquake source and epicenter intensity as well as Xie's statistical result.Finally,the authors discussed the influences of different ground conditions on the textual research and identification of historical data.

In 1556, a strong earthquake with magnitude 8¹/₄ occurred in Huaxian, Shaanxi Province. The earthquake destroyed many places in Shaanxi Province and its adjacent regions, and also affected above 100 counties in Gansu Province, Hebei Province, Shandong Province and etc. According to historical documents, an earthquake was also recorded in Yueyang district, about 700km away from the Huaxian earthquake epicenter on the same day. Was it an isolated event or influenced by the Huaxian earthquake? It is a controversial problem.A lot of historical earthquake records of Yueyang district and its adjacent regions have been collected and studied, including the border area between Hunan Province and Hubei Province. The records for the 1556 AD Yueyang earthquake in “Annals of Yuezhou” in Longqing time of Ming Dynasty were judged to be more accurate than other historical documents. Earthquake recorded in 1556 AD in Yueyang district was a felt event with less strength. In analyzing historical materials, two principles for cases of earthquake records lacking have to be adopted: 1)Local annals to be used shall have other earthquake records. That means the annals writers paid attention to earthquake events; 2)Other types of disasters were recorded in the period of time when the earthquake occurred. That means there were no other kinds of historical disaster events influencing the records of earthquake.There were no earthquake records in counties like Tongshan, Tongchen, Puxi, Chongyang, Gongan, Shishou in the 34th year of Jiajing reign (1556 AD). It can be concluded that the earthquake records in Yueyang district in 1556 were not the effect of the Huaxian earthquake in Shaanxi Province in the same year. It was an isolated event. At last, according to records of local annals, the earthquake epicenter and magnitude are determined.

Diffuse earthquake refers to the maximum earthquake potential not associated with the identified seismogenic structures. The assessment of diffuse earthquake is a key point in nuclear power plant siting. As one part of the regional seismotectonic model,the diffuse earthquake plays an important role in SL-2 determination,because nuclear power plant is usually sited in the region of lower seismicity. Diffuse earthquake usually consists of moderate or small earthquakes of magnitudes less than 6,and the evaluation of diffuse earthquake is carried out in the seismotectonic province. The concept of“seismotectonic province”has been used to represent diffuse seismicity for the purpose of seismic hazard assessment,with each seismotectonic province assumed to encompass an area of equal seismic potential,that is to say the“diffuse earthquake”. The principles and methods of delineating seismotectonic province are discussed in detail in this paper. According to the related nuclear safety statutes,it is necessary to make reasonable assessment of diffuse earthquake for each seismotectonic province,especially for area where the nuclear power plant is sited. After discussing the principles for diffuse earthquake assessment,the paper puts forward the methods for evaluating the magnitude of diffuse earthquake based on the concept of diffuse earthquake by using seismic and geological data. The paper has also discussed how to determine the specific distance of the diffuse earthquake to the site in the seismotectonic province where the nuclear power plant is located.

In recent years, many fractal studies have been made in aspects of time, space, and intensity of earthquakes, but most of them are limited to single fractal (capacity dimension D₁ and information dimension D₂). It is found that single fractal is not enough to characterize the temporal and spatial fractal properties of earthquakes and commonly has a very narrow linear non-scale range. Moreover, single fractal dimensions cannot be obtained in some cases. In this paper, therefore, the correlation fractal dimension is adopted to analyze spatial characteristics of seismicity in Taiwan area. By use of plentiful earthquake records from the last century in Taiwan area, the correlation fractal dimensions for each seismic region, zone, and segment in this area are calculated, and then the fractal characterization for hypocentral distribution of earthquakes can be quantified. Finally, the relationship between the spatial correlation dimensions and the seismogenic environments in this area is construed synthetically. The major results are as follows: 1) the correlation fractal dimensions of the eastern seismic region of Taiwan area differs from that of the western region, owing to the different tectonic settings of the two regions; 2) each seismic region can be subdivided into different seismic zones based on the difference of plate tectonics, crustal structure and the distribution of active faults. Each zone has its distinctive correlation fractal dimension; 3) the different segments of each seismic zone have different characteristics of earthquake clustering, and hence have different correlation fractal dimensions. The results of this study indicate that there exists a good correlation between the spatial images of seismic activity obtained from correlation fractal dimension analysis and the corresponding seismo-tectonic settings.