关键词: 地震资料, 完整性分析, 川滇藏交界地区

Abstract:

Earthquake catalog is the foundational data for analyzing seismic activity, assessing seismic hazard, and studying earthquake prediction. The majority of historical earthquake records are sourced from historical documents, with a significant portion of these records found in local gazetteers. Compiling historical literature is an essential way in analyzing seismic activity because historical accounts of earthquakes often provide more detailed and accurate information than geological data. Among these sources, analyzing relevant content in local gazetteers, such as the historical development of local governance, military garrison, official records, and descriptions of disasters and auspicious events, plays a crucial role in seismic activity research. This article aims to acquire historical earthquake records by consulting local gazetteers, folk books, and other historical sources containing natural, social, and political records. These records serve as historical foundations for analyzing the completeness of seismic data records.

The border region between Sichuan, Yunnan, and Tibet is located in the northwest secondary block of the Sichuan-Yunnan block, which is one of the areas with frequent strong earthquakes in China. The Xianshuihe fault zone and the Jinshajiang fault zone are the northeastern and northwestern boundary faults of the Sichuan-Yunnan block, respectively. They are large-scale and highly active fault zones formed due to the eastward escape of the Tibetan plateau caused by the relative movement between the Indian and Eurasian plates. Previous studies on active tectonics have shown that major earthquakes with magnitudes of 8 and above, as well as over 80% of strong earthquakes with magnitudes of 7, mainly occur in the boundary zones of active blocks with intense structural deformation and high stress accumulation. Moreover, the known active faults in the study area, such as the Batang fault and Litang fault, are also major faults that significantly have influence on the occurrence strong earthquakes. The Sichuan-Yunnan-Tibet adjacent region is home to significant infrastructure, including the Sichuan-Tibet railway and hydropower stations. Analyzing the completeness of earthquake data in the border region of Sichuan, Yunnan, and Tibet can contribute to the assessment of fault hazards and the analysis of regional seismic activity trends. This, in turn, can help minimize the damage caused by earthquakes to critical infrastructure and further enhance the safety and security of people’s lives and properties.

This study reviewed the local gazetteers of 44 counties in the border region between Sichuan, Yunnan, and Tibet, and summarized the establishment and historical evolution of each county. Based on the analysis of the road evolution from Sichuan to Tibet and from Yunnan to Tibet, we examined the significant roles of important transportation hubs and nodes, such as stations, pond flood, and grain platforms, in regarding of recording earthquakes. Combining various historical sources and previous research on the completeness of earthquake data in the region, we conducted a comprehensive analysis to determine the probable starting years for the availability of seismic records of magnitude 7 and above in the Xianshuihe area and the three parallel rivers area. Additionally, based on the data of the length and short axis of isoseismal lines from 88 earthquakes, an elliptical model was used to derive the seismic intensity attenuation relationship for the Sichuan-Yunnan block. By placing the fitted isoseismal lines of magnitude 6 and 7 earthquakes in the study area, we analyzed their impact range, providing a spatial dimension basis for the completeness analysis of seismic data.

This article provides a comprehensive analysis and demonstration of the complete starting years of seismic data in the border region between Sichuan, Yunnan, and Tibet from both temporal and spatial perspectives. The results indicate that due to the establishment of grain stations and Tangxun along the Sichuan-Tibet road, as well as the appointment of officials, several counties in the Xianshuihe area, including Kangding, Luhuo, Garzê, Litang, and Yajiang, were developed between 1719 and 1736. At the same time, there are relatively abundant historical documents related to earthquakes in the Xianshuihe area. Local chronicles, reports from governors and resident ministers, written records in Tibetan temples, and accounts from lamas have documented earthquake surveys, disaster assessments, and relief efforts. By combining these historical sources with the analysis of intensity attenuation relationships in the Sichuan-Yunnan block, the affected areas of earthquakes with magnitudes 6 and 7 can be determined that the period from 1719 to 1736 marks the starting years with complete M≥7 earthquake data in the Xianshuihe area. The towns of Batang, Mangkang, and Changdu in the three parallel rivers area are also significant nodes and hubs along the road to Tibet. They were established with administrative institutions and granaries between 1719 and 1728, and the road network extensively covered Tangxun in the region. In considering the seismic records and historical sources in the three parallel rivers area, as well as referencing the recording capabilities of granaries, administrative institutions, and Tangxun in the Xianshuihe area, and estimating the potential recorded seismic magnitudes based on the intensity attenuation relationships of the Sichuan-Yunnan block, it can be suggested that the period from 1719 to 1728 is a possible starting point for complete earthquake data with M≥7 in the three parallel rivers area. In areas farther away from the road to Tibet, such as Jiangda, Gongjue, Baiyu, Xinlong, and the northern regions of Batang and Litang, as well as the large contiguous regions of Derong, Xiangcheng, Daocheng, and Jiulong, the eastern boundary is the Xianshuihe fault zone, while the area between the two zones is divided by the northeast-oriented Batang fault. Previous seismic geological investigations have found that within the aforementioned regions, the influence of the Jinshajiang fault zone extends along the Batang-Derong-Benzilan line. In remote areas away from the road and with sparse population, the possibility of individual earthquakes with magnitudes above 7 occurring but being missed cannot be ruled out. However, in other areas not located on active fault zones, it can be considered unlikely to experience earthquakes with magnitudes above 7. Based on the analysis of the data, the starting years of earthquakes with a magnitude of 6 and above should be the same as those of earthquakes with a magnitude of 7 and above. However, according to the analysis of the average occurrence rate of earthquakes per year, there is a significant lack of records for earthquakes of magnitude 6 and above. This may be due to the sparsely populated and vast nature of the Tibetan region during historical times, limited administrative capabilities of officials, and lack of earthquake historical records and documents. Therefore, it is not possible to determine the exact starting year for complete data on earthquakes of magnitude 6, which would be the same as for earthquakes of magnitude 7 and above.

Key words: seismic data, integrity analysis, the juncture region of Sichuan, Yunnan and Tibet Provinces