NEW NATIONAL SEISMIC ZONING MAP OF CHINA
Institute of Geophysics, China Seismological Bureau, Beijing 100081, China
I．BACKGROUND OF THE NEW NATIONAL SEISMIC ZONING MAP
The policy of seismic disaster mitigation in the mainland of China is “prevention first”. According to the law, the earthquake design for ordinary structures must fit the demand of national seismic zoning map. Seismic zoning map is the basis of the earthquake design (Tang, 1998; Wu 1998).
The seismic zoning map must be updated with the progress in methodology and accumulation of the data. There are three generations of national seismic zoning map before this new one. Prof. Li Shanbang published the first seismic zoning map in 1957. This map demonstrated the maximum affected intensity of China. The second edition of national zoning map was published in 1977. The map was provided by using the methodology of long term and middle term earthquake prediction. The map demonstrated the maximum encountered intensity in the forthcoming 100 years. This map was adopted by the building code. The third edition of national seismic zoning map was provided in 1990. The probabilistic method was used in preparing the map. The intensity with exceeding probability 10% within 50 years was given in the map. It was used in the building code and other regulation related with earthquake design. The new zoning map is the normal update of the national zoning map.
Intensity was adopted in these zoning maps. But up to now, earthquake design using the response spectrum is popular in China and also in the world. It is very hard to formulate response spectrum from the single parameter of intensity. It is widely accepted to use ground motion parameter to make zoning map in many countries. China is in the stage of fast economic development and urbanization. New structures and middle to high-rise buildings spring out everywhere and every day. So it is necessary to provide the response spectrum related with the earthquake surroundings to fit the needs of earthquake design in the new situation. The national seismic zoning map must provide the zoning parameters of ground motion. With these parameters, design response spectrum should be formed easily. Intensity zoning map should be replaced to fit the demand of society. The update of this edition of seismic zoning map is driven by social development.
The deadline of the data used to the last national zoning map including geological data, seismological data, and geophysical data, is Dec. 31, 1987. After that time, there is a lot work concerned with the basic data. The instrumental earthquake database is doubled because the increasing monitoring stations. Nearly a hundred thousand events are recorded. 451 damage events with magnitude 5 were recorded. A lot of work has been done about the detailed investigation of active faults in western, southwestern, and northern part of China. There are also deep sounding studies in the eastern part of China to understanding the earthquake source. The updating of the national seismic zoning map is also driven by the new basic data available.
Another reason for the updating of the national seismic zoning map is the progresses in seismic hazard assessment for important projects and urban microzoning and improvement of the methodology of the seismic hazard assessment (China Seismological Bureau, 1998). The seismic ground motion parameters including seismic response spectrum and peak acceleration were widely used in the work. We already have experiences in the determination of the attenuation of the ground motion parameters. This makes it possible to provide zoning map with ground motion parameters.
The project for providing the new zoning map started in the end of 1996. Five institutes, 28 local seismological bureaus and 166 scientists involved in this project. The zoning map was finished in 2000. During the project, there were a series of interactive meetings with the users and local seismological bureaus.
The probabilistic approach was used for the zoning maps. In order to consider the uncertainties in the probabilistic analysis, logic tree methodology was adopted. Four working groups are responsible for delineation of potential sources under their knowledge base and database. These four sets of potential sources were used in logic tree method. The new results from the studies of active fault and the instrumental earthquake records were carefully used in the determination of potential sources and seismicity parameters.
The strong ground motion records were collected and used for the determination of attenuation relations. The strong ground records from the western United States were used to establish the original attenuation. Then we modified the original attenuations by the comparison of attenuation of intensity in western United States and that in east part of China and west part of China.
The country was divided into 40000 grids. The seismic hazard analysis for every grid was performed. The ground motion parameters with exceeding probability 10% within 50 years were determined. From this database, the final two zoning maps were made.
GIS technology was used in the projects, in different stage, including database management, delineation of potential sources, demonstration of the zoning results and the study of the reasonability of zoning results.
II. THE MAIN FEATURES OF THE NEW ZONING MAP
The probabilistic methodology was adopted in compiling the new zoning map.
The exceeding probability of the new zoning map is 10% within 50 years. The scale of the map is 1:40000000. The soil condition is hard (Yuxian Hu, 2001).
The new zoning map includes two specific maps (1) Acceleration (a) related to the platform of the response spectrum, which is defined as the platform value of the response spectrum over 2.5; (2) The characteristic period of the response spectrum (Tg). From these two parameters, the design response spectrum can be determined easily. Figure 1 is the illustration forming the design response spectrum. The decay rate from the characteristic period is 1/Tg. Considering the syntonic frequency of most structures, these parameters are much better than intensity and peak acceleration.
1. Acceleration Zoning Map
In the acceleration map, the territory is divided into seven zones. Zone 1 is with acceleration less tham 0.05g; zone 2 with acceleration 0.05g; zone 3 with acceleration 0.10g; zone 4 with acceleration 0.15g; zone 5 with acceleration 0.20g; zone 6 with acceleration 0.30g; zone 7 with acceleration equal to or greater than 0.40g. Compared with the intensity zoning map, we have two more zones. There are the zone with acceleration 0.15g and the zone with acceleration 0.30g. On the consideration of “concept design”, the zone 3 and zone 4 are considered as intensity 7; the zone 5 and zone 6 are considered as intensity 8.
The zone 7 (≥0.40g) is highest seismic potential, and includes 16 regions. These regions are located in the main seismic provinces and the potential sources with maximum magnitude 8. These regions are part of western Taiwan, Gulang in Gansu Province, Haiyuan in Ningxia Autonomous Region Maduo and Dolan in Qinghai Province, Tashkuegan and Xinyuan of Xinjiang autonomous Region, Yangbajing in Tibet Autonomous Region.
The zone 6(0.30g) is also high seismic hazard region. It includes 22 regions and covers 2.6% of the land territory of the country, nearly 210,000 km2. There are mainly distributed in Taiwan, North and south Tianshan, Silk Corridor, Tibet, South-north seismic Province. Haikou, the capital of the Hainan Province, and the Baotou of Inner Mongolia are also located in zone 6. Linyi of Shandong Province is a region with high-density population in zone 6.
Fig.1. Illustration of the zoning parameters.
The zone 5 (0.20g), zone 4 (0.15g) and zone 3 (0.10g) cover 46% of the land territory. Most of the densely populated regions are in these zones. The most part of western China, some regions of the northern China are in zone 5 (0.20g).
Zone 2 (0.05g) regions covers 20% of the land territory. It mainly distributed in the eastern part of China.
Earthquake design must be considered in 80% of the country.
Most cities and Towns in China are located in the high seismic hazard regions.