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《Earthquake》 1986-01
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Jiao Yuanbi Wu Kaitong Yang Mandong (Center for Analysis and Prediction, State Seismoiogical Bureau)  
We colleted and sorted out the date from the seismic network in North China, consulted some observation reports of some local networks, and determined the relations between amplifications of different instruments and effective distances of monitoring so as to define the detecting range of individual stations. The region which can be detected by four or more stations simultaneously is termed as an effective monitoring region. The detectability of the network during four different periods is calculated by using an IBM computer. The results are as follows:1. Before the Xingtai earthquake of Feb. 1966, only a Halin type seismograph was installed at the Baijiatan seismic station in Beijing in the North China region, so the entire region could not be basically detected by the seismograph. Earthquakes which occurred within a certain range could be recorded by single station, with the earthquakes of M3.5-4.0 registered in the Xingtai seismic region. If the directions of first motions were clear, single stations could be used to locate the epicenters, but the accuracy was not so high.2. From Dec. 1970, the seismic network was capable to detect earthquakes of M1.5 -2.0 in the.Beijing-Tainjin and Xingtai regions, and was generally able to monitor eartquakes of ML≥3.0 in the whole North China region, while the network had poor de-tectability to monitor earthquakes in the southern part of the region.3. Before the Tangshan earthquake of July 1976, the network was able to observe earthquakes of ML≥2.5 throughout the region. The area in which earthquakes with ML ≥ 1.5 could be detected in the Beijing region was expanded obviously compared with that in 1970. The earthquakes of ML≥1.5 could be observed in Linyi area of Shandong province, and earthquakes of ML≥2.0 could be monitored in the Tangshan seismic re-gion.4. From Dec. 1984, the range in which earthquakes of ML ≥ 1.5 in the Beijing-Tiaiijin-Tangshan-Zhangjiakou region could be detected by the network was greatly expanded. Earthquakes of ML=1.0 could be monitored in the Beijing area. The capability for observing earthquakes in Handan area, Hebei province, and Linfen area, Shanxi province were also improved. Earthquakes of ML ≥ 2.0 in the whole North China region could be observed except in the east part of Bohai area and the southeast past of Henan province. If the data from stations in Anhui province are included, the detectability in the south part of Henan province could be also improved to monitor earthquakes of ML≥ 2.0.
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