Abstract: The line-strain observation at Yixian and Yanqing stations shows that large change of compression has appeared in the Capital Circle Area since 2005. The strain in NS turned back at Yanqing station in June and July, 2006. The parameters of plane strain are calculated using extensometer data in 3 directions at Yixian station. The result shows the parameters of plane strain changed in reverse 1~2 days before the M_S5.1 Wen′an earthquake (July 4th 2006) from. The strain has been accelerated in the western part of the Capital Circle Area since 2005 based on analysis, when Yuxian 4.0 earthquake (Aug. 31, 2005) occurred, Wen'an earthquake occurred in course of plane strain turning back. It may indicate that plane strain enters a stage for strain accumulation after Wen'an earthquake in Capital Circle Area.

Key words: Strain change, Digital observation, Wen′an earthquake, Capital Circle Area