Abnormal Analysis of Averaged Midnight F before the Ms 5.1 Chuxiong Earthquake Rong Zhu 1, Dong-mei Yang 1, Yan Zhang 2, Jian-jun Wang 3, Yu-fei He 1 1. Institute of Geophysics, China Earthquake Administration, Beijing, China, 2. Chuxiong Staion, Yunnan Earthquake Administration, Yunan, China 3. Gansu Province Earthquake Administration, Lanzhou, China A shock with magnitude Ms 5.1 occurred at 12:56 Beijing local time on 25 Feb 2010 Yunnan Province China (25.4°N, 101.9°E). Geomagnetic data are analyzed and the averaged midnight F of Chuxiong station clearly increased one month before the day of earthquake. Figure 5. The Dst index in Jan 2010 ( u.ac.jp/dst_realtime/201002/index.html) Figure 1. The location of the 4 points Figure 2.The location of the there epicenters and fault lines. Above all, the big changes in Chuxiong station on Jan 2010 might have relation to the earthquake occurred on Jan , the mechanism and process should be further studied. Figure 3.a, b, c, d respectively show the variation curve of the averaged midnight F from Jan.1, 2009 to July.7, 2010:Chuxiong, Quanzhou, Tonghai and Yongning. Data Selection Thinking of the data availability and the distribution of the stations and observatories in China, Chuxiong station, Quanzhou, Yongning and Tonghai observatories were selected. Figure1 shows the location of Chuxiong station, Quanzhou, Yongning and Tonghai observatories. The instruments in Chuxiong and Quanzhou are Didd Overhauser magnetometer （ GSM-19FD ）, while in Yongning and Tonghai observatores are Geomannetic filld F and components measuring system (FHDZ-M15), all are in second sampling in DHZF. The calculation of midnight average value based on minutes data from 0 to 3 local time. Data Analyze Figure 3 shows the variation curve of the averaged midnight F from Jan.1, 2009 to July.7, The 4 curves have the same variation trend, we can not find any obvious differences. When we made Chuxiong’s value as reference, use Quanzhou, Tonghai and Yongning’s value minus Chuxiong’s value, the result is in Figure4. The black, red and green lines shows the different of the averaged midnight F between Quanzhou and Chuxiong, Tonghai and Chuxiong, Yongning and Chuxiong. It is very clear that there is a big change on Jan, 2010, which means the values of Chuxiong suddenly increase in the middle of Jan, 2010, compared to the averaged geomagnetic midnight F values of the other 3 observatories, the amplitude is about 3 nT. Discussion There are some possibilities to explain why the averaged midnight F of Chuxiong station clearly increased about one month before the earthquake: instrument, artificial disturbance, external field, earthquakes and so on. 1. It was confirmed that the instrument in Chuxiong worked well. 2.There was no artificial disturbance around the station. 3. The Dst index in Jan 2010 (Figure 5) shows the geomagnetic is rather quiet. As the big change only appeared in Chuxiong Station, the possibility of effect from external field could be eliminated. If it is because of the earthquake occurred on Feb. 25, 2010, how to explain that there is no similar change before other earthquakes? For instance, one shock occurred on July 9, 2009, Ms 6.0 in Yao’ an region (25.6°N, 101.1°E), on the day after a shock magnitude of Ms 5.2 occurred between Yao’ an and Xiangyun region (25.6°N, 101.0°E ) in Bai minority region. The distance between Chuxiong station and a b c d Figure 4. The black, red and green lines shows the different of the averaged midnight F between Quanzhou and Chuxiong, Tonghai and Chuxiong, Yongning and Chuxiong. epicenters are less than 100 km. Figure2 shows the location of the there epicenters and the fault lines in that area ( ). In Figure4, no big change was found before these earthquakes. It ’ s a pity there is a deficiency in Yongning ’ s data. The epicenter of the earthquake on Feb. 25, 2010 locates on a north-south fault line Yuanmou- Lvzhijiang, the other two earthquakes ’ epicenters were not so close to it, this might explain the obvious change of the averaged midnight F observed by Chuxiong Station. Maybe only around the Yuanmou-Lvzhijiang fault line the seismic-geomagnetic effect existed before the earthquakes. The seismogenic process affected that zone and some changes are formed as a result. Acknowledgement Thanks for all participate colleagues hard work and thank those who work for Chuxiong station, Quanzhou, Tonghai and Yongning observatories.