2 Earthquake Administration of Yunnan Province, Kunming 650224, China;
3 Hangzhou Aadtech Co. Ltd., Hangzhou 310030, China
The earthquake precursor monitoring can be assessed in four ways. Undoubtedly, the first is earthquake precursor monitoring ability, i.e., whether the anomalies related to the seismic activities can be recorded before the occurrence of destructive earthquakes. The second is whether the changes of the earth solid tides can be recorded. The third is whether co-seismic response can be recorded and the last is the SNR value. The study of co-seismic effect has attracted great attention both from abroad and at home with a view to exploring the indirect assessment of precursor monitoring due to the global continuous MS≥7.0 strong earthquakes in the past decade.
A large amount of co-seismic effect data of well water level and water temperature has been recorded in China's earthquake underground flow observation for 50 years. According to the statistics of the Wenchuan MS8.0 earthquake in Sichuan on May 12, 2008, co-seismic effects in water level observations were recorded in 194 wells and 132 in well water temperature observations (Liu Yaowei, 2009b). However, not many co-seismic responses were recorded about geochemical quantity measurements, and only in the radon vapor observations in the Taidiangou spring in Wudu of Gansu, Shengli well in Ningxia and wells in Ningde, Fujian, were the first batch of radon vapor co-seismic effect recorded (Liu Yaowei et al., 2009b). There are no reports at home or abroad about the co-seismic effect of mercury vapor. The first mercury vapor co-seismic effect in China was recorded at the Mile monitoring well in Yunnan during the Nepal MS8.1 earthquake on April 25, 2015.
Seismic mercury observation has a history of 30 years in China, and digital mercury vapor observation started over a decade ago. Mercury observation plays an important role in seismic precursor monitoring and prediction explorations. Medium and short-term data or short-term precursory anomalies were captured before several earthquakes. Some wells even recorded suspected dynamic solid tides (Jin Yangfen et al., 1987; Zhang Wei, 1989; Wei Jiazhen et al., 1994; Ren Jia et al., 2005; Qian Xiaodong et al., 2008; Zhao Xiaomao et al., 2010). However, no co-seismic effect data was ever recorded before the Nepal MS8.1 earthquake.
The Mile well is located in the hot spring area at Meihua village, Mile County, Yunnan Province with well coordinates of 103°26'E, 24°24'N and well elevation of 1416m. The depth of the monitoring well is 614.4m and the observation layer is the self running confined water in triassic limestone karst with the water temperature of 23℃. Besides the physical observations such as dynamic water level and water temperature, digital chemical observations of radon vapor, mercury vapor and helium were also carried out during the Tenth "Five-year Plan". The monitoring instruments were a SD-3A radon detector, RG-BQZ mercury detector, and WGK-1 helium detector. In May 2013, an ATG-6138M online mercury trace analyzer (mercury detector) was installed in the Mile well which was designed with the National Twelfth "Five-year Plan" Fund (2012BAK19B02) by the Hangzhou Aadtech Co. Ltd., subordinate to Hangzhou Electronic Polytechnic University. The absolute detection limit of the mercury detector is 5×10-13 gHg, and the relative detection limit is 0.1pgHg/L(10-4 ngHg/L). This is two orders of magnitude above that of the RG-BQZ digital mercury detector (0.008ng) widely used in China. The comparison between this new type of detector and RA915am detector of Russia which is internationally recognized as the best, reveals that there is a high consistency between the two. During the tentative mercury vapor monitoring in the Mile well, a clear co-seismic effect was recorded when the Nepal MS8.1 earthquake occurred (Fig. 1). No co-seismic effect was recorded by the simultaneously monitoring SD-3A radon detector, RG-BQZ mercury detector, and WGK-1 helium detector during that period.
Fig. 1 shows that the normal background value of the mercury vapor in the Mile well is around 30ng/m3 with some daily variations, which is in a fall-rise-fall-rise mode in a range of 10-30ng/m3. After the earthquake, the value rose sharply from less than 20ng/m3 to above 100ng/m3 which is 3-8 times the normal daily value, which indicates that there should be clear co-seismic anomalies. There are clear anomalies. The high value of 80-110ng/m3 remained for about 20 hours after the earthquake and then fell to the normal background value of 20-30ng/m3.
The chemical monitoring of the earthquake underground flow observation network in China plays an active role in the earthquake precursor monitoring and seismic prediction, but most of the observations cannot record earth solid tide effects and there is little data for co-seismic effect. The reason for such a phenomenon was widely believed to be due to a low sensitivity to crust dynamic response. However, both the co-seismic effect of radon vapor at the Wudu spring in the Wenchuan earthquake or that of mercury vapor at the Mile well in the Nepal earthquake prove that chemical observation can record the data of crust dynamic effects. The failure to record earth solid tides or co-seismic effects through chemical monitoring by most wells or springs can be caused by the imprecision of chemical monitoring instruments and low sampling rate. Therefore, it is necessary to develop chemical instruments with better precision and higher sampling. This effort is not only for the monitoring of solid tide and co-seismic effect, but is also for a significant improvement of earthquake precursor monitoring with chemical observation.
ACKNOWLEDGEMENTS: The authors owe their gratitude to research professor Liu Yaowei at Institute of Crustal Dynamics, CEA and research professor Fu Hong at the Earthquake Administration of Yunnan Province for their valuable support to this research.
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2 云南省地震局, 昆明北辰大道 650224;
3 杭州超距科技有限公司, 杭州振华路212号 310030