Gas Geochemitry In Western Sichuan Related To 12 May 2008 Wenchuan Ms8.0 Earthquake

The Wenchuan Ms 8.0 earthquake occurred on 12 May 2008 killed about 100 thousand people, left millions of homelessness and destroyed many towns. To accurately predict earthquake is one of the most important measurements for mitigating disaster of earthquakes, which demands to understand the genetic process of earthquake and genetic mechanism of precursors. The fluids in the deep earth play a very important role in the pregnancy and occurrence of earthquake. The hot spring fluids carry a lot of information of the inner earth. After the Wenchuan Ms 8.0 earthquake, it is very significant to conduct investigation of gas geochemistry in the epicenter region and vicinity, which can obtain the information related to the Wenchuan Ms 8.0 earthquake.The spring gases were repeatedly sampled three times from 32 hot springs in western Sichuan in June and October 2008 and June 2009. The concentrations of chemical components, isotopic ratios of helium and neon (³He/⁴He(R), ²⁰Ne/²²Ne), ²¹Ne/²²Ne) and stable carbon isotope ratio of carbon dioxide (δ¹³C_CO2) of ninety six gaseous samples were measured. In the corresponding period, the concentrations of He, H₂, CO₂, CH₄, Rn, Hg and N₂/O₂ ratio in soil gas were measured repetitionally three times in the field at 828 sampling sites in eight short profiles across the seismic rupture zones in June and December 2008 and July 2009.The gas geochemical characteristics were investigated at the seismic belts in the western Sichuan according to the data of the chemical components, isotopic ratios of helium, neon and carbon of 96 samples. The evolution of gas geochemistry in the western Sichuan was revealed by comparison between the data of this study and those of 2000 reported in the literature (XX et al., 2006). The parameters of ²⁰Ne/²²Ne)-²¹Ne/²²Ne, ³He/⁴He-⁴He/²⁰Ne, CO2/³He-³He/⁴He,δ¹³C_CO2-N₂,δ¹³C_CO2-CO₂ and CH₄/³He-³He/⁴He indicated that the spring gases have the multiple sources. The heat flow values in the western Sichuan were calculated by the equation of correlating helium isotope ratio with heat flow.The data showed that a lot of fluids derived from the mantle containing a great deal of He and CO2 migrated into the crust and mixed with fluid originated from the crust when the Wenchuan Ms 8.0 earthquake occurred, which resulted in obvious increase of ³He/⁴He andδ¹³C_CO2 values of the hot spring gases in the Xianshuihe and Minjiang fault zones in the western Sichuan. The maximum of helium isotope ratio was 5.3 Ra (Ra=1.4×10^-6, atmospheric ³He/⁴He), indicating 44.1% helium derived from the mantle in the Kangding district. As time goes by, contribution of the mantle fluids to the hot spring gradually decreased, but the crustal gas components, radiogenic helium and CO₂ from organic origin increased relatively. In the third measurement, values of ³He/⁴He andδ¹³C_CO2 became as large as those of 2000 in most hot springs. However, there were still a lot of mantle He and CO₂ at the Erdaoqiao and Guanding hot springs in Kangding county located in the Xianshuihe fault zone. The higher values of ³He/⁴He and heat flow were found in the region where the layers of high conductivity and low wave velocity exist in the lithosphere. The ³He/⁴He value observed at the Wenchuan Seismic Station in the Longmenshan fault zone obviously increased, indicating 30% of total helium was derived from the mantle. The geochemical data indicated that the mantle fluids upwelling into crust could play an important role in generation and occurrence of the Wenchuan Ms 8.0 earthquake.The concentrations of He, H₂, CO₂, O₂, N₂, Rn and Hg in soil gas indicated obviously temporal and spatial variations. The concentration anomalies of He, H₂, CO₂, O₂, N₂, Rn and Hg in soil gas occurred near the seismic scarps produced by the Wenchuan Ms 8.0 earthquake. The magnitudes of the He and H₂ anomalies decreased significantly with decreasing strength of the aftershocks with time. The maximum concentrations of He and H₂ (40 and 279.4 ppm, respectively) were found in three replications at the south part of the rupture zone in the epicenter region. The spatio-temporal variations of CO₂, Rn, and Hg concentrations in the north parts of the seismic rupture zone differed apparently from those in the south parts. The higher concentrations of He and H₂ in soil gas occurred near the segments of the rupture zone where vertical displacements were larger. The anomalies of He, H₂, CO₂, Rn, and Hg concentrations could be related to the variation in the regional stress field and the aftershock activities.