【作者】 周晓成；

【导师】 杜建国；

【作者基本信息】 中国科学技术大学 ， 固体地球物理学， 2011， 博士

【摘要】 2008年5月12日汶川Ms8.0地震给人民生命财产带来巨大的损失。减轻地震灾害的关键之一是对地震作出准确的预测。准确地震预测需要对地震的孕育和发生机理有清楚的认识。深部流体对地震的孕育和发生起很重要的作用;溢到地表的流体携带了大量来自深部的信息。汶川Ms8.0地震后,开展震中及其周边地区的气体地球化学研究,获取与汶川Ms8.0地震相关的深部地球化学信息,对认识汶川Ms8.0地震及其余震具有重要的意义。于2008年6月、2008年10月和2009年6月,在川西地区32个温泉点进行3次气体重复采样,测量了96个温泉气体样品的气体组份、氦同位素和氖同位素比值(³He/⁴He（R）和²⁰Ne/²²Ne)、²¹Ne/²²Ne)、二氧化碳的稳定碳同位素组成(δ¹³C_CO2 PDB‰);同期,在汶川Ms8.0地震地表破裂带上8个地点,三次重复测量了土壤气体中He、H₂、CO₂、CH₄、O₂、N₂、Rn和Hg的浓度（2008年7月、2008年11月、2009年7月）,共取得828个土壤气测点浓度数据。根据三次测量的32个温泉气体的组分和氦、氖、碳同位素分析数据,对川西主要地震带进行了气体地球化学研究。为了讨论川西温泉气体地球化学演化,将所获数据与2001年测量的³He/⁴He值、δ¹³C_CO2值进行了对比分析。利用²⁰Ne/²²Ne)-²¹Ne/²²Ne、³He/⁴He-⁴He/²⁰Ne、CO2/³He-³He/⁴He、δ¹³C_CO2 -N₂、δ¹³C_CO2 -CO₂、和CH₄/³He-³He/⁴He等指标判定了研究区温泉气体的来源;同时利用³He/⁴He值与大地热流值的关系,计算了川西地区大地热流值。研究表明:汶川Ms8.0地震时,在川西鲜水河断裂带和岷江断裂带,有含大量He和CO₂的幔源流体涌入地壳,与壳源流体混合,使温泉气体中³He/⁴He和δ¹³C_CO2值大幅度上升。在康定地区R/Ra（Ra为大气的³He/⁴He）最高达到5.3Ra,幔源氦的贡献率最高达到44.1%;随着时间推移,幔源流体供给逐渐减少,壳源流体中放射成因的He和有机成因的CO₂在温泉气体中相对含量增加,³He/⁴He （Ra）值、δ¹³C_CO2值逐渐减小。在第三次测量时,大部分温泉恢复到汶川Ms8.0地震前的地下流体的平衡状态;但是,在鲜水河断裂带上康定地区二道桥和灌顶温泉仍然有大量来自幔源的He和CO₂。³He/⁴He值和大地热流值的空间分布与川西下地壳的高导低速层的分布有很好的一致性。在龙门山断裂带上的汶川地震台测点,³He/⁴He值也有明显升高,幔源氦贡献率最高达到30%。根据以上结果推测,幔源流体上涌可能对汶川Ms8.0地震孕育和发生起到很重要的作用。汶川Ms8.0地震地表破裂带土壤气体中He、H₂、CO2、O₂、N₂、Rn和Hg的浓度存在明显的时空变化。结果表明:土壤气中He、H₂、CO₂、Rn浓度和N₂/O₂值在汶川Ms8.0地震陡坎的附近有明显高值异常。土壤气中He和H₂的浓度异常幅度随着余震强度的衰减而降低。在三次测量当中,土壤气中的He和H₂的最大浓度（40和279.4 ppm）都出现在靠近震中的地段。土壤气中的CO₂、Rn和Hg浓度的时空变化在地震破裂带南部和北部存在明显差异;土壤气中的He和H₂的浓度高的区域,地震陡坎垂直位移也较大。汶川Ms8.0地震破裂带土壤气中的He、H₂、CO₂、Rn和Hg的浓度异常可能与余震活动和区域应力场的变化有关。更多还原

【Abstract】 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.更多还原