在理論上確認鹽水於地震帶的導電度

原始網址：www.sciencedaily.com/releases/2016/06/160613131100.htm

Electrical conductivity of salt water in seismogenic zones theoretically determined

在理論上確認鹽水於地震帶的導電度

A joint research team consisting of Hiroshi Sakuma, senior researcher, Functional Geomaterials Group, Environment and Energy Materials Division, National Institute for Materials Science (NIMS), Japan, and Masahiro Ichiki, assistant professor, Graduate School of Science, Tohoku University, Japan, succeeded in theoretically determining the electrical conductivity of NaCl solution (salt water) in a high-temperature and high-pressure environment at ground depths ranging from 10 to 70 km. By comparison with electrical conductivity data collected underground, the theoretical approach indicated the presence of salt water deep underground. This discovery may reinforce the theory that underground salt water influences the occurrence of earthquakes and volcanic eruptions.

包括日本國家物質材料研究機構環境與能源部，基礎地質材料研究團隊的高級研究員 Hiroshi Sakuma，以及日本東北大學理學院的助理教授 Masahiro Ichiki在內的聯合研究團隊，成功在學理上確立 NaCl溶液(鹽水)在地下10至70公里處高溫高壓環境下的導電度。藉由比對從地下蒐集到的導電度資料，這項理論可以指出在地底深處是否有鹽水的存在。此發現或許能更加鞏固地下鹽水會影響地震及火山爆發形成的學說。

It is commonly said that the presence of salt water in bedrock makes a fault prone to slide, influencing the occurrence of earthquakes, or decreases the melting points of rocks, influencing volcanic eruptions.

一般認為存在於岩床中的鹽水會造成斷層更容易滑動而影響地震發生，或者降低岩石的熔點而影響火山爆發。

However, it is difficult to directly verify the presence of salt water through drilling surveys deep underground. Since liquids including salt water have electrical conductivity about six orders of magnitude higher than that of solids, surveys involving the measurement of electrical conductivity are often carried out to detect the presence of salt water. However, because the electrical conductivity of salt water under the high-temperature and high-pressure conditions occurring in such environments as crustal seismogenic zones is unknown, it had been impossible to associate electrical conductivity measurements with the presence of salt water.

然而，要透過鑽井探測直接證實地底深處是否有鹽水存在是相當困難的。既然包括鹽水在內的流體導電度一般都比固體高出大約百萬倍，因此經常會施行包含導電度測量在內的探勘來偵測是否有鹽水存在。然而，由於在像地殼內的地震生成帶這種高溫高壓環境下的鹽水導電度會是多少仍未知曉，因此要直接將導電度測量結果跟鹽水的存在與否畫上等號是不可行的。

The research team developed a molecular model to reproduce the supercritical state of water. Using the model, the team successfully calculated electrical conductivity of salt water with NaCl concentrations ranging from one-sixth to triple that in seawater at high temperature and high pressure (temperature: 673-2,000 K, pressure: 0.2-2 GPa), conditions that are difficult to simulate in experiments. These electrical conductivity data indicated that high electrical conductivity measured under the ground in the Tohoku region may be explained by the presence of salt water with salt concentrations equivalent to seawater.

研究團隊發展出一個分子模型可以重現超臨界狀態下的水。團隊利用這個模型而成功計算出在實驗難以模擬的高溫高壓環境下(溫度：絕對溫度673至2000 K，壓力：0.2-2百萬帕)，NaCl濃度為海水濃度六分之一至三倍的鹽水的導電度。計算出的導電度資料指出在東北區域地下測量到的高導電度，或許可用有濃度等同海水的鹽水存在來解釋。

In future studies, we will combine these results with the electromagnetic crustal observations across Japan to identify the presence of salt water deep underground where seismic and volcanic activities are high, as in subduction zones, and conduct research in order to understand the mechanism of the outbreak of earthquakes and volcanic eruption.

在未來的研究中，我們會將這些結果跟日本各處地殼電磁的觀測數值結合起來，以確認隱沒帶這類地震及火山活動頻繁的區域地下深處是否有鹽水存在，並進行相關研究來瞭解地震和火山爆發的發生機制。

This research was conducted as a part of the projects "Geofluids: nature and dynamics of fluids in subduction zones," (Grant-in-Aid for Scientific Research on New Academic Related Areas) and "Research on understanding supercritical fluid properties in crust through molecular dynamics calculation and its influence on earthquake occurrence," (Grant-in-Aid for Challenging Exploratory Research) supported by the Ministry of Education, Culture, Sports, Science and Technology.

這項研究為以下兩計畫的一部分，分別是「地質流體：流體在隱沒帶的性質和力學」(由Scientific Research on New Academic Related Areas 補助)以及「透過分子動力計算模型以瞭解超臨界流體在地殼中的性質，及對地震活動影響的研究」(由Challenging Exploratory Research補助)，並在文部科學省的支持下進行。

This research was published in the online version of Journal of Geophysical Research: Solid Earth, on January 20, 2016.

此篇研究於2016年1月20日刊登在線上版的《地球物理研究期刊：固體地球》(Journal of Geophysical Research: Solid Earth)。

引用自：National Institute for Materials Science. "Electrical conductivity of salt water in seismogenic zones theoretically determined." ScienceDaily. ScienceDaily, 13 June 2016.