Physics and Chemistry of the Earth, Parts A/B/C, Volume 34, Issues 6-7, Pages 341-516 (2009) Special Issue Electromagnetic Phenomena Associated with Earthquakes and Volcanoes Edited by M. Hayakawa, J.Y. Liu, K. Hattori and L. Telesca

劉正彦は近日全世界GPS衛星地上ステーションの監視測定データの発見を分析して、四川の大地震の発生のその前の4日間、その前の5日間とその前の6日間、お願いする近くの地区の上空の電離層密度に震動してすべて非常に発生をかき乱すことがいて、下降幅と福衛の3番の観測の結果は近くて、皆半分で左右します。現象を類似して921集集の大地震の発生のその前の4日間、その前の3日間とその前の1日間でもかつて現れたことがあって、電離層が地震の前兆をかき乱してと確かに「ある種類が緊密です」の関連性があることを表示します。

劉正彦は過去の数年、全球で発生する地震の報告で、震動した後で全世界GPS衛星地上ステーションの監視測定データに対してに比べて発見を分析して、80％規模5以上の地震、曾在震はその前の5日間以内電離層の密度の明らかな下がる現象が現れます；もし範囲を規模6以上の強震まで縮小するならば、9割に達してその前の5日間震動する内に電離層が現れて非常にかき乱します。

電離層電子密度の観測は、近年注目されている地震前兆研究の１つです。左図は今年5月の台湾上空の電離層電子密度観測図で、赤線は毎日の電離層電子密度変化を示し、上図は5月に発生したマグニチュード5以上の地震の報告です。両図を比較すると、マグニチュード5以上の地震発生の1～5日前に、台湾上空の電離層電子密度は皆明らかに減少する現象があります。 (中央気象局提供)

Over the six days preceding the Sichuan quake, Taiwan's Formosa-3 satellite observed a noticeable decline in the electron density of the ionosphere above Sichuan. And America's National Aeronautics and Space Administration (NASA) reported observing unusual infrared radiation above the quake's epicenter before the quake occurred. NASA planned to follow up on its discovery by working with scientists in various countries to study the feasibility of constructing a comprehensive earthquake warning system based on atmospheric observations.

Liu Jann-yeng, a professor in the Graduate Institute of Astronomy at National Central University, made a similar discovery of his own-on three of the four days immediately preceding the Chichi earthquake, the electron density of the ionosphere above Taiwan dropped significantly below the median for the preceding 15 days. When Liu went on to look at the atmospheric data on 184 Taiwanese earthquakes of magnitude 5 or greater, he found that significant declines occurred one to five days prior to 90% of the quakes with hypocenters at depths of 35 km or less.

The ionosphere is the portion of the atmosphere 50-2,000 km above the Earth's surface. X-ray, ultraviolet, and far ultraviolet radiation from the Sun ionize molecules within it, creating a plasma of free electrons and positively charged ions.

Liu says that there are a number of hypotheses about why earthquakes cause a drop in electron density. One theory suggests that changes in stress resulting from the relative movement of the crustal plates prior to an earthquake generate electric current, like a battery. This creates a positive charge (electrical "holes") that flows to the surface, creating an electrical field that stretches up into the sky. The field attracts free electrons, reducing their density in the ionosphere.