In recent years, scientists have been studying changes in the electron density of the ionosphere as a possible earthquake precursor. In the May 2008 electron density graph at left, the red lines represent daily changes in the ionosphere. The image above shows the magnitude five or greater earthquakes from the same month. A comparison of the two reveals significant declines in the electron density above Taiwan one to five days before quakes.

Electron density

Though research into earthquake precursors is still a long way from producing reliable predictions, there have been some tantalizing preliminary results.

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.

Another hypothesis holds that rock fractures and pressures between rocks generate electric currents that give rise to powerful electric and magnetic fields and large electromagnetic waves. When these waves reach the ionosphere, they cause rapid heating and expansion of the ionospheric plasma, reducing the density of particles in the ionosphere.

The CWB and National Central University are now making daily observations of the ionosphere above Taiwan from latitudes 21 to 25 North in the hope that changes in the ionosphere will let them know when an earthquake is imminent.

Kuo Kai-wen, head of the CWB's Seismological Center, admits that this research suffers from numerous limitations. The principal one is that after you exclude geomagnetic storms, violent thunderstorms, and typhoons, we don't know why most of the remainder of the sudden declines in the electron density of the ionosphere occur.

"A scientist who fired projectiles into the bedrock at medium to low speeds discovered that doing so caused fluctuations in the electron density of the ionosphere above the spot," says Kuo. "This suggests that these electron densities are very sensitive to change."

The fact that major quakes can occur anywhere within very large ranges-including the entire length of faults or even whole tectonic plates-is another problem. For example, the 2008 Sichuan quake could have arisen anywhere within a 2900-km-long fault zone. Consequently, the areas above which the ionosphere might change are also very large. Even if we were to measure a sharp decline in the electron density over Taiwan, with present-day technology, we wouldn't be able to tell whether the coming quake was going to be centered in Taipei, Pingtung, or Hualien.

"If we were to observe electron densities dropping to significantly less than their 15-day median all we could say was there was a 50% likelihood of a magnitude 5 or greater quake somewhere in Taiwan over the next five days," says Kuo. "At this stage, the information is so imprecise and has such a margin of error that it wouldn't make any sense to issue a public warning. The only thing we could do would be to remind our Seismological Center colleagues to be extra vigilant."