
Researchers in Taiwan monitored 144 earthquakes between 1997 and 1999, and
they found that for those registering 6.0 and higher the electron content of the ionosphere changed significantly one to six days before the earthquakes. Earthquake forecasters can also watch for changes in the ionosphere by monitoring very-low-frequency (3- to 30-kilohertz) and high-frequency (3- to 30-megahertz) radio transmissions. The strength of a radio signal at a receiver station changes with the diurnal cycle: it is greater at night than in daylight, as anyone who listens to late-night radio from far-off stations knows. The altitude of the ionosphere, which moves lower as the positive holes migrate to the surface, also has an effect on radio signals; the lower the ionosphere, the stronger the signals. So at dawn on an earthquake day, a curve drawn to represent the drop-off in radio signal strength will appear markedly different from the normal curve for that signal at that location.
The connection between large earthquakes and electromagnetic phenomena in the ground and in the ionosphere is becoming increasingly solid. Researchers in many countries, including China, France, Greece, Italy, Japan, Taiwan, and the United States, are now contributing to the data by monitoring known earthquake zones.
Using these phenomena for earthquake prediction will take a combination of satellite and ground-based sensors. Satellites can cover most of the planet, but at ELF frequencies signal sources are hard to pinpoint. Ground-based monitors have smaller detection ranges, up to 50 km, depending on the sensitivity of the magnetometer and the size of the quake, but are far more precise. With a network of such sensors, forecasters looking at the amplitude of signals received at each sensor might be able to locate a quake within 10 to 20 km. This means that, for an area as large as California, accurate earthquake detection might require that forecasters distribute 200 to 300 magnetic-field and air-conductivity sensors on the ground.
For decades, researchers have detected strange phenomena in the form of odd radio noise and eerie lights in the sky in the weeks, hours, and days preceding earthquakes. But only recently have experts started systematically monitoring those phenomena and correlating them to earthquakes.
A light or glow in the sky sometimes heralds a big earthquake. On 17 January 1995, for example, there were 23 reported sightings in Kobe, Japan, of a white, blue, or orange light extending some 200 meters in the air and spreading 1 to 8 kilometers across the ground. Hours later a 6.9-magnitude earthquake killed more than 5500 people. Sky watchers and geologists have documented similar lights before earthquakes elsewhere in Japan since the 1960s and in Canada in 1988.

Earthquake Lights 1966 - Kobie, Japan
SourceAnother sign of an impending quake is a disturbance in the ultralow frequency (ULF) radio band—1 hertz and below—noticed in the weeks and more dramatically in the hours before an earthquake. Researchers at Stanford University, in California, documented such signals before the 1989 Loma Prieta quake, which devastated the San Francisco Bay Area, demolishing houses, fracturing freeways, and killing 63 people.
Both the lights and the radio waves appear to be electromagnetic disturbances that happen when crystalline rocks are deformed—or even broken—by the slow grinding of the earth that occurs just before the dramatic slip that is an earthquake. Although a rock in its normal state is, of course, an insulator, this cracking creates tremendous electric currents in the ground, which travel to the surface and into the air.
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