[Original headline: Sparks that signal the shock ]
The earth's surface is a jigsaw of tectonic plates, each a slab of rock hundreds of kilometres across. Pressure builds up in the fault lines between these plates, and when a section of rock slips suddenly the result is an earthquake.
It appears to be a purely physical process, so it is surprising to discover there is an electrical dimension. Not only do earthquakes cause a variety of electrical effects, but we now know how to trigger them using a pulse of artificial lightning.
A surge of low-frequency electromagnetic radiation, known as a precursor signal, is sometimes detected before an earthquake. These can show up in unexpected ways. When the radiographer Mike Grayson was carrying out a CAT scan in Newcastle, New South Wales, he noticed unusual streaks appearing on the image.
Seventeen minutes later, the city was struck by a major earthquake. The occupants of the hospital were unharmed, though parts of it, including the scanning room, were demolished. Grayson said afterwards: "If I ever see that image again I'd be the first out of the building."
In 1991, a group of American scientists were investigating how natural background noise interferes with satellite communication. A detector in Corralitos indicated ultra low frequency radio noise more than 20 times stronger than usual. It looked like an equipment fault and the signal disappeared. A few hours later an earthquake struck less than five miles away. Not all earthquakes seem to produce precursor signals, so they are unreliable as a means of earthquake prediction.
When they do occur they can cause spectacular effects, described by one Japanese researcher as Alice In Wonderland syndrome. Before the Kobe earthquake, there were reports of U-shaped candle flames, clocks whose second hands suddenly went around very quickly or stopped, swarming earthworms and schools of fish aligning themselves in the same direction.
Such stories were once dismissed as folk myths, but researchers at the quantum geophysics laboratory in Osaka found they could all be duplicated using electrical equipment. There is also a long history of stories of earthquake lights. These range from glowing fireballs popping out of the ground to a diffuse sparkling in the sky.
There is no accepted explanation, but they are thought to be related to St Elmo's fire, the glowing ionisation sometimes seen on ships' masts in stormy conditions. This is caused by a strong electric field in the air, which Japanese researchers believe may be associated with the precursor signal.
The Soviets set out to map the conductivity of the earth's crust in an area of seismic activity in Tajikistan. Their experiments, carried out in the 1970s, involved firing massive charges of electricity into the earth. There were tremors but nobody paid much attention. In 1993, Nikolai Tarasov of the institute of earth physics in Moscow analysed data from the trials and found something remarkable.
The electromagnetic pulses from the experiment were followed by tremors, each one releasing up to a million times as much energy as the pulse that preceded it. Tarasov went on to look at the results of similar experiments carried out at Tien Shan in the 1980s, and found that two-thirds were also followed by tremors. Tarasov is now part of a team being funded by the EU to investigate how electricity can cause earthquakes.
There is nothing in the standard geological model to explain it. One suggestion is that the electric pulse may be heating up groundwater and increasing pressure. We know about the impor tance of underground water pressure from the Rocky Mountain waste disposal project in the 1960s. A deep well was drilled at Rocky Mountain for the disposal of chemical waste. Millions of litres were injected, and the area, which had been quiet for almost a century, started to suffer hundreds of small earthquakes, with most activity in the months when disposal was taking place.
The pressure of the fluid injected into the well is thought to have made it easier for rock to slip along the fault lines. Electrically heating the water might have a similar effect. Other theories focus on the rocks themselves. Many minerals are piezoelectric, which means that when they are put under pressure, as in a fault zone, they can produce an electric potential.
It also means that when electricity is applied they vibrate slightly. An electric pulse could set a large mass of rock vibrating, enough perhaps to start slippage and trigger an earthquake. A lthough the underlying physical principles may not be understood, the implications are literally earth-shattering. We now have a relatively simple and easy means of inducing earthquakes.
This could be used to prevent major disasters. Tectonic strain builds up gradually, increasing to a point where a major earthquake will occur. Instead of waiting for a big quake it would be possible to release the strain in hundreds of smaller tremors instead. An earthquake has been building in California for decades, but there is no intention of forestalling with a series of mini-quakes. Anyone setting off a minor tremor risks being sued for the damage it causes. McDonald's was famously forced to pay $3mto a customer scalded by their hot coffee; even a small tremor could make a lot of people spill their drinks. More seriously, once started, a quake cannot be stopped.
There is no guarantee that instead of a small tremor, an earthquake machine would not set off the apocalyptic Big One along the San Andreas fault. There are also implications for nuclear testing. Geologists used to claim that nuclear tests could not be linked to earthquakes because the energy levels were comparatively low.
However, a nuclear explosion generates an intense electromagnetic pulse - possibly powerful enough to trigger an earthquake. This might encourage politicians to shelve plans to resume nuclear testing. We have the technology to trigger earthquakes, even if we do not fully understand the process. Whether this knowledge can be used constructively remains to be seen.
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