葡京官网

Well water warned of Kobe quake

作者:罗靛唣    发布时间:2019-02-28 06:16:02    

By Peter Hadfield GROUNDWATER beneath the city of Kobe may have given out several warning signals prior to the devastating earthquake of 17 January, according to Japanese researchers. Changes in the composition of groundwater have long been considered as possible warnings of large earthquakes, but few tremors have occurred conveniently close to monitoring instruments. The Kobe quake itself was more than 300 kilometres from a network of instruments scrutinising Japan’s most active tectonic area, near Tokyo. But by chance, George Igarashi of Hiroshima University had begun monitoring the groundwater near Kobe only three months before the earthquake struck. Igarashi was not expecting an earthquake. He simply wanted to gather information on the concentrations of radon in groundwater and chose Kobe because its groundwater – which is extracted from wells for use in the brewing of sake – had been extensively studied in the past. Radon is formed from the radioactive decay of uranium in the Earth’s crust. Minute quantities seep into deep groundwater from uranium-bearing rocks. If stress in the rocks opens up fissures and cracks, then more radon can leak into the groundwater. Because rocks can become stressed immediately before an earthquake, a sudden increase in the concentration of radon-222 in groundwater may herald a quake. Another Japanese study conducted 20 years ago at the Izu peninsular near Tokyo, linked groundwater radon levels with impending earthquakes. In late 1993 Igarashi and his colleagues, made a few initial measurements of the concentration of radon in water from a well 30 kilometres northeast of the epicentre of the Kobe quake. On 27 October 1994, when the radon levels were already slightly higher than those recorded in 1993, the team began monitoring the well continuously. In last week’s issue of Science (vol 269, p 60) the researchers say that levels of radon began steadily rising over the following few weeks. Then, on 7 January 1995, radon levels surged to around 12 times their 1993 levels, before falling back to slightly below the 1993 figures on 10 January (see Graph). The researchers were unable to find an explanation for the results other than seismic activity. Igarashi says that many seismologists were sceptical of the earlier Izu peninsular study, but argues that the new data are much more convincing. “The anomaly was very clear,” he says. “After the earthquake, the radon concentrations returned to stable levels.” In a second paper in the same issue of Science (vol 269, p 61), Urumu Tsunogai and Hiroshi Wakita of the University of Tokyo claim they have identified another earthquake precursor. Prior to the quake, they say, concentrations of chloride and sulphate ions in Kobe’s groundwater increased. Unlike Igarashi’s team, Tsunogai and Wakita had not begun collecting data prior to the earthquake. Instead, they bought bottles of commercial mineral water that came from wells in the Kobe region and were date stamped. The researchers found that the concentration of chlorides in these samples remained around 13.9 parts per million from June 1993 to July 1994. From August 1994, however, the concentration started increasing, reaching a figure 10 per cent above its stable level a few days before the Kobe earthquake. A similar correlation was found for sulphates. The supply of bottled water was disrupted immediately after the quake, but from later samples Tsunogai and Wakita know that levels of chlorides and sulphates continued to rise, reaching a peak in mid-February, before dropping back to normal from the middle of March. Tsunogai and Wakita suggest that the chemical changes are due to sulphate and chloride-rich groundwater leaking into the Kobe area. The researchers say these high concentrations of chlorides and sulphates could not have come from the granite that lies immediately beneath Kobe. But groundwater rich in these ions lies in a zone of geological faults near the mountains behind Kobe. Tectonic stress prior to the Kobe earthquake could have created microscopic cracks in the rocks, argue the researchers, allowing water from the fault zone to contaminate Kobe’s wells. Many seismologists hope that the two groups’ results will eventually lead to effective ways of predicting major earthquakes. But Igarashi says this will require much more research. “I don’t think our studies can be applied to earthquake prediction over the next few years,” he says. But from a scientific viewpoint,

 

Copyright © 网站地图