Is the world entering a seismically active phase?
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A series of major earthquakes, measuring 7.3 in Haiti, 8.8 in Chile and 6.7 in Taiwan, have triggered widespread debate among experts as to whether the earth is entering a seismically active period.
Major quakes occur in 100-year cycles
Sun Shihong, a research fellow at the China Earthquake Networks Centre believes earthquakes occur in cycles of 100 years.
In the first 60 years of the 20th century there were seven earthquakes above 8.5 on the Richter scale. In the following 40 years there were no major quakes. Sun believes a new cycle began at the end of 2004 with a massive 9.1 magnitude quake in Indonesia.
Sun says major earthquakes occur in the early years of each century and are followed by several decades of relatively stability. But he admits his theory needs further research.
Another expert who refused to give his name said current knowledge did not support the idea of 100-year cycles.
Earthquakes becoming more frequent
Geophysicist Lei Jun from Peking University told the Guangzhou Daily a week ago, "Several major earthquakes over 8.5 in magnitude have occurred in the last few years. This shows the earth is very active. But theories and observations do not prove the earth has entered a seismically active period. And it is impossible to give a clear answer to the public on whether we will see more major earthquakes in the near future."
"Observations suggest seismically active periods usually alternate with the seismically quiet periods. It takes a long time to build up the energy required to trigger an 8.5 magnitude earthquake."
Lei had a relatively reassuring message for the public: "It is very likely that earthquake activity is becoming quieter."
Stephen S. Gao, a geophysicist at Missouri University of Science & Technology, said, "Relative to the 20-year period from the mid 1970's to the mid 1990's, the Earth has been more active over the past 15 or so years.
"We still do not know the reason for this yet. It could simply be the natural temporal variation of the stress field in the earth's lithosphere," he said.
Advice: prepare for the worst
Can earthquakes be scientifically predicted? At a conference of experts held in London in 1996, a broad consensus was reached that earthquakes are inherently unpredictable. They are not predictable now and will not be predictable in the future.
The US Geological Survey states categorically that it does not make predictions. The most it offers are long-term forecasts about the probability of quakes and assessments of likely damage. For example, they recently said there is 99.7 percent probability of a major earthquake in California in the next 30 years, but did not specify exactly when and where.
Lei's advice is that we should concentrate on constructing earthquake-proof buildings and improving rescue capabilities, rather than worrying about when and where the next major quake will strike.
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