Steve Connor The Independent 10 Dec 11;
Deforestation and extreme weather may later cause earthquakes, scientists believe.
Their findings suggest that cutting down trees on steep slopes may increase the risk of not only landslides but earthquakes in heavily deforested places such as Haiti, which suffered a devastating magnitude 7 quake in 2010.
Geologists have previously discounted the idea that low atmospheric pressure associated with tropical cyclones can influence the timing of earthquakes. But the new study suggests a different mechanism based on changes to the weight of soil and other ground material bearing down on a geological fault under seismic stress.
"Very wet rain events are the trigger. The heavy rain induces thousands of landslides and severe erosion, which removes ground material from the Earth's surface, releasing the stress and encouraging movement along faults," said Shimon Wdowinski of the University of Miami in Florida.
"The 2010 earthquake in Haiti occurred... 18 months after the same area was hit by two tropical storms and two hurricanes," he said. "It can happen in other mountainous areas affected by cyclones, such as Japan, the Philippines, and maybe Central America," he told the American Geophysical Union in San Francisco.
The idea the weather can play a role in triggering earthquakes is controversial. But Dr Wdowinski said an analysis of the timing of earthquakes and cyclones in Taiwan over the past 60 years has demonstrated a statistical correlation, with a significant number of quakes bigger than 6 occurring within four years of major cyclones – known as typhoons in the Far East.
Taiwan was hit in 1969 by Typhoon Flossie, then a 6.2 quake hit Taiwan in 1972. In 2009, Typhoon Morakot was followed by a 6.2 quake in the same year, and a 6.4 quake in 2010. Typhoon Herb, in 1996, was followed by a 6.2 quake in 1998 and a 7.6 quake in 1999.
Dr Wdowinski said rapid soil erosion on steep slopes caused by tropical cyclones changed the stress on the geological fault over a period of months or years, which can trigger an earthquake.
"Statistical analysis showed that the timing of the earthquake is above the expected. It is way above background. If it was a random process and there was no relation between earthquakes and cyclones... there was less than 1 per cent probability of this occurring," he said.
"It is not that it happens during a cyclone but that there is a delay, and the delay of between three months and three years is due to the ground erosion. The delays can be due to the time it takes for the erosion to wash the material to the ocean," he said.
An independent analysis of ground movements caused by summer monsoons in the Himalayas also suggested a link between extreme weather and earthquakes.
A study by Thomas Ader of the California Institute of Technology in Pasadena found that earthquakes were more likely to occur in dry winter months after the monsoon period.
