Evidence needed to judge new earthquake cycle

SAN FRANCISCO - Scientists still need to seek comprehensive and robust scientific evidence to decide whether the Earth has entered a new cycle of large earthquakes, a geophysicist told Xinhua on Wednesday.

After the 9.2-magnitude earthquake off Sumatra in 2004, the global earthquake energy released indeed has been much greater than the preceding eight years as several great earthquakes approaching or above 9.0 magnitude happened, said Dr Jian Lin, senior scientist at the Woods Hole Oceanographic Institution in the United States.

However, the repeat "cycles" of large or great earthquake are often longer than one century, but scientists only have instruments to record earthquakes since about 1900 and the understanding of global earthquake activity prior to 1900 is still extremely limited.

"We should exercise caution when judging if the Earth is indeed entering a new seismic cycle," Lin said in an email to Xinhua.

There have been concerns that the Earth may enter a new cycle of seismic activity following some recent earthquakes in different regions.

In Italy, death toll of a 5.8-magnitude earthquake rose to 16 Tuesday night while one person is still missing and some 350 others were injured.

It came nine days after another 6.0-magnitude quake hit the same region.

According to Lin, the compression between the Eurasian plate and the African plate is the direct cause for the 6.0-magnitude earthquake near Camposanto, Italy on May 20, as well as the 5.8- magnitude earthquake near Medolla, Italy on Tuesday.

Known as a highly active earthquake area, the Mediterranean region, which, including part of Italy, is sandwiched between the two large tectonic plates: the Eurasian plate to the north and the African plate to the south, he explained.

"As the two plates are now compressing each other at a rate of about four to 10 millimeters per year, we should not be surprised to see more earthquakes in the region," said Lin.

Meanwhile, he noted that it is the same compression between the Eurasian plate and African plate that caused the recent earthquakes in Italy and earthquakes in Algeria in the past.

Though it is often not mentioned, it is important to point out that Northern African nations boarding the Mediterranean Sea are also vulnerable to deadly earthquakes, the geophysicist said.

In a research paper published in the Journal of Geophysical Research last year, Lin and other researchers pointed out a significant risk of damaging earthquakes to northern Algeria, including its capital city of Algiers where the population is over three millions.

The compression has accumulated to produce a possible magnitude 6.6-6.9 earthquake at most near the city of Algiers, said Lin, who is the leading author of the research paper.

In 2003, more than 2,000 people were killed and over 10,000 injured when a magnitude-6.8 earthquake hit northern Algeria. The country has a long seismic history for the past hundreds of years with several earthquakes more than magnitude 7.0.

According to the US Geological Survey, the deadliest documented earthquake in Europe happened in the Mediterranean region when a 7.2-magnitude quake hit Messina on the Sicily Island of Italy in 1908.

The earthquake and a local tsunami took some 100,000 to 200,000 lives.