by Alessandra Cardone
ROME, Aug. 27 (Xinhua) -- The seismic event in central Italy was evolving as expected, with a sequence of hundreds tremors taking place since after the 6.0-magnitude quake hit early on Wednesday, an Italian expert said.
"Earthquakes tend to take place in clusters, which means several aftershocks follow a major event," Warner Marzocchi, chief scientist of the National Institute of Volcanology and Seismology (INGV), told Xinhua in an interview.
"These clusters become clearer after a large quake, and aftershocks are not necessarily milder than the first tremor, some might also be of the same intensity or stronger," he added.
Since the major temblor struck at 3:36 a.m. local time on Wednesday, and up to mid-Thursday, an earthquake swarm has produced some 640 tremors, according to the INGV.
Some 88 of these events were between magnitude 3.0 and 4.0, nine between magnitude 4.0 and 5.0, and one only exceeded magnitude 5.0, it added.
The most affected areas were the towns of Amatrice and Accumoli in Rieti province, which were the closest to the epicenter, and Arquata del Tronto in Ascoli Piceno province.
The death toll reached 250 on Thursday, according to the Italian civil protection.
Yet, figures were still provisional, since dozens of people remained unaccounted for, and search efforts were underway, officials warned.
"It may sound paradoxical, yet the risk of another quake is higher now than a week ago," Marzocchi said.
"This is true in any seismic event: when no seismic activity is registered in an area, the chance of a quake is low... But, as soon as a first major event tremor takes place, the risk of further tremors rises precisely because earthquakes tend to occur in clusters," he explained.
Italy, and especially its central regions crossed by the Apennines chain, has long been at high seismic risk. A violent quake, for example, had already destroyed the town of Amatrice in 1639.
"The problem lies in the fact that the earth's crust moves, and especially the African tectonic plate is shifting towards Europe," the head of Italy's quake institute explained. "The whole Mediterranean area is in relative motion."
"For the Apennines in particular, there is a sort of stretching activity: the mountainous chain is like splitting in two, because the Adriatic ridge tends to move toward the northeast while the Tyrrhenian ridge is more stationary," Marzocchi said.
