Unusual large number of tremors stirs fear in northern Chile

Living in one of the world's most earthquake-prone zones, CHILE (CHILE.CHL)(CHILE.CHL)(CHILE.84)ns are accustomed to the regular tremors that ripple through their country, which has strict quake-proof regulations for construction.

But the far northern region of Tarapaca has undergone an unusually high number of tremors in a small area around the port towns of Iquique and Pisagua since a relatively strong magnitude 6.7 tremor hit on March 16.

"This is already unusual," said Mario Pardo, a director at the University of Chile's seismic centre, on Tuesday. "There are too many aftershocks to be only associated with the 6.7 quake."

"This is an anomaly that has been extensive in terms of time and persistent in terms of magnitude."

Over 40 of the more than 300 tremors since the quake were strong enough to be felt by residents, according to experts. None have been large enough to cause damage, although a 6.1 tremor over the weekend caused an electricity outage.

Iquique is one of the chief ports responsible for exporting copper from Chile, serving an arid region that is home to many of the country's top copper mines - including Glencore Xstrata and Anglo American's massive Collahuasi - as well as significant reserves of lithium, used in batteries.

Local media reported that sales of emergency kits, batteries and tents had risen in the area, while beach hotels and restaurants reported a drop in visitors as fears of a tsunami kept people away.

"At this time we have reinforced seismic monitoring in all the northern zone," said Jean-Marie Walker, an official at emergency office Onemi.

SEISMIC GAP

Iquique is located in a so-called 'seismic gap'. Stress builds up where tectonic plates (in this case, the Nazca and South American plates) rub up against each other, and if there is no quake to release the tension, the stress increases.

Sooner or later that tension will be released, seismologists say, and with no major quake in the area since 1877, when it does come, it will be big - possibly a magnitude of at least 8.5.

The 2010 earthquake and resulting tsunami that devastated central Chile was a magnitude 8.8, while in 1960 southern Chile was hit by a 9.5 quake, the biggest in modern history.

"The only thing we can be sure of ... is that we are expecting a large earthquake in the area," said Pardo.

But this month's tremors do not necessarily mean that 'the big one' is due imminently. Understanding the link between the recent activity and when a mega-quake will occur is complex, said Pardo.

"We are working on that, how to understand better this seismic activity in the context of the model that we are expecting a significant earthquake in the region."

(Reporting by Rosalba O'Brien, Anthony Esposito and Alexandra Ulmer; Editing by Bernard Orr)