WASHINGTON, June 19 (Reuters) - A slow-moving earthquake that could barely be felt despite its 7.8 magnitude caused a stealthy tsunami that killed more than 600 people in Indonesia last July, U.S.-led researchers reported on Tuesday.

A survey of the area affected by the tsunami shows that early-warning systems and even common sense would not likely have helped prevent the tragedy, the Georgia Institute of Technology team reported.

"The general assumption was that if you were near the coast where the earthquake took place, you would feel it and be able to run to higher ground," said Hermann Fritz, who led the study published in the journal Geophysical Research Letters.

"This event caught people by surprise and showed that it's not always that simple."

No sirens alerted south Java coastal communities after a quake struck around 110 miles (180 km) offshore in July 2006. The resulting tsunami swept away unwary beachgoers.

The quake itself was little noticed, and researchers say quakes are sometimes not felt, depending on their depth and what kind of terrain the quake waves pass through.

In the 2006 quake, government ministers blamed a slow and bureaucratic process for initiating a tsunami alert.

Earthquakes are frequent in Indonesia, which lies in the so-called Pacific ring of fire.

An Indian Ocean tsunami triggered by a giant undersea earthquake off Indonesia's Aceh province in December 2004 killed tens of thousands of people across the region.

Much work went into installing tsunami warning systems afterwards. But Fritz said they cannot always be relied upon to save lives, especially in places near where earthquakes hit.

The survey team interviewed survivors and studied evidence left behind by the tsunami and discovered signs a 65-foot-high (21-meter-high) wave hit one area.

"This event indicates that there was likely a combination of both a tectonic tsunami and a submarine landslide or a canyon failure triggered by the earthquake," said Fritz.

"The runup was unusually high along one portion of the coast, too much for a 7.8 magnitude earthquake. The only explanation we could think of is that a submarine mass movement triggered by the earthquake could have added to the effect of the earthquake," he added.

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