MEXICO CITY (Reuters) - A strong earthquake of magnitude 6.4 shook southwestern Mexico on Tuesday, the latest in a series of temblors in recent days, but there were no initial reports of damage.

The earthquake hit at 6:50 a.m. local time (1250 GMT) and was centered about 23 miles northwest of Arriaga, in the state of Chiapas, at a depth of 72 miles, the U.S. Geological Survey said.

"We have no material or human damage," said civil protection official Zaydi Garcia in Tuxtla Gutierrez, capital of Chiapas state and 65 miles from the quake's center.

"We felt it fairly strongly here but we have no reports so far of damage or fallen buildings," said civil protection official Jose Basan in Oaxaca, a colonial city with cobblestone streets popular with tourists roughly 160 miles (260) from the quake's center.

The earthquake, which the USGS revised down from a preliminary reading of 6.6, was barely felt in the capital Mexico City.

Since Saturday, three moderate quakes have rattled the northwestern Mexican city of Mexicali.

Mexico is prone to earthquakes, especially along its Pacific coast. Those above magnitude 6.0 are the most dangerous.

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