A series of earth tremors centred in northern Tanzania has caused alarm in Kenya and Tanzania. The most powerful quake, on the afternoon of 17 July, was estimated at 5.9 by the US Geological Survey (USGS) on the Richter scale.

The USGS reported that the 'swarm' of earthquakes was close to the Ol Doinyo Lengai mountain, an active volcano on the floor of the Rift Valley in northeastern Tanzania, close to the Kenyan border. However, the agency stated that information so far available was "not sufficient to determine if the current Tanzania swarm activity reflects a geologic process that might lead to a change in the eruptive behavior of Ol Doinyo Lengai". The last major eruption was in 1966.

Geology professor Eliud Mathu said at least 10 major tremors were felt in Tanzania and Kenya between July 12 and 18.

Mathu also said investigations into the tremors, which he described as "abnormal and strange", would continue.

Fred Belton, a mathematician at Middle Tennessee State University (USA), has climbed Ol Doinyo Lengai 11 times and spent more than 100 nights on the mountain, studying the relations between barometric pressure and lunar cycles and the volcano.

He said: "It is extremely interesting that the quakes are centred near Ol Doinyo Lengai but this does not mean it will erupt. The quakes may just be tectonic, indicating movement in the Rift Valley, and do not necessarily mean lava is moving."

No major damage has been reported, but several of the tremors caused panic in the Kenyan capital, Nairobi, when buildings shook violently. Rumours of a looming major earthquake caused the evacuation of workers from several high-rise buildings in Nairobi city centre on 18 July, as uncertainty spread.

"We should all be diligent and watch out for signs of any earthquake. We should not panic. Life should continue as normal," said Kenyan government spokesman Alfred Mutua.

The tremors also affected the northern Tanzanian town of Arusha. The building housing the International Criminal Tribunal for Rwanda was evacuated on 17 July, said Danford Mpumilwa, the tribunal's spokesman.

Kenya's main international seismic station at Kilimambogo, about 40km northeast of Nairobi, recorded the tremors and data was being analysed by both Kenyan experts and the USGS.