From Thursday to yesterday morning, the centre detected 31 quakes on the seabed with magnitudes ranging from 4 to 5.3 on the Richter Scale. They occurred 400-600 kilometres west of Ranong's coast, the centre said.

The epicentre was along a fault line where lava had seeped out, it said.

The quakes could lead to the formation of undersea volcanoes and a powerful eruption could trigger a tsunami.

The centre said the quakes also indicated that the Bahrain Volcano, which is about 110km from the fault line, or 670km from Thailand, might erupt in the future.

Smith Thammasaroj, the PM's Office assistant minister who oversees the centre, urged the public to pay attention and await further news about the quakes.

He also asked crews aboard ships to immediately report any unusual changes in the sea - such as changes in its colour or unusual behaviour among marine animals - to the centre's hotline 1860 or related agencies.

Changes in the smell of phosphorous or bubbles rising to the surface could also indicate danger.

On December 26, 2004, the Andaman side of Phuket, Phang Nga, Krabi, Ranong, Trang and Satun were devastated by a tsunami triggered by an earthquake that measured 9.1 on the Richter Scale. It killed hundreds of thousands of people in Asia.

Phuket Governor Udomsak Assawarangkura said the island's alarm system was not set off because the intensity of the quakes was not considered dangerous.

However, authorities were closely monitoring the situation and were prepared for an emergency evacuation if necessary, he added.

Disaster Prevention and Mitigation officials said Phuket's 18 warning towers were connected and ready to send warnings that should give people at least 40 minutes to scramble to safe areas.

Tourists at Patong Beach appeared calm. Although shops remained open, some owners had packed items as a precaution.

In Phang Nga's Takua Pa district, 500 villagers from Ban Nam Khem fled in panic for Bang Muang shelter, official Maitree Jongkraijak said.

The villagers returned home in the late afternoon.

Some said they did not trust the province's 16 warning towers although authorities said the system was ready.

Many villagers complained that the news presentation on television made them panicky, while the warning towers did not sound an alarm.

 "Presenting the news the way some channels did could have caused heart attacks because most of us can still recall the horror of the [December 26] tsunami," one villager said.

Satun Governor Manit Wattanasaen said he had warned residents in Muang, Thung Wa, Tha Phae, and Langu districts to closely follow developments and be prepared for any emergency.

Ranong residents sent children and the elderly to safe ground as a precaution.

Southern Fishermen Federation president Samae Jaemudor said the group was told to watch for unusual occurrences at sea and prepare to evacuate people if necessary.