PHUKET: People on Phuket should be prepared for the next tsunami, according to scientists who have settled the debate about whether another big wave will follow the one that smashed the region in 2004.

Nine years on, researchers have discovered a cave on the Indonesian island of Sumatra that provides a ''stunning'' record of Indian Ocean tsunamis over thousands of years.

It's only a matter of ''when'' not ''if '' another tsunami strikes, they say.

Authorities on Phuket are notoriously lax at allowing awareness of the potential for a tsunami to dim with time. Most evacuation signs on Phuket are being allowed to fade.

Now, the scientific results indicate, resort managements and local Phuket officials would be wise to always advise guests to be prepared for the next big wave.

They say layers of tsunami-borne sediments found in the cave in northwest Sumatra suggest the biggest destructive waves do not occur at set intervals - meaning communities in the area should be ready at all times for a tsunami.

''It's something that communities need to know,'' research team leader Charles Rubin said, adding that the team wanted to ''promote safety of coastal communities''.

More than 240,000 people were killed around the Indian Ocean in the December 26, 2004 tsunami, including 5400 tourists and residents in Thailand.

Professor Rubin and other researchers from a Singapore institute were working with scientists from an Indonesian university when they discovered the cave, south of Banda Aceh, the capital of Aceh province.

Inside the cave the researchers found layers of sandy sediment, which had been washed in by tsunamis thousands of years previously, Mr Rubin said.

The layers, which contained small fossils from the seabed, were well-preserved and separated by droppings deposited by bats in the cave, he added.

''This is a beautiful, stunning record of tsunamis that you just don't have very often,'' Mr Rubin said.

Only huge tsunamis and storm surges can get into the cave, which has a raised entrance. Afterwards the sediment is protected inside from erosion by wind or water.

Mr Rubin said the scientists dated the layers and believe they show that between 2800 and 3300 years ago, some four to five tsunamis battered the area.

Before the 2004 tsunami, it had been hundreds of years since such a huge destructive wave had hit Aceh, the scientist said.

But he said the new discovery suggests that tsunamis are not evenly spaced through time, which provoke those involved in policy and planning in the region to greater preparedness.

Two tsunami alarms in 2012 that were not part of a planned drill illustrated that little preparation had gone into thinking about evacuations and other requirements on Phuket.

Memorial ceremonies are held on Phuket and in Krabi and Phang Nga each December 26.

The Phuket tsunami memorial wall at the north of the island and a cemetery in Phang Nga where 380 unidentified tsunami victims are buried are usually cleared of weeds once a year, just in time for the anniversary.

A two-day Phuket region coastal safety summit is scheduled for January at the Royal Phuket Marina.