A 7.4-magnitude earthquake struck beneath the Andaman Sea at 06:12 on Thursday, prompting authorities to urge residents and visitors in six coastal provinces to move inland or to higher ground. By midday, instruments had recorded a wave of 0.4 metres (16 inches) at one offshore gauge; officials said no destructive surge had reached shore.
Researchers at the Andaman Marine Institute say the quake was followed by an unusual shift in the seafloor, adding urgency to an already volatile situation. They caution that no one can tell whether it will precede a larger rupture, but say the coast has entered a period of heightened risk. The government’s warning office says the immediate threat is easing.
The anniversary calculation
The alarm coincides with the 20th anniversary of Thailand’s coastal hazard atlas, presented on 21 October 2006 as the country’s definitive planning guide. A fresh analysis released Thursday by researchers who re-ran its published calculations found that the model cut the active fault from 1,100 km (684 miles) to 650 km (404 miles), reducing its projected maximum wave at Khao Lak from 8.2 metres (27 feet) to 3.1 metres (10 feet).
The atlas assumed the rupture would stop at a boundary chosen for the planning exercise, rather than continue through the connected fault system, said Renuga Shobi, a geophysicist who led the reanalysis. A technical note from the launch year described the shorter rupture as a “manageable planning case”. Shobi said the public presentation did not explain that a longer rupture had been left out.
“The coast was told the number was a ceiling. It was only one scenario, and not the worst one.” — Renuga Shobi, geophysicist
The anniversary was meant to celebrate two decades of better warnings. Instead, the comparison has reopened painful memories of the 2004 Indian Ocean tsunami, which killed more than 5,000 people in Thailand. For researchers, the correction is a hard-won victory; for residents, it means the safety figures repeated for years did not cover the most severe scientifically credible case.
Warning against certainty
Shobi stressed that the new calculations do not predict a magnitude 9 earthquake, or establish that one is imminent. Earthquake science cannot reliably forecast the date of a rupture. But if the full fault broke, her team’s model shows a first tsunami wave could reach parts of the Thai coast in about 18 minutes, leaving little time for a second warning.
The Thai Coastal Warning Office said its atlas was built to support practical evacuation planning and that Thursday’s earthquake did not, by itself, prove a larger one was coming. “We are acting on the measurements we have now, not on the largest number in a research model,” said deputy director Eduarda Perez. The office said sirens and phone alerts were activated in coastal districts.
Local officials have directed people from low-lying areas towards marked assembly points, including schools and municipal buildings on higher ground. About 320,000 people live in the areas covered by the precautionary notice, according to provincial estimates. In Khao Lak, hotel staff were telling guests to keep shoes, water and travel documents ready while buses waited inland.
The warning has also revived a basic question about the atlas: why was the longer rupture not shown alongside the planning scenario? Perez said the office would publish its technical materials and convene outside specialists next week. Shobi said the coast needed updated maps before the next tourist season, not another anniversary presentation.
Seismologists will continue monitoring the aftershocks and seafloor gauges through Friday. For now, officials have asked residents to follow alerts rather than return to beaches; researchers say the data will decide whether the warning can be lifted. The corrected model, they argue, is not a prediction but a clearer account of what the coast must be ready to face.
