An undersea earthquake jolted the southern provinces of Phang Nga, Phuket and Krabi yesterday, causing widespread alarm but no harm.
As soon as the 4.6-magnitude quake erupted at 4.18am in Phang Nga's Koh Yao district, messages about it flooded social-media platforms.
"It shook so much that I woke up," said a Twitter user who lives in Krabi.
A Thai tourist in Phuket, meanwhile, posted an update on social media saying, "At about 4.20am, I suddenly woke up because floor, tables and chairs shook seriously."
The Meteorological Department said the quake was in the Klong Marui Fault Zone, which is a sea zone and still active. This zone is near Phuket and Krabi too.
The department's deputy director-general Burin Wechbunthung said this fault zone was not a big threat because it did not cause any vertical slip.
"Big quakes usually take place in subduction zones like the one around Sumatra Island," Burin said.
"We are monitoring that situation around the clock because any big quake there could also affect Thailand."
Associate Professor Panya Charu-siri, a geology lecturer at Chulalong-korn University, said people need not panic about yesterday's quake because it was not a powerful one.
"Definitely, it won't cause any tsunami," he said.
Still, many local fishermen were worried about the shaking and the impact it might cause.
There was another mild aftershock, measuring 3.2 magnitude, around 12.25pm.
"After the quake, many small fishing trawlers were afraid to go out to sea," Trang Fishermen Club president Aren Phrakong said.
His club had advised its members and local people to keep abreast of news updates from the Meteorological Department.
This year, Koh Yao has seen several earthquakes. A 4.0-magnitude event erupted in the area on February 20. Then on March 25, a 3.8-magnitude quake hit the area.
Dr Paiboon Nuannin, a seismology academic, said the world experienced between 15 and 20 magnitude-4 quakes each day.
"When it happens in the sea - like what happened in Koh Yao - it's unlikely to cause any structural damage on nearby land," he said.
He advised people to stay outdoors or get out of the moving vehicle when they started feeling vibration.