The 6.8 magnitude earthquake that hit Myanmar has killed at least 75 people and injured 40, with the death toll expected to rise.
At least five monasteries and 35 buildings were also destroyed.
Authorities in Thailand have also confirmed one woman was killed on the Thai side of the border.
Witnesses have said tremors were felt in Bangkok, central Myanmar and as far away as the Vietnam capital of Hanoi where people were evacuated from tall buildings.
An estimated 5.5 magnitude quake also hit northern Thailand this afternoon, but authorities reported little damage.
The quake was centered 111 km north of the town of Chiang Rai,
Thailand's northernmost province.
Witnesses in northeast Myanmar and northern Thailand said they felt the earthquake strongly and tremors caused widespread panic. Ten aftershocks further jolted the area.
"In my 40 years, I never felt an earthquake this strong. A glass broke and I had to hold on to a pillar," Thanawan Sisukniyom, a retired teacher in Mai Sai, told Reuters by telephone.
Witnesses in Chiang Mai, the country's second-largest city,
reported no immediate damage, but said the earthquake was felt
A witness in the Myanmar town of Tachilek, which borders Chiang Rai, said parked motorcycles fell to the ground and cracks were seen in the road.
Thai broadcaster TPBS said electricity was cut off in parts of Mae Sai. The earthquake was shallow at a depth of 10 km.
"Many people fled their homes and lay down on the ground outside, away from the buildings," said a resident of Kentung, 80 km west of the epicentre.
"We are still sitting on the ground since there are several aftershocks. In some buildings, TV sets fell off the tables and shrine altars fell down."
People also left their homes in Myanmar's capital Naypyitaw and its biggest city, Yangon, some 550 km away.
Earthquakes of magnitudes of around 5 and as high as 7 have hit northern Myanmar and Thailand several times in the past 15 years, but damage and casualties have been limited and the areas are thinly populated.
Chiang Rai province is a sparsely populated, hilly area that forms part of the famous "Golden Triangle", known for growth of illicit opium and where Myanmar, Laos and Thailand meet.
Somchai Baimuang, deputy director of the Thai meteorological department, urged the public not to panic.
Residents left their homes in the Myanmar capital Naypyitaw and the biggest city, Yangon. No deaths or injuries were reported.