Residents in Chiang Mai province felt a long period of shaking but saw no major damage. High-rise buildings swayed slowly for at least half a minute in Bangkok, startling residents.
The US Geological Survey said the 6.1 magnitude was about 10 kilometers below the surface.
It was centered in northwestern Laos, about 31 kilometers from Chaloem Phrakiat district in Nan province.
Moderate quakes of 4.6 and 5.7 magnitude shook the same area overnight.
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