The Department of Mineral Resources is studying whether to include a fault responsible for recent tremors in Phitsanulok and Phichit on a list of the country's active fault lines.

"We have previously said that the hidden fault is still inactive, but we may have to put it into the active fault list, though we need to wait for official confirmation based on our study first," a department spokesman said.

The epicentre of the earthquake, measuring 4.5 on the Richter scale, was in Phitsanulok's Bang Krathum district, which the department said has not felt a vibration in 100 years.

Only one aftershock, recorded at 1.6 on the scale, was detected just 20 minutes after the main shock. 

The hidden fault runs north-south in the Phetchabun Mountains zone.

"Although there are recorded tremors this century, it [the fault] was written about during the reign of King Naresuan in Ayutthaya over 440 years ago," the spokesman said.