The Department of Mineral Resources is studying whether to include a hidden fault responsible for tremors of 4.5 on the Richter scale in Phitsanulok and Phichit provinces that caused minor damage to houses, temples, schools and a hospital on a list of the country's active fault lines.
Suvapak Imsamut, the department's deputy chief and spokesman, on Monday told a press conference that the department is highly aware of the earthquake that rocked Phitsanulok and Phichit provinces in the North around midnight last Thursday.
"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."
However, Mr Suvapak said that Phitsanulok province is now a green zone under the Department of Public Works and Town & Country Planning, which means that it is in a risky area that needs soft quake-led damage preventive measures.
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 incident caught geologists off guard as the 100-metre-deep fault was largely unheard of until now, he said.
Suwit Kohsuwan, a department specialist, said his team had evaluated information from all 20 earthquake measurement stations nearby and found that the hidden fault runs north-south in the Phetchabun Mountains zone.
As a result, the department has set up 26 mobile earthquake measurement stations to ascertain the length of the fault line, as this will give an indication of the general magnitude of future events.
"The length is associated with the scale of vibration, so a long fault will cause greater vibrations," he said.
"Finding the fault will not be an easy job, but it is vital that we do due to its proximity to communities.
"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."
Mr Suwit said the case is particularly "interesting" as it marks the first time a hidden fault has produced a reading of 4.5. Similar undiscovered faults usually only score 1-2, he said, adding that there had been damage to 22 houses, temples, schools and a hospital.
