Director of the department's Chiang Mai office Suparerk Tansrirattanawong said 69 aftershocks had been recorded.

"It has sent a signal that more earthquakes may hit Mae Rim and San Sai districts," Suparerk said.

Yesterday's earthquake measured 5.1 on the Richter scale and occurred just after midnight. Its epicentre was in Mae Rim district. The earthquake was about 33 kilometres deep.

"It's the first time an earthquake has occurred so close to Chiang Mai city," Suparerk said.

An official at the Meteorological Department said the earthquake was caused by the Mae Tha Fault, which was about 30 kilometres away. Stretching about 55 kilometres in length, this fault has generated 23 quakes during the past 28 years. Prior to yesterday, the most recent quake was in March.

The impact from yesterday's quake was felt by many. Mae Jo University students evacuated dormitories and engineers spent yesterday checking buildings for structural damage.

Staff at the Lotus Hotel Pang Suan Kaew said the earthquake panicked guests for a short time. Other hotels reported calm.

Mineral Resources Department director-general Apichai Chawacharoenphan warned residents not to get overly alarmed. "Panic often leads to injuries during an evacuation," he said.

The earthquake was one of the strongest recorded in Chiang Mai. The largest was 5.2 on the Richter scale in December 1995.

Apichai said while yesterday's earthquake was not alarming, the number of aftershocks was "unusually high".

"Department officials will look into this and work with other authorities," he said.

Information will be passed to the Public Works and Royal Irrigation Departments.

Regulations in Chiang Mai, Chiang Rai, Phayao, Lamphun, Lampang, Mae Hong Son, Tak, Nan, Phrae and Kanchanaburi require buildings to be able to withstand earthquakes.

"The Public Works Department is seeking to enforce similar rules in Bangkok and its four adjacent provinces," Apichai said.

National Disaster Warning Centre chief Smith Dharmmasaroj said there were nine active fault lines in Thailand and earthquakes along some could affect Bangkok.