A succession of two earth tremors rocked Sofia, the first a mild quake of four on the Richter Scale just after 10pm on November 15 2008, the second a 3.5 tremor at 7am on November 16.

Minutes after the first tremor, which took place at 10.08pm and was measured by seismologists as having lasted 1.8 to two seconds, residents of some buildings went out into the streets, alarmed by the buildings having shaken. The city's taller buildings were described as having rocked violently.

Some residents described having heard a loud noise preceding the first incident. There was no official confirmation of this.

The Bulgarian Academy of Sciences said that the November 15 incident measured four on the Richter Scale. In official Richter definitions, this is categorised as a light earthquake.

Some areas reported problems with mobile phone and landline communications immediately after the first quake. Communications were resorted soon afterwards.

Soon after the first tremor, the switchboards at emergency number 112 and other official institutions were jammed. Most information that came out after the tremor was passed through the internet.

There was no information whether there had been any injuries or significant damage after the two tremors. On Sunday morning, officials were conducting inspections of major hotels, shopping malls, residential and office buildings.

Bulgarian news agency BTA, quoting the Emergency Situations Ministry, said that Kremikovtzi steel mill and the wall of the Iskar Dam were confirmed to be undamaged.

Reports said that the epicentre of the first tremor had been in Sofia's southern regions, with some describing the epicentre having been Sofia's South Park, while other reports said that it had been at a point between Bistritsa and Zhelenitsa. According to the website of the European-Mediterranean Seismological Centre, the earth tremor had been registered at 42.57 N, 23.35 E, at a depth of five km, 14km south of Sofia. According to Google Earth, however, this map reference is a point about halfway between the towns of Svoge and Botevgrad.

After the first tremor, the Bulgarian Academy of Sciences said that there would not be aftershocks or a second tremor. About nine hours later, many residents were rattled awake by the 3.5 tremor.