6.0 on the Richter scale is not what one would refer to as "little". If that happened in the U.S, a lot of wooden built structures (oh, wait, almost all are built from wood!) would have gone "bye-bye". You didn't feel it as you live in a very sturdy concrete structure (as all blocks in Mladost and pretty much rest of Sofia) and a lower floor. I bet that those living on the 18th floor had different experiences.