A 4.0 magnitude earthquake was recorded in Turkey's northwestern Marmara Sea on Monday at around 5.45 p.m.Shocks were also felt in Istanbul, since it was the closest palce to the center of the quake, which occurred 6.99 km beneath the Marmara Sea, the Disaster and Emergency Management Presidency of Turkey (AFAD) announced.No casualties or property damage has been reported so far.Turkey sits on three active fault zones running through the north, east and the Aegean Sea; the fault lines being 2,900 kilometers in length, in addition to many smaller fault lines traversing the country. Over 33 million people live in regions under the risk of an earthquake of a high magnitude while some 23.3 million people inhabit regions exposed to the risk of earthquakes of lesser magnitude. Overall, they make up two-thirds of the total population, which currently stands at 77.6 million.AFAD figures show 10 million buildings are situated in high-risk areas and another 5 million are in secondary risk areas. The most active fault line, in the country's north, caused a major earthquake in 1999, killing tens of thousands of people.