JAKARTA, Indonesia (CNN) -- A powerful earthquake has shaken buildings and caused panic on the densely-populated Indonesian island of Java, but there have been no reports of casualty and no tsunami alert.

The quake, measured at magnitude 7.5 by the U.S. Geological Survey, struck shortly after midnight. According to CNN's Kathy Quiano, it was felt in the capital Jakarta where many fled their homes.
""We felt the earthquake in Jakarta, it was pretty strong and went on for at least a minute,"" Quiano said.
Financial Times journalist John Aglionby described feeling the incident in his house in a suburb of the Indonesian capital.
""I was just getting into bed on the second floor of my house. You rarely feel these things in Jakarta so I knew something was wrong immediately,"" he told CNN.
According to the U.S. Geological Survey, the quake was located 70 miles (110 km) east-northeast of Jakarta, at a depth of 175 miles (282 km). The Pacific Tsunmai Center in Hawaii said the seismic jolt was too deep to generate deadly waves.
Aglionby said there were still concerns for the many oil rigs exploiting the resource rich waters off the north coast of Java.
Indonesia, the world's largest archipelago, is prone to seismic upheaval due to its location on the so-called Pacific ""Ring of Fire,"" an arc of volcanos and fault lines encircling the Pacific Basin.
In December 2004, a massive earthquake struck off Sumatra island and triggered a tsunami that killed more than 230,000 people in a dozen countries, including 160,000 people in Indonesia's westernmost province of Aceh.
In 2006 more than 5,000 people were killed by an earthquake near the Java island city of Yogyakarta.
U.S.G.S. geophysicist John Bellini told CNN that 7.5-magnitude quakes strike around the world roughly 18 times per year, affecting Indonesia approximately twice annually