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

The quake, measured at magnitude 7.5 by the U.S. Geological Survey, struck shortly after midnight in a part of the sea dotted with oil rig platforms, Financial Times journalist John Aglionby told CNN International.

He described feeling the quake 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. Video Watch Aglionby describe what he felt »

According to CNN's Kathy Quiano, the shaking caused many in the capital to flee their homes. "We felt the earthquake in Jakarta, it was pretty strong and went on for at least a minute," Quiano said.

The Pacific Tsunami Warning Center said there was no threat of a tsunami, because the quake was too deep.

According to the U.S. Geological Survey, the quake was centered 100 kilometers (62 miles) east of Jakarta, at a depth of 289 kilometers (180 miles). It occurred about 32 kilometers (20 miles) from shore.

"Earthquakes of 7.5 and over happen approximately 18 times a year," said John Bellini of the USGS, "and in the area around Indonesia that varies. You might see one or two a year, or you might not see any."

About 15 minutes after the quake struck, Quiano drove through Jakarta, and said she saw no signs of damage. She said residents -- especially those living in high-rises, ran from their homes when they felt the quake, but returned fairly quickly.

"The earthquake went on for at least a minute here. It was pretty strong and scary," Quiano said. "People are shaken, that's for certain."

And Aglionby said there were still concerns for the many oil rigs exploiting the resource rich waters off the north coast of Java.