It sounded like the roar of a high-speed train—but it caused far more devastation. The earthquake that hit northwestern Turkey at Izmit, near Istanbul, on August 17, 1999, measured 7.4 on the Richter scale and killed 17,000 people. The tremors destroyed entire buildings, collapsed bridges, burst dams and caused landslides. Three months later, another earthquake with a magnitude of 7.1 struck about 60 miles to the east.
Both quakes occurred on what is called the North Anatolian Fault, which runs across northern Turkey. Since 1939, a series of large earthquakes has worked its way roughly from east to west along the 650 miles of thisfault line. Such an “earthquake sequence” occurs when the strain on a fault has accumulated gradually over a period of relative inactivity, sometimes lasting a few hundred years. This strain is then released in a series of earthquakes, each one triggering the next, rather than in a single large earthquake.