Introduction
Turkey's tragedy
Long time shaking
Plate tectonics
Finding faults
Proving the theory
Perilous predictions
Safe buildings
1999-2004 Activity
Tectonic tragedy
Courtesy of M. Nafi Toksšz, Massachusetts Institute of Technology, Earth Resources Lab.
The dots indicate quakes in Turkey since 1977. Most are shallow and weak; color indicates depth of the center of the earthquake.
USGS National Earthquake Information Center.
Verging closer to disaster
By any standard, Turkey is a dangerous place to be, seismically speaking. In 1939, 30,000 people died in a giant quake measured at 8.0 on the Richter scale. The 1939 quake, like the one in Aug. 1999, was along the North Anatolian Fault, a 900-mile crack that's near the junction of the huge African and Eurasian plates.
The 1999 quake, rated at 7.4 on the Richter scale, was, despite the death toll, considerably weaker than the disastrous 1939 earthquake. That's because the Richter scale is logarithmic; an 8.0 quake has ten times as much ground motion -- and releases 30 times as much energy -- as a 7.0 quake. Explain this to me again! And while we are at it, here is an explanation of the different kinds of seismic waves unleashed by an earthquake.
By definition, earthquakes are caused by a significant slipping along adjacent sections of the Earth's crust. Geophysicists believe underground stress is relieved when one section of a fault ruptures in an earthquake, but the rupture increases stress at either end of the section that moved. Three times in the past 1,100 years, the North Anatolian Fault has ruptured. During the sequence that began in 1939, the progression has been steadly westward. And despite the inability to pinpoint the next earthquake, there were predictions that a quake would strike where last month's occurred. The Boston Globe reported that geophysicists from the Massachusetts Institute of Technology had marked the zone "as an area to be watched closely." (See "MIT Effort... " in the bibliography).
M. Nafi Toksoz, a Turkish-born MIT professor of geophysics and seismology who has studied earthquakes in Turkey since 1971, told the Globe that data from global positioning system receivers on the fault showed that "this was the region where most of the stresses were accumulating."
If the next quake follows the historic "heading west" sequence, Toksoz says it could strike even closer to giant Istanbul, which is just 65 miles from the epicenter of the August quake. "Since the fault extends westward under the Sea of Marmara, south of Istanbul, it is of great concern that if that particular segment ruptures in another earthquake, it could directly affect Istanbul, a metropolis of 12 million people." By putting more force on the area near Istanbul, the August quake was "increasing the likelihood of a future earthquake in this vulnerable area."
Earthquakes, when they occur below and rupture the bottom of a sea or ocean basin, may also cause tsunamis -- giant, deadly waves.
Before we look at the science of predicting earthquakes, let's get back to basics:
What are tectonic plates, and how do they move?
