Hours After the First Quake, the Region Was Hit by an Unusually Large Aftershock.

Like all large earthquakes, the powerful 7.8 magnitude temblor that struck Turkey and Syria on Monday was followed by dozens of aftershocks, secondary quakes that occur when the movement of the first causes changes in stresses underground.

One of those aftershocks, which struck about nine hours after the initial quake, was nearly as strong as the first and measured at magnitude 7.5. Powerful aftershocks like that can add to the destruction, shaking buildings and infrastructure that have already been badly weakened by the initial quake.

The 7.5 aftershock was well above average, and was about one-third as powerful as the 7.8 quake. From statistical analyses of quakes worldwide, the most powerful aftershock Monday would have been expected to be about magnitude 6.8, or only about one-thirtieth as powerful as the first quake.

“There’s nothing magic about aftershock magnitude,” said Susan Hough, a seismologist with the United States Geological Survey. Sometimes an aftershock is even larger than the initial quake, Hough said, in which case the aftershock is considered the main quake and the first one is referred to as a foreshock.

Aftershocks can occur on the same fault as the main earthquake, or on nearby faults that are affected by the stress changes. This aftershock was centered about 60 miles north of the initial quake.

Aftershocks also can continue to occur for weeks or months after a strong quake, with their frequency and strength gradually declining.

The earthquakes on Monday occurred on the East Anatolian Fault Zone, part of a complex system of faults in Turkey that make the region one of the most seismically active in the world. Another fault zone, the North Anatolian, has caused many large earthquakes, including a 1999 quake centered about 60 miles from Istanbul that killed more than 15,000 people.

All of these are strike-slip faults, meaning the blocks of crust move horizontally relative to one another when strains between them reach a breaking point. Hough said that given the magnitude of the first quake on Monday, it’s likely that the movement occurred along 120 miles or so of the fault.

After the initial break, near Gaziantep in south central Turkey, the rupture would have propagated along the fault at about 2 miles a second, Hough said. This would help account for the lengthy shaking, which some witnesses said lasted for 90 seconds or longer.

Large strike-slip fault zones on land are not common, Hough said; the San Andreas Fault in California is another example. But those on land can be extremely destructive, because they tend to be close to population centers, and they can be fairly shallow, increasing the shaking felt at the surface.

Some quakes are centered more than 100 miles below the surface, but the initial shock on Monday occurred at a depth of about 11 miles.