Horizontal Earthquakes

Report on rare earthquakes due to sudden movement between two parallel, but horizontal, geological features (as opposed to two plates trying to slide past one another, or at a subduction zone). From MSNBC:

That's what happened to the city of Kohat, Pakistan, in 1992. A magnitude-6.0 earthquake moved a 30-square-mile (80-square-kilometer) swath of land one foot (30 centimeters) horizontally in a split second, leveling buildings and killing more than 200 people.

The area hadn't experienced many temblors before, making the earthquake an unusual occurrence. Now, 20 years later, geologists have used satellite and seismic data to track down the cause of that rare quake — an equally rare type of fault.

"The pattern we saw was absolutely a dead ringer for a horizontal fault," said Roger Bilham, a geophysicist at the University of Colorado at Boulder. "But here's the problem: How do you get a horizontal earthquake?"

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"The fault is like the contact layer between a carpet and the floor beneath it — perfectly horizontal," Bilham told OurAmazingPlanet.

To understand what happened at Kohat, you need to picture a waterbed, Bilham explained. If the waterbed is sitting on concrete, it's nearly impossible to push. But if you put the waterbed on a slippery surface — say, an ice rink — it becomes a little easier to move. It may crumple up at one end, and some patches may get a little stuck, but if you push slowly and surely, you can move the waterbed.

Now imagine that waterbed is the Kohat Plateau, a 3,800-square-mile (10,000-square-km) slab of earth that lies just southwest of the Himalayas. As the Eurasian plate pushes the plateau southward, it slides along (or creeps) on its own sort of ice rink, a lubricating layer of salt separating the plateau from the underlying layer of rock.

Every once in a while, though, a patch of the plateau sticks against the bedrock below. And while the rest of the plateau slides southward, "the surrounding creep loads up around the one stuck patch, then boom! Earthquake," Bilham said.