Imagine that, as you sit at your desk or in your living room reading this story, your entire city suddenly snaps a foot to the south.
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?"
Most earthquakes occur at near-vertical faults, such as the strike-slip San Andreas Fault or the thrust fault that caused the 2011 Japan earthquake. The Kohat Plateau earthquake occurred on a horizontal fault — something that scientists have rarely, if ever, seen before.
"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.
Because this type of earthquake is so unusual, Bilham and colleagues used interferometric synthetic aperture radar, a type of satellite data, to confirm their suspicions. This type of radar uses microwaves to map a section of the Earth's surface at different times (in this case, the images were taken nearly 20 years apart), and then compares the two maps to very accurately measure seismic deformation and movement in the area.
"That sewed it up very nicely," Bilham said. His team's findings will appear in the February issue of the journal Nature Geoscience.
A caterpillar's crawl
The Kohat Plateau is creeping south at a speed of about 1 to 2 millimeters per year, Bilham said. Over the course of about 200 years, patches of the plateau can build up enough stress to cause localized earthquakes, like the one in 1992.
Over time, the plateau's movement is much like a caterpillar's crawl — a combination of slow gliding and sudden jerks.
This type of earthquake will likely become more frequent in the region, Bilham said, as the plateau slowly squeezes out its underlying layer of lubricant. Although Bilham thinks this layer is likely salt, he says no one will know for sure until scientists drill below the plateau to take samples. But whatever it turns out to be, residents of the Kohat Plateau should hold on tight.