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On April 15th and May 12th 2015, Nepal was hit with two large earthquakes, killing and injuring thousands. The Himalayas are in this region, and include Mt. Everest, the highest point on Earth. They are still growing due to the forces of plate tectonics. At the same time, due to gravity and erosion, they are trying to reach equilibrium, ultimately becoming gently rolling hills like the North Carolina Piedmont. To learn more
I spoke with Dr. Kevin Stewart, Associate Professor in the Department of Geological Sciences at the University of North Carolina in Chapel Hill and co-author of Exploring the Geology of the Carolinas. Dr. Stewart discussed plate tectonics previously on SciWorks Radio.
The Nepal Earthquake occurred on an active plate boundary which separates India from Asia. It is a convergent plate boundary where India is trying to subduct or sink beneath Asia, and it is that continental collision that essentially caused the earthquakes. There is a major fault that is separating the two continents. A lot of that contact between the two is stuck, and so although India is pushing, its not really slipping underneath Asia. Eventually the stress will build up and the fault will rupture. And a lot of the slip that has been accumulating for decades will then be released in a couple of minutes.
This is how mountains are built in a continental collision. In fact, the region containing Kathmandu rose by 3 feet. But when crust moves along a fault, it will change the points of stress, resulting in additional ruptures called aftershocks.
The main shock at the end of April had an aftershock sequence and this is common for earthquakes that you have a lot of aftershocks. The very recent 7.3 Earthquake was larger than one expects for a simple aftershock.
The epicenter of an earthquake is the point on the Earth's surface directly above the rupture where the quake originated known as the hypocenter. For the April quake, the epicenter was in the Gorkha region of Nepal, about 90 miles west of Kathmandu.
The depth of the hypocenter has a pretty strong control on the amount of damage. This particular earthquake occurred at about 15 kilometers depth, about 10 miles or so, which is shallow. And so because the primary energy release was at a relatively shallow depth, that means that a lot of that energy gets transmitted to the surface pretty efficiently. So when you have a shallow earthquake, the damage is often much greater because the shaking is often much more intense. When we're looking at that large aftershock on May 12, that was a 7.3, so both quakes the 7.8 and the 7.3 are very strong earthquakes, and that's why we did see so much damage.
The 2 quakes were 7.8 and 7.3. Those are both big, but this .5 difference represents almost about a 6-fold increase in energy released.
If we remember back to the 1989 Loma Prieta Earthquake in the San Francisco Bay area, the one that happened during the World Series, that was a 6.9. So here we're looking at a magnitude almost one full magnitude tick above that earthquake. One whole step in the magnitude scale, we typically think of that as about a 30 fold increase in the energy release. A magnitude 7.8 earthquake is a whole lot more shaking and a whole lot more energy release than even a 6.9.
This kind of mountain building has happened before, including right here in North Carolina.
The Appalachians and most of the topographic relief in the Appalachians was created about 300 million years ago when Africa was colliding with North America. And that was the same kind of plate tectonic setting that is now happening in the Himalayas, where India is colliding with Asia. And during these continental collisions you get very large mountain ranges. So, 300 million years ago when we were sort of at the peak of the collision between Africa and North America, we had what was probably a Himalayan scale mountain range running through Western North Carolina, and no question we were having earthquakes like this and larger during that time.