Thursday, May 21, 2015

The Nepal Earthquake Part 1: Plate Tectonics

May 7, 2015
12:28 pm

I still remember the Nisqually Earthquake of 2001. Ironically enough, we had just finished an "earthquake drill" when our second-grade teacher shouted "Earthquake! For real!" We all immediately bolted under our desks, not necessarily because we were concerned about falling debris, but because we just liked bolting under desks. You could say it was a guilty pleasure of ours, I guess.

Many of those who have not been in an earthquake or who have not studied them equate them to the shaking and rattling of the Earth, but when I was under my desk, the movement seemed more akin to organized waves propagating throughout the room. When an earthquake occurs, it sends seismic waves from its hypocenter that travel thousands of miles per hour through the ground, so I have a feeling I was sensing the motion of these waves. It was actually pretty fun... I clearly remember having this vision of being in a boat on the water navigating through heavy seas, and these 'seas' were the seismic waves. Nothing was being damaged or falling down... it was just a little 15 second roller-coaster ride. Afterwards, we resumed school as if nothing had happened. The only place in Seattle that had sustained significant damage was Pioneer Square, which has old buildings that are not built to the same engineering standards as the other, more modern buildings in our area.

The Nisqually Earthquake may have been fun for 8-year-old Charlie, but the Nepal earthquake is horrible for everybody. There have been a number of severe earthquakes that the world has experienced over the past decade (Indonesia, Haiti, and Japan are just a few that come to mind), and the Nepal earthquake is right up there with them. Although it did not have the destructive tsunami that the earthquakes in Indonesia and Japan had, the damage that it has caused is simply unbelievable. Whole villages have been turned to rubble. Langtang Village, a small settlement of 435 people in the Langtang Valley north of Kathmandu, was buried under a landslide that, according to evewitness reports, was 2-3 kilometers wide and 100 meters deep. Seeing some of the destruction through pictures online makes me very sad, and I can't imagine how the friends and families of those affected must be feeling right now. A girl I grew up with and her friend are still missing from the quake and are presumed to have died (I wrote a blog post about it here), and the earthquake has directly impacted my community in a way I never thought it would.

I've always been not only a weather buff but an earth science buff in general, and that extends to geology. I think we can all agree that the Nepal Earthquake is terrible and we wish it never happened. But earthquakes themselves do not have any regard for life or property; they are just a natural consequence of plate tectonics. In that aspect, I think the Nepal Earthquake is a fascinating geological event, and that's what I'd like to blog about here since you hear so much about the death and destruction caused by the earthquake in the media. I hope you can join me in taking an interest in the geological nature of the earthquake and the processes that led to it.

This will be a three-part blog; the first part will explain some of the basics of plate tectonics, the second will explain the basics of earthquakes, and the third will look specifically at the earthquake in Nepal and how it compares to some recent earthquakes around the world.

Mt. Everest from Gokyo Ri, November 5, 2012. Retrieved from Wikimedia Commons.

In order to understand the Nepal earthquake, we first have to understand the geology of the Himalayas. The Himalayas are the world's tallest mountain range; out of the world's 10 tallest mountains, 9 reside in the Himalayas. The Himalayas are so tall for a variety of reasons, and not all of them are related to plate tectonics. They are young and located at low latitudes, and as such, they have not been heavily eroded by glaciers. The dominant reason why they are so tall, however, is due to the type of plate boundary that forms them... a continental-continental convergent boundary.

There are two general types of plates: oceanic plates and continental plates. Oceanic plates are thinner, denser, and comprised of basalt. Continental plates are thicker, less dense, and are mainly composed of granite. When one of these plates hits another plate, the less dense plate generally goes under the more dense plate. The characteristics of all plates are not the same, so some oceanic plates are denser than others, and the same is true for continental plates. There are many types of earthquakes, but the largest and most destructive ones are generally caused due to different plates smashing into each other. There are three kinds of these "convergent plate boundaries:" oceanic/oceanic, continental/oceanic, and continental/continental, so let's take a brief look at each of them and how they differ from each other.

The Aleutian Island chain is a great example of a convergent oceanic/oceanic plate boundary. The Pacific Plate is being subducted under the North American plate, creating a beautiful arc of volcanic islands and a deep ocean trench in the process. Note that the plates themselves are NOT the oceanic/continental crust; they are subdivisions of the lithosphere, which is the crust and the upper, rocky portion of the mantle. The movement of these plates is driven by convection currents on the asthenosphere, which, while still solid, is more viscous and deformable, and is liquid at certain locations, like mid-ocean ridges (more about those later).

Oceanic/oceanic convergent plate boundary. Retrieved from USGS.

Map of the Aleutian Trench and Islands. Retrieved from Wikipedia Page on Aleutian Trench.

The continental/oceanic convergent plate boundary is the one that we see along the West Coast of not only North America but South America as well. Our Cascades were formed by this plate boundary, and we will certainly have an extremely powerful earthquake within the next several hundred years, likely on par with the Japan earthquake back in 2011, and perhaps even stronger. The Andes are a textbook example of this type of plate boundary, as they have an extremely deep trench and very high (over 20,000 feet!) mountains in close proximity to each other. They have the strongest earthquakes in the world; the 1960 Nazca Earthquake was a 9.5 magnitude earthquake, which is approximately 800 times more powerful than our relatively puny Nisqually earthquake in 2001 (remember, earthquakes are measured on a logarithmic scale). These cataclysmic earthquakes are not just limited to continental-oceanic plate boundaries though, they occur at oceanic-oceanic convergent plate boundaries too. A 7.9 earthquake actually just struck the Aleutian Islands less than a year ago.

Continental/oceanic convergent plate boundary. Retrieved from USGS.

Continental-continental convergent boundaries create very high mountain regions for two reasons: no subduction takes place because the plates are too buoyant, and the plates have much more mass and are far thicker than oceanic plates. This leads to extremely high mountain ranges like the Himalayas of Nepal/Tibet and the Karakorum of Pakistan. These boundaries often create a massive plateau behind the main mountain range due to rock being pushed upwards instead of subducted. The Tibetan Plateau, which is five times the size of France and, on average, taller than Mt. Rainier, is a textbook example. These boundaries do not tend to have volcanoes and "megathrust" earthquakes (earthquakes approaching a magnitude of 9.0 or greater), but as we have seen in Nepal, they can still generate extremely powerful quakes.

Continental/continental convergent plate boundary. Retrieved from USGS.

These convergent boundaries are responsible for creating most of our mountain ranges. It makes sense intuitively; if you have a plate subducting underneath another, it's going to push that other plate up, and in the case of a continental-continental plate boundary, when subduction is not an option, the ground has nowhere to go but up!

As well as I'm discussing plate boundaries, I might as well briefly touch on the other two, divergent and transform, as earthquakes occur on all plate boundaries, not just convergent ones. Divergent plate boundaries arise when hot masses of the asthenosphere rise and burst through the lithosphere. This mass then cools, creating a new basalt crust and lithosphere. This formation of new lithosphere pushes each plate away from each other, causing them to diverge; hence the name divergent plate boundary.

Divergent plate boundary. Retrieved from USGS.

Ironically enough, divergent boundaries can create mountain ranges as well due to the magma rising through the lithosphere and cooling, forming a range. However, most of these ranges are underwater, because when plates diverge, seawater inevitably flows in after the rift between them has gotten large enough that seawater can flow through. Ever notice how the east coast of South America and west coast of Africa look like they fit with each other? Well, it is widely accepted that they were once connected in Pangea, which was a "supercontinent" composed of all the current continents, and that they began to break up 180 million years ago. In the middle of the Atlantic between these continents is the Mid-Atlantic Ridge. The Mid-Atlantic ridge is only one of many; the ocean is filled with these ridges.

Not all divergent plate boundaries occur underwater though. The East African Rift is a divergent boundary along equatorial Eastern Africa from Ethiopia to Malawi. There are several very deep lakes and stunning volcanoes in this area, including one that's becoming famous for its stark decrease in snow coverage (note: it's not due to global warming, it's due to deforestation).

Kilimanjaro's retreating glaciers. Credit: Penn State Department of Meteorology

Transform boundaries neither converge nor diverge; they slide right past each other. They differ from convergent and divergent boundaries in they are conservative plate boundaries, meaning lithosphere is neither created (divergent plate boundary) or destroyed (subduction zones). When plates are subducted or created, they are generally not done so in a uniform fashion, and rather have many individual transform faults connecting the overall subducting or diverging structure of the plate boundary itself. While they are most common around mid-ocean ridges, the most destructive ones occur near subduction zones on land. The San Andreas Fault in California is one of the most famous transform faults in the world, and is responsible for delivering many catastrophic earthquakes to the sunshine state.

The San Andreas Fault, an example of a transform boundary. Retrieved from USGS.

The San Andreas Fault has a striking structure and is something I would like to see in real life. Take a look at this photo below!

Aerial photo of the San Andreas Fault in the Carrizo Plain. Retrieved from Wikipedia

Southern California and Baja California are currently sliding northwards, and in approximately 50 million years, they will become subducted under the Aleutian Trench. So in the very very slight possibility that Los Angeles is still a bustling city 50 million years from now, citizens there will have to move somewhere else.


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