Understanding The Mechanics Of Earthquakes

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The "faille des Causses", a geological fault in the Grands Causses, as seen from Bédarieux (Hérault, France).

With the August 23, 2011 earthquake on the East coast of the United States, the internet has been a-buzz. We had quite the interesting discussions, behind closed doors, here at GeekMom. GeekMoms Patricia Vollmer and ChaosMandy started the discussion on the site.

As someone who lives in the Pacific Ring and an area that has tremors, of a variety of magnitudes, pretty much daily–by mid-day on August 23, 2011, my area had eight earthquakes and California had quite the number as well, with three of those earthquakes, ranging from a magnitude 3.6 – 4.2, reported to be felt within a 12 hour period, between August 23, 2011 at 11:37 PM PDT – August 24, 2011 at 9:58 AM PDT. On the less frequent side of things, Colorado also experienced a magnitude 5.3 earthquake on Tuesday, August 23, 2011–plus as someone who took Geology as their science in university, I find this topic beyond fascinating!

I think the first thing people need to understand is that the earth is still forming. It may not be changing in the same extreme as it was 600+ million years ago, but she is changing just the same. Mountains are still building. Continents are still moving. Some plates are migrating faster than others. The East Pacific Rise is moving at a rate of 15 cm per year, while The Arctic Ridge is moving at a rate of less than 2.5 cm per year. But just the same, they are moving. And part of the result of these movements are earthquakes, volcanoes, tsunamis, landslides and more.

This subject can also be rather daunting to navigate, especially when words, such as, “dip-slip fault”, “strike-slip fault”. “oblique-slip fault”, “divergent boundary”.  “convergent boundary”, “transform boundary”, “normal fault”, “thrust (reverse) fault”, with an added dash of “depth”, “length”, and “magnitude” thrown in. What on earth–not to be punny–does any of this mean and how does it help us to understand what occurred on the East coast of the United States?

How one will experience the result of this movement and how far the waves will travel depend on a variety of factors. They include the depth of the quake, fault type and nature of the surrounding area.

I am going to try my best to explain this as simply as possible without the use of correct scientific phrasing. I don’t want to confuse something that can already be quite confusing, especially if you do not have any type of background in science. The names given to the types of faults or boundaries has to do with the direction of motion.

If two plates are moving towards each other, they are convergent. If two plates are moving away from each other, such as in Iceland, they are divergent. If two plates are moving in opposite direction, horizontally beside each other, they are transform. The “slip” has to do with movement along the other axis.  “Dip” is when one plate is moving under, and in some cases is thrust over, another plate. “Strike” is when they rub each other, horizontally. “Oblique” is when there is movement both horizontally and vertically.

One of the nearest faults to where I live is a subduction zone, dip-slip and falls under the title of convergent boundary. As I said, it can be confusing. Basically, what that means is one piece of the earth is moving under another. The plate is always moving and, as a result, we have a lot of seismic activity, even if we do not feel it. Convergent boundaries and subduction zones are associated with tsunamis and volcanoes. The Chili earthquakes, the huge 1964 Alaskan earthquake and resulting tsunami that devastated my coast, the 2011 Tōhoku earthquake, Mount St Helen and Mount Fuji are examples of the types of activity occurring on and near subduction zones.

The San Andreas fault is a transform boundary and strike-slip. Think of the two plates rubbing each other and moving in opposite direction of each other, north-to-south. The San Andreas fault is particularly interesting because most transform boundaries are found on the ocean’s floor. Transform boundaries are also associated with shallow quakes. Some strike-slip earthquakes can really transform the earth, in quite measurable ways, such the magnitude 7.2 Baja California earthquake of April 4, 2010, which moved things in a downward and southerly direction up to 80 centimeters (31 inches).

Iceland, which is on two plates being pulled away from each other, is a marvel. The ground cracks, the fissure widens, magma rises and new land is form. Aside from this process, there is also a lot of volcanic activity.

Sometimes, the plates can lose their ‘lubrication’ and get stuck. This will cause pressure and stress to build. I’m sure you know what happens when you don’t release stress. When finally you uncork, it can be explosives. Think of earthquakes as a person who hasn’t taken the time to properly vent as small issues start to pile-up. When they finally unleash, watch out. These will cause masses to be pulled closer together or ripped apart at a great force. Even if you live in an area that has a lot of seismic activity, stress can still build and the results, explosive.

Trying to predict these things is rather difficult. Even in my area, where we have constant seismic activity, we are waiting for the next “big one”. Despite the little releases of stress on a daily basis, stress is still building. Because of the types of faults I live near, not only do I get to worry about large magnitude quakes but I also get to worry about tsunamis.

Are Virginia and other places in the US less prone to earthquakes? Yes. Does that mean they do not and cannot happen? No. Since Virginia became a state, there have been approximately 300 earthquakes recorded. Virginia is not alone there. To be even more specific, 39 of the 50 United States have a moderate to high risk for earthquakes.

How will this help you to know what type and size of damage an earthquake may cause in your area? I don’t think it will. There are so many variables involved in determining this: magnitude, distance to the earthquake focus or source, focal depth, type of faulting and type of material/ nature of the surrounding area. The best thing you can do is be prepared. Some of the most powerful earthquake in North American history have occurred inland and along the boundaries of The North American Plate. Not only do we have the major plates to contend with, but fracture zones all over the place.

And remember, magnitude isn’t always related to damage or severity, even in areas that are considered prepared for earthquakes. I’ve experiences earthquakes over a magnitude of 6 and hardly felt them. I’ve experienced a quake that was just under a magnitude of 5 that caused my apartment building to sway and leave permanent ripples in the ceiling.

But the earth is awesome, in the true sense of the word. The mechanics of how she functions, causes me to geek-out to no end. Don’t even get me started on how all of the above is one of the earth’s ways that it recycles and, as a result, has slower changing landmasses.

 

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