A bit of an off-topic article about earthquakes / by The Green Medium

About twice a minute, two massive plates somewhere on the planet slip past each other slightly faster than usual, releasing a little bit of tension and vibrating the area around it ever so slightly.

Luckily for everybody, most of those vibrations are so tiny that we can’t even feel them. And even then, most of the remainder won’t even deal more damage than, say, a few smashed plates, or falling over at inconvenient times because wow why is the ground suddenly a lot closer to my knees.

All of these tectonic jerks — earthquakes, as literally all of you know, it’s in the title and I have no idea why it took me so long to get here — release energy when they happen. They have to, in fact, because conservation of energy stops for nothing. There’s a few ways that it can happen, too, because the arrangement of tectonic plates on top of and around one another can vary drastically.

Ignoring other causes of earthquakes like volcanism and fracking, there’s two (three?) general ways that plates can shift to cause earthquakes. But first: terminology.

The interface between the two plates has two axes (maybe? I’m not entirely certain that geophysicists actually use them as axes): the strike, which lies pretty much tangential to the surface of the earth, and the dip, which lies at an angle from perpendicular to the surface, and 90º from the strike. The angle of the dip from normal to the surface is called the dip angle. If the dip angle is not zero, then one of the two plates is on top of the other — this plate is called the “hanging-wall” and the face it rests on is the “footwall”.

Here’s a terrible diagram to go with that terrible explanation:

I can just hear my drafting professor now: "What's going on with those hatch lines, Ben" "What are you doing in my house," I reply

I can just hear my drafting professor now:

"What's going on with those hatch lines, Ben"

"What are you doing in my house," I reply

 

The way that the two plates actually stick together is due to asperities on the surfaces of the two plates. Asperities are the roughness on any surface that end up causing friction— but something tells me that they might be a lot larger on, well, continental sized rocks. The asperities of two plates lock together and prevent slipping, so they prevent continental drift from taking its due course. Eventually the force becomes too great, and slipping occurs, causing an earthquake.

If the plate slips along the strike, it’s appropriately called a strike-slip. Kindly, the term dip-slip is used when the direction of movement is along the dip. While direction doesn’t matter for strike-slipping, it does for dip-slips: “normal” is used if the hanging-wall slides down relative to the footwall, and “reverse” refers to the hanging wall moving up.

The three different modes are important in terms of energy released, as well. Normal earthquakes generally release the least, then strike-slips, then reverse. In particular, reverse slips with dip angles greater than 45º are called thrust earthquakes, and are responsible for the most violent and destructive events.

HOW DO WE MEASURE THIS?

There’s a few different ways to measure the intensity of an earthquake, and so several different scales have been developed and used to help describe them. Subjective systems like the 12 point Mercalli scale have been used because they convey the qualitative, tangible effect of a particular quake. This, however, doesn’t provide a ton of understanding in regards to the actual mechanics and as a result, more rigorous and quantitative scales have become more preferable over the past century.

Two magnitude scales are particularly important. The first, developed by (and named for) Charles Richter in 1935 uses the amplitude of seismic waves in order to measure the intensity. This kind of amplitude can vary pretty massively from earthquake to earthquake, so he used logarithms (base 10) to compress the scale into a length that makes more sense for actual use, with the interesting consequence that negative magnitudes can be found if your instrumentation is sensitive enough. Richter magnitudes work pretty well, but run into issues because the shaking amplitude can vary based on, say, location, without giving an amazing indication of how much energy was released. Additionally, while the scale is mathematically unbounded, it ends up saturating at around magnitude 7 because of physical limitations on how much the ground can vibrate. Most earthquake magnitude scales end up saturating after a while, causing significant problems in differentiating between the most massive earthquakes.

The exception is the second scale: the moment magnitude. Instead of relying on the amplitude of shaking, it uses more complicated calculations to find the seismic moment (which is the moment created by all the forces along the fault area) and takes the logarithm of that moment to give a scale that doesn’t saturate and is directly tied to the energy release. Particularly, a jump of 2 magnitudes on the moment magnitude scale is a 1000-fold increase in the amount of energy. 

There’s some fun patterns to how earthquakes distribute themselves in terms how much energy is released vs. how frequently they happen. It’s been found that the logarithm of the number of times an earthquake above a certain Richter magnitude in a year is a linear function of Richter magnitude. This law has been found to hold everywhere on earth, and even on the moon, and this apparently has a few implications for how earthquakes and moonquakes actually happen.

Easily the funnest fact of the article is, however, in this chart:

This provides a very, very clear indication as to just how flipping massive earthquakes over magnitude 9 are. The fact that one earthquake, that in Chile in 1960, accounts for around 20% of the moment release of all earthquakes in that 100 year period. There’s an extent to which I’d be skeptical about the exact precision of this chart, because there isn’t really an indication of how widespread the use of accurate seismometers was in the early 1900s, not even the Richter scale would be conceived for another 30 years.

The last thing that I find super cool about this is that the fact that each step down in magnitude appears to release about half as much energy. This is probably due to the same law law about magnitudes and frequencies that was seen earlier, but it’s much better when its visual.

 

uuuuh, so that went a bit off the rails. Here’s the information that I got bogged down in, in a slightly less compressed form:

 

http://www.geo.mtu.edu/UPSeis/intensity.html

http://www.weather.gov.hk/education/edu02rga/article/ele-EarthquakeMagnetude_e.htm

http://www.ucl.ac.uk/EarthSci/people/sammonds/6%20Seismic%20moment.pdf

http://www.geos.ed.ac.uk/homes/scrampin/opinion/Crampin_Gao%282015%29.pdf

I also went super old school and read the Earthquake section of the Encyclopedia Britannica from 1988 because I don’t know what decade it is, apparently

Next up: Hopefully I can focus on what the blog is actually supposed to be about

-Ben