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Today, on #SeismogramSaturday: what are all those strangely-named seismic phases described in seismograms from distant earthquakes? And what do they tell us about Earth’s interior? 
In the past couple of weeks we’ve discussed body waves (P and S waves, which move through the body of the Earth) and surface waves (Love and Rayleigh which, unsurprisingly, only travel across Earth’s surface).
But in some seismograms, notably those recorded far away from the earthquake epicenter, we see lots of other arrivals. What are these, and why do they have such strange names?
First, we have to discuss what lies below us, in the planet’s interior. You may know that the Earth has a layered structure, with a crust (5-70 km thick), a mantle (~2900 km thick), and a core (~3500 km in radius). iupui.edu/~g115/mod04/po…
The core is further subdivided into a solid inner core and a liquid (really!) outer core. Note that the outer core is the ONLY part of Earth’s interior that is liquid—the mantle is solid (a common misconception is that it is made of magma—it is not!)
When seismic waves are generated by an earthquake, they travel in all directions, so most go down into the Earth, where they will encounter all of these different layers. Because waves travel at different speeds in each of these layers, the wave path bends (this is refraction).
Waves are lazy and prefer to travel where they move slowly. This means that in most cases they refract back towards Earth’s surface where they can travel more slowly (the exception is when they hit the outer core, where they bend the opposite way).
Note that S waves cannot travel through liquid because liquids have no shear strength. Since direct S waves don't show up on the far side of the Earth, we know the outer core is liquid. iris.edu/hq/inclass/ani…
Ultimately, all of these waves will eventually return to the Earth’s surface where they are recorded by seismometers. Those are the seismograms you see here (in this case, these are waves from Thursday’s M7.5 quake in Ecuador, as recorded in Canada).
Seismologists need to know what path a wave took, because we use these waves to study the regions through which they have traveled. To make this clear, we give them names that describe where they’ve been.
Each leg of the journey is given a letter designation. A P wave traveling in the mantle is called P. But when it enters the outer core, we call that legs of its journey K. And if it then goes through the inner core, we call that branch I.
The wave has to come back out again, through the outer core again (K) and then back through the mantle (P). So its full journey is described by its new name: PKIKP (mantle-outer core-inner core-outer core-mantle).
Waves don’t just pass through these layers, though—some of the energy reflects off of them. We give reflections a name too: c is a bounce off of the outer core, and i (note the small letters) is a bounce off of the inner core.
PcP is thus a P wave that travels through the mantle, bounces off of the outer core, and returns to the surface. ScS does the same, but is an S wave. media.springernature.com/m685/springer-…
Can we take a moment to think about how cool that is? These waves are BOUNCING OFF OF THE EARTH’S CORE.
In fact, it was one was one of these reflected waves (PKiKP), recorded in 1929, that provided seismologist Inge Lehman with the data she needed to show that Earth’s core had a solid inner layer.
There’s one more crazy thing—depending on the angle at which a wave hits the boundary, some of the energy can also convert from P to S, or from S to P. Thus you can have PcS (a wave that starts as a P wave, bounces off of the outer core, and converts to an S)!
SKKP is an S wave that travels through the mantle, converts to a P at the outer core, bounces within the outer core (so it travels two legs of the journey within the outer core), and then travels as a P in the mantle.
Things can get kinda crazy—anyone want to tackle SKiKPPcS?
There are many more phase names, for waves that bounce off of more subtle layers in Earth’s interior (P660, anyone?) or for waves that travel in the oceans or atmosphere. These are all described here: ds.iris.edu/data/vocab.htm
So when an earthquake is recorded far from its source, you’re not just getting P and S waves, you’re getting all of these bouncing, refracted waves. Each one shows up at a different time, and this creates the complex waveforms we see.
Remember, the important thing isn’t the name of the phase, it’s the fact that its speed, its amplitude, and its path all tell us what Earth’s interior is like. This is immensely powerful, since we can’t travel that deep underground (the movie The Core, notwithstanding).
A really great tool for visualizing all of this is Seismic Wave. You can see how the waves move in the Earth, what their seismograms look like on the surface. ds.iris.edu/seismon/swaves/
As ever, post your questions here! And enjoy the rest of your #SeismogramSaturday.
